national lighting solutions
LED Drivers for High-Brightness Lighting
Solutions Guide
national.com/LED
2009 Vol. 2
Temp Sensor
PWM Dimming Signal
MCU
3.3V
48V
General Illumination LED Drivers
Technology Overview
Product Highlights
Application Information
Design Examples
Articles
AC/DC
LED Driver
LED Driver
LED Driver
High-Brightness LED Lighting
Overview
Regardless of type, color, size, or power, all LEDs work best
when driven with a constant current. LED manufacturers specify
the characteristics (such as lumens, beam pattern, color) of
their devices at a specified forward current (IF), not at a specific
forward voltage (VF).
Most power supply ICs are designed to provide constant voltage
outputs over a range of currents (see below), hence it can be
difficult to ascertain which parts will work for a given application
from the device datasheet alone.
With an array of LEDs, the main challenge is to ensure every LED
in the array is driven with the same current. Placing all the LEDs
in a series string ensures that exactly the same current flows
through each device.
High-Brightness LEDs: Input Voltage and
Forward Voltage
Sources of input voltage for LED arrays come from batteries or
power supplies that have a certain tolerance. An automotive
battery, for example, may supply 8V to 16V depending on the load
and the age of the battery. The "silver box" power supply inside a
desktop CPU may supply 12V ±10%.
High-brightness (HB) LEDs also give a range of forward voltage.
A typical HB LED might be characterized at a forward current
of 350 mA. The forward voltage of the LED when IF = 350 mA is
specified with a range that includes a typical value as well as
over-temperature maximum and minimum values. To ensure that
a true constant current is delivered to each LED in an array, the
power topology must be able to deliver an output voltage equal
to the sum of the maximum forward voltages of every device
placed in the string.
Manufacturers bin their devices for color, brightness, and
forward voltage. Binning for all three characteristics is
expensive, and forward voltage is often the specification that
is allowed to vary the most. Adding this to the shift in forward
voltage as the LED die temperature changes gives rise to the
need for constant-current regulators that have a wide range of
output voltage.
Constant Voltage Regulator
2
Constant Current Regulator
When Input Voltage Exceeds
LED Voltage
When Input Voltage is Less than
LED Voltage
If input voltage always exceeds the sum of the maximum
forward voltages of every LED in a string, then two options are
available: linear regulators and buck regulators.
When the minimum forward voltage of all the LEDs in a string will
always exceed the maximum input voltage, a step-up, or boost,
regulator is needed.
A linear regulator introduces efficiency and thermal drawbacks,
but is the simplest design option. In order to provide constant
current, the linear regulator must be an adjustable type that
uses a pair of feedback resistors. Replacing the top feedback
resistor with the LED string and placing a current-sensing
resistor in the bottom position "tricks" the former constant
voltage source into adjusting the output voltage until enough
current flows through the current sensing resistor to equal the
feedback voltage of the IC.
The inductive-boost converter is the simplest regulator that can
deliver currents above 350 mA with a varying output voltage.
As with linear and buck regulators, a boost converter with a
feedback-divider network can be modified to become a constant
current source. One important distinction between the buck
regulator and boost regulator must be made when the power
switch is internal to the control IC. Such monolithic systems have
a fixed current limit.
Linear regulators have the advantages of simplicity, low parts
count, and very little Electromagnetic Interference (EMI).
They can deliver constant current as long as the VF in the LED
string does not exceed their dropout limited output voltage.
The disadvantage lies in efficiency and thermal dissipation.
Loss in a linear regulator LED driver is approximately equal to
(VIN – n x VF) x IF , where "n" is the number of LEDs in the string.
At currents of 350 mA and above, the linear solution may require
a heatsink, adding cost and size to the design.
The more efficient option when input voltage always exceeds
the LED voltage is a step-down or buck regulator. As with
linear regulators, this must be an adjustable type, and the same
method can be used to turn almost any buck regulator into a
constant current source for LEDs. Buck regulators enjoy high
efficiency and eliminate the need for a heatsink, at the cost of a
more complex circuit and the addition of switching noise. Many
recent buck regulators switch at 1 MHz and above, making their
external components so small that at currents under 1A they
may actually use less space than a linear regulator.
national.com/LED
In buck regulators, the internal switch passes the same DC
current as the LED. A boost converter differs in that the internal
switch sees a higher current that varies with input voltage;
the greater the difference between VIN and VOUT, the higher
the internal switch current. Care must be taken to evaluate a
monolithic boost regulator-based LED drive to make sure that it
will not hit the fixed current limit over the range of input voltage.
When Input Voltage Range Overlaps LED
Voltage Range
As HB LEDs are adopted into more and more applications,
situations will arise when the input voltage varies above and
below the forward voltage of the LED string. For these cases,
a current regulator is needed that can both buck and boost as
required by the input and output conditions. Possible topologies
include the buck-boost, SEPIC, Cuk, flyback, and VIN referenced
buck-boost (also called the floating buck-boost).
In all of these topologies, the power-switch current exceeds the
LED current and varies with input voltage. The same attention to
peak switch current must be made over the full range of input
voltage, especially if a regulator with an internal power switch
and fixed current limit is implemented. For more information about
National’s LED products, samples, design simulation tools, and
more, visit: national.com/LED.
3
LED Drivers Selection Tables
Buck (Step-down) High-Brightness LED Drivers
!
NEW
!
NEW
Product ID
VIN Range (V)
VOUT (V)
ILED
(A)
No. of LED
LM3407 E, W
4.5 to 30
Up to 27
0.35
LM3402/HV E, W
6.0 to 42/6 to 75
Up to 37/67
LM3404/HV E, W
6.0 to 42/6 to 75
LM3405A E, W
Multi-Output
Internal
SWITCH
1 to 7
—
0.425
1 to 9/15
Up to 37/67
1
3.0 to 22
Up to 20
LM3406/HV E, W
42/6.0 to 75
LM3401 E, W
Topology
Key Features
4
Floating
Buck
Constant frequency PWM with true average
current control
—
4
Buck
200 mV feedback voltage, fast PWM dimming
1 to 9/15
—
4
Buck
200 mV feedback voltage, fast PWM dimming
1
1 to 3
—
4
Buck
200 mV feedback voltage, fast PWM dimming,
thin package
Up to 37/67
1.5
1 to 9/15
—
4
Buck
200 mV feedback voltage, fast PWM or two-wire
dimming, true average current control
4.5 to 35
Up to 35
3
1 to 9
—
—
Buck
Dual-side hysteresis, very low reference voltage
and short propagation delay
LM3409/HV E, W
6.0 to 42/6.0
to 75
Up to 42/75
3.0+
1 to 9/15
—
—
Buck
External high-side P-FET current source with
differential current sensing and analog current
adjust
LM3421 E, W
4.5 to 75
Adjustable
3.0+
1 to 16
—
—
Floating
Buck
20 mV to 1.235V adjustable differential current
sense voltage, 50 kHz max PWM dimming
LM3423 E, W
4.5 to 75
Adjustable
3.0+
1 to 16
—
—
Floating
Buck
20 mV to 1.235V adjustable differential current
sense voltage, 50 kHz max PWM dimming; fault
timer; LED ready flag; high-side dimming
LM3424 E, W
4.5 to 75
Adjustable
3.0+
1 to 18
—
—
Buck
Temperature foldback, synchronizable 50 kHz max
PWM dimming
LM3429 E, W
4.5 to 75
Adjustable
3.0+
1 to 20
—
—
Buck
50 mV to 1:25 adjustable high-side current-sense
voltage, analog and PWM dimming
LM3433 E, W
-9.0 to -14
Up to 6
20+
—
—
—
Negative
SYNC
Buck
Negative output voltage capability allows LED
anode to be tied directly to chassis for max heat sink
efficacy
Boost (Step-up) High-Brightness LED Drivers
!
NEW
!
NEW
Multi-output
Internal
SWITCH
3 x 10
4
2.1(1)
1 to 5
Adjustable
3.0+
4.5 to 75
Adjustable
LM3424 E, W
4.5 to 75
LM3429 E, W
4.5 to 75
Product ID
VIN Range (V)
VOUT (V)
ILED (A)
No. of LED
LM3431 E, W
5.0 to 36
40
0.15
LM3410 E, W
2.7 to 5.5
24
LM3421 E, W
4.5 to 75
LM3423 E, W
Note (1) Specified in ISW
4
Topology
Key Features
—
Boost
LED protection: short, open, and thermal
—
4
Boost
Ultra-low stand-by current of 80 nA, internally
compensated
1 to 20
—
—
Boost
20 mV to 1.235V adjustable differential current
sense voltage, 50 kHz max PWM dimming
3.0+
1 to 20
—
—
Boost
20 mV to 1.235V adjustable differential current sense
voltage, 50 kHz max PWM dimming; fault timer;
LED ready flag; high-side dimming
Adjustable
3.0+
1 to 18
—
—
Boost
Temperature foldback, synchronizable
50 kHz max PWM dimming
Adjustable
3.0+
1 to 20
—
—
Boost
50 mV to 1:25 adjustable high-side current-sense
voltage, analog and PWM dimming
PowerWise® product
E Evaluation
board
W WEBENCH
enabled
Buck-Boost High-Brightness LED Drivers
!
NEW
!
NEW
Product ID
VIN Range
(V)
VOUT (V)
ILED (A)
No. of LED
Multi-Output
Internal
SWITCH
LM3410 E, W
2.7 to 5.5
24
2.1(1)
1 to 5
—
LM3421 E, W
4.5 to 75
Adjustable
3.0+
1 to 20
LM3423 E, W
4.5 to 75
Adjustable
3.0+
LM3424 E, W
4.5 to 75
Adjustable
LM3429 E, W
4.5 to 75
Adjustable
Topology
Key Features
4
SEPIC
Ultra-low stand-by current of 80 nA, internally
compensated
—
—
Floating Buck-Boost
SEPIC
20 mV to 1.235V adjustable differential current
sense voltage, 50 kHz max PWM dimming
1 to 20
—
—
Floating Buck-Boost
SEPIC
20 mV to 1.235V adjustable differential current
sense voltage, 50 kHz max PWM dimming; fault
timer; LED ready flag; high-side dimming
3.0+
1 to 18
—
—
Floating Buck-Boost
SEPIC
Temperature foldback, synchronizable
50 kHz max PWM dimming
3.0+
1 to 20
—
—
Buck-Boost
Flyback SEPIC
50 mV to 1:25 adjustable high-side current-sense
voltage, analog and PWM dimming
Note (1) Specified in ISW
Offline High-Brightness LED Driver Solutions
Product ID
VIN Range (V)
VOUT Max
(V)
ILED
(A)
No. of LED
LM3445 E, W
80 to 270
Adjustable
1+
1 to 14+
PowerWise® product
E Evaluation
national.com/LED
board
W WEBENCH
Multi-Output
Internal SWITCH
Topology
Key Features
—
—
Floating
Buck
Integrated TRIAC dim decoder circuit for
LED dimming. Adaptive programmable offline
allows for constant ripple current. No 120/100
Hz flicker
enabled
5
Key Products Overview
LM3409 – PowerWise® PFET Buck Controller for High-Power LED Drivers
Theory of Operation
The LM3409/09HV are P-channel MOSFET (PFET) controllers for
step-down (buck) current regulators. They offer wide-input voltage
range, high-side differential current sense with low adjustable
threshold voltage, fast-output enable/disable function, and a
thermally enhanced eMSOP-10 package. These features combine
to make the LM3409/09HV ideal for use as constant-current
sources for driving LEDs where forward currents up to 5A are
easily achievable.
Typical Application Circuit
R UV2
R UV1
RADJ
VIN
UVLO
10
VIN = 36V
CF
2
IADJ
VCC
C IN1
C IN2
9
C F2
3
R OFF
The LM3409/09HV uses Constant Off-Time (COT) control to regulate
an accurate constant current without the need for external controlloop compensation. Analog and Pulse-Width Modulation (PWM)
dimming are easy to implement and result in a highly linear dimming
range with excellent achievable contrast ratios. Additional features
include programmable Under-Voltage Lockout (UVLO), low-power
shutdown, and thermal shutdown.
1
LM3409
EN
CSP
VO = 24V
8
R SNS
4
CSN
COFF
7
DAP
C OFF
5
PGATE
GND
6
Q1
ILED = 700 mA
maximum
L1
D1
CO
LM3406/06HV – PowerWise® 1.5A Constant-Current Buck Regulator for
Driving High-Power LEDs
Theory of Operation
The LM3406/06HV is a buck regulator with a wide-input voltage
range, low-voltage reference, and a two-wire dimming function.
These features combine to make the LM3406/06HV ideal for use
as a constant-current source for LEDs with forward currents
as high as 1.5A. The controlled on-time architecture uses a
comparator and a one-shot on timer that varies inversely with
input and output voltage instead of a fixed clock.
The LM3406/06HV also employs an integrator circuit that
averages the output current. When the converter runs in
continuous conduction mode (CCM), the controlled on-time
architecture maintains a constant switching frequency
over changes in both input and output voltage. This gives
the LM3406/06HV an accurate output current, fast transient
response, and constant switching frequency over a wide range
of conditions.
6
Typical Application Circuit
V IN
CB
VIN,VINS
BOOT
L1
SW
R ON
C IN
D1
RON
F
LM3406/06HV
VOUT
CS
DIM
R SNS
COMP
CC
GND
VCC
CF
LM3407 – PowerWise® 350 mA, Constant-Current Output Floating Buck Switching Converter
for High-Power LEDs
Theory of Operation
Typical Application Circuit
The LM3407 is a constant-current output floating buck switching
converter designed to provide constant current to high-power
LEDs. The device is ideal for automotive, industrial, and general
lighting applications.
The LM3407 has an integrated power N-MOSFET. An external 1%
resistor allows the converter output voltage to adjust as needed
to deliver constant current accurately to a serially connected LED
string. The switching frequency is adjustable from 300 kHz to 1 MHz.
The LM3407 features a dimming input to enable LED brightness
control by Pulse-Width Modulation (PWM).
LM3421/23/24/29 – PowerWise® N-Channel Controllers for Constant-Current LED Drivers
Theory of Operation
The LM3421/23/24/29 devices are versatile high-voltage LED
driver controllers and can be configured in a buck, boost, buckboost (Flyback), or SEPIC topology. These controllers are ideal for
illuminating LEDs in a very diverse, large family of applications.
Typical Application Circuit
10V – 70V
V IN
L1
C IN
1
RUV 2
The PWM controller is designed for adjustable switching
frequencies of up to 2.0 MHz. Additional features include fast PWM
dimming, cycle-by-cycle current limit, over-voltage protection, and
input under-voltage protection.
2
CCMP
RCSH
RT
3
4
5
VIN
LM3424
HSP
HSN
EN
COMP
SLOPE
CSH
IS
RT/SYNC
VCC
20
19
18
RHSP
1A
ILED
RHSN
COUT
RSLP
17
CFS
RSNS
VIN
16
RFS
C BYP
The LM3424 includes an integrated thermal foldback feature to
provide a more robust thermal design to extend the life of the LED
and increase system reliability.
6
RUV 1
CSS
RGAIN
7
8
9
GATE
nDIM
GND
SS
15
14
ROV2
RLIM
13
TGAIN
TSENSE
OVP
VIN
Q2
12
DAP
R REF 1
10
TREF
VS
C OV
11
R REF 2
C REF
C NTC
national.com/LED
ROV1
RBIAS
NTC
7
Key Products Overview
LM3431 – PowerWise® 3-Channel Constant-Current LED Driver
with Integrated Boost Controller
Theory of Operation
The LM3431 is a 3-channel linear current controller combined with
a boost switching controller ideal for driving LED backlight panels in
space-constrained applications. The LM3431 drives 3 external NPN
transistors or MOSFETs to deliver high-accuracy constant current to
3 LED strings. Output current is adjustable to drive strings in excess
of 200 mA.
The boost controller drives an external NFET switch for step-up
regulation from input voltages between 5V to 36V. The LM3431
features LED cathode feedback to minimize regulator headroom
and optimize efficiency.
A DIM input pin controls LED brightness from analog or digital
control signals. Dimming frequencies up to 25 kHz are possible
with a contrast ratio of 100:1. Contrast ratios greater than 1000:1
are possible at lower dimming frequencies.
The LM3431 eliminates audible noise problems by maintaining
constant output voltage regulation during LED dimming.
Additional features include LED short and open protection,
fault delay/error flag, cycle-by-cycle current limit, and thermal
shutdown for both the IC and LED array.
Typical Application Circuit
LM3433 – PowerWise® Common-Anode-Capable High-Brightness LED Driver with
High-Frequency Dimming
Theory of Operation
Typical Application Circuit
The LM3433 is an adaptive constant on-time DC-DC buck
constant-current controller (a true current source). It provides a
constant current for illuminating high-power LEDs.
0.1
µF
8
VIN
-12V
BST2
HO
LED
HS
0.47µF
BST
VCC
6 µH
Q2
LO
0.01
µF
Q3
Q1
LS
DIM
SS
DIM
+3.3V
VEE
EN
4.7 µF
DMO
DIMR
LM3433
EN
GND
The PWM functions by shorting out the LED with a parallel
switch allowing high PWM dimming frequencies. Additional
features include thermal shutdown, VCC UVLO, and logic-level
shutdown mode.
TON
ADJ
ADJ
LED ANODE
COMP
270 pF
-12V
CGND
The output configuration allows the anodes of multiple LEDs to
be tied directly to the ground-referenced chassis for maximum
heat sink efficacy. The high-frequency capable architecture
allows the use of small external passive components and no
output capacitor while maintaining low LED ripple current.
CSP
CSN
44.2k
2.2 µF
22 µF
0.01
LED CATHODE
LM3445 – PowerWise® TRIAC Dimmable Offline LED Driver
Theory of Operation
Conventional TRIAC dimmers are designed to interface to a
resistive load (halogen or incandescent bulb), while today’s LED
driver solutions interfaced to a standard wall dimmer produce
120 Hz flicker of the LED and/or do not allow 100:1 dimming.
National’s LM3445 LED driver decodes the TRIAC chopped
waveform and translates the signal to dim the LEDs, achieving a
full, wide dimming range without flicker.
Industry-leading TRIAC dimmable offline LED driver solution is
perfect for any application where an LED driver must interface
to a standard TRIAC wall dimmer. National’s new TRIAC
dimmable LED driver delivers a wide, uniform dimming range
free of flicker, best-in-class dimming performance, and high
efficiency—all while maintaining ENERGY STAR® power factor
requirements in a typical application.
Typical Application Circuit
V+
VBUCK
D3
D7
BR1
R2
TRIAC
Dimmer
D4
Q1
VAC
D1
D9
D8
C10
C12
C9
+
VLED
-
R4
VLED-
D2
D10
R5
Q3
C5
L2
ASNS
U1
BLDR
ICOLL
R1
FLTR1
C3
VCC
LM3445
DIM
GATE
COFF
ISNS
FLTR2
GND
Q2
R3
C4
C11
national.com/LED
9
High-Brightness LED Applications
General Illumination
MR16
MR16 Basic Architecture
12 VAC
MR16 LED Circuit
DC-DC
LED Driver
12/24 VDC Input
from AC-DC Adaptor
1W LED x 3/
3W LED x1
MR16 Driver Solutions
VIN
No. of LED
LED Type
ILED (mA)
Recommended Part No.
Key Features
12 VDC/12 VAC
3
1W
350
LM3405A XMK
Small size, tiny SOT23-6 package
12 to 24 VDC
3
1W
350
LM3407
High efficiency, high precision of LED current
12 VDC/12 VAC
1
3W
600
LM3405A XMK
Small size, tiny SOT23-6 package
12 VDC/12 VAC
1
3W
750
LM3405A XMY
Thermally enhanced package, eMSOP-8
12 to 48 VDC
1
5W
350
LM3406
Two-wire dimming, high efficiency
12 to 24 VDC
3
1W
350
LM3401
100% duty cycle
12 to 48V
3
5W
350
LM3409
100% duty cycle, analog dimming
12 to 24 VAC-VDC
1 to 3
1W to 5W
>1
LM3421/29
Buck-boost architecture
12 to 24 VAC-VDC
1 to 3
1W to 5W
>1
LM3424
Buck-boost architecture, thermal foldback
10
Design 1: MR16 Using LM3405A
Description:
• This circuit is designed to drive a 3W high-brightness LED from an input of
12 VDC/12 VAC for halogen MR16 lamp replacement.
VIN
VIN
VOUT
Test Data:
1: Output Voltage & Current
Parameter
Reading
VIN
Loading
VOUT
ILED
12 VDC
1 LED
3.8V
0.70A
2: Efficiency
Reading
Input Voltage
VIN
IIN
VOUT
ILED
Efficiency
12V
12V
0.274A
3.80V
0.70A
80.9%
BOM (Main Component)
Item
Designation
Description
Part No.
Vendor
1
U1
LED driver IC, LM3405A
LM3405A (eMSOP-8)
National
2
C1
16V, 220 µF, 8 x 7 mm
SG or YK, 220 µF, 16V
Lelon or Rubycon
3
L1
Inductor 6.8 µH, 0.095 Ω, 2.6A
LPS6225-682MLB
Coilcraft
4
Co
CAP0805, 0.47 µF
GRM188R71C474KA88
Murata
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11
High-Brightness LED Applications
General Illumination
Design 2: MR16 Using LM3407
Description:
• This circuit is designed to drive an array of 3 series-connected 1W LEDs
from an input of 12 VDC/12 VAC for MR16 halogen lamp replacement.
3 LEDs
COUT
IOUT
L1
DIM
EN
LM3407
FS
RISNS
D1
LX
ISNS
GND
VCC
CVCC
VIN
RFS
VIN
CIN
Test Data:
1: Output Voltage & Current
Parameter
Reading
VIN
Loading
VOUT
ILED
12 VDC
3 LEDs
9.71V
0.35A
2: Efficiency
Reading
Input Voltage
VIN
IIN
VOUT
ILED
Efficiency
12.00V
12.01V
0.30A
9.71V
0.35A
94.06%
BOM (Main Component)
Item
Designation
Description
Part No.
Vendor
1
U1
LED driver IC, LM3407
LM3407 (eMSOP-8)
National
2
L1
Inductor 33 µH, 0.58A
LPS-4018-333ML
Coilcraft
3
CIN
Cap MLCC 50V, 4.7 µF, X7R
GRM32ER71H475K88L
Murata
4
COUT
Cap MLCC 25V, 2.2 µF, X7R
GRM31MR71E225MA93
Murata
12
Design 3: MR16 with Two-Wire Dimming Driven by the LM3406
Description:
• This circuit is designed to drive a 1.5A high-brightness LED from an input
of 12 VDC for MR16 halogen lamp replacement.
• The two-wire dimming feature of LM3406 enables PWM dimming over the
power input line.
Dimming Control
U1
VIN
D22
Cin1 Cin2 RCC
C21
(optional)
Duty Cycle
Control
GND
L1
VIN
SW
VIN
SW
RON
LED+
D1
CB
BOOT
CO
LM3406
VINS
DIM
DIM1
R23
CS
COMP
CC
ROUT
VOUT
LED-
VCC
GND
R35
CF
NC
Test Data:
1: Output Voltage and Current
Parameter
Reading
VIN
Loading
VOUT
ILED
12 VDC
2 LEDs
4.20V
1.50A
2: Efficiency
Reading
Input Voltage
VIN
IIN
VOUT
ILED
Efficiency
12V
12V
0.62A
4.20V
1.50A
84.68%
BOM (Main Component)
Item
Designation
Description
Part No.
Vendor
1
U1
LED driver IC, LM3406
LM3406 (eTSSOP-14)
National
2
L1
15 μH, 2.2A, 47 mΩ
SLF10145T-150M2R2-P
TDK
3
Cin1
3.3 μF, 50V
C3225X7R1H335M
TDK
4
Co
0.15 μF, 50V
C3216X7R1H105M
TDK
national.com/LED
13
High-Brightness LED Applications
General Illumination
QR111, PAR30 and PAR38
Description:
• This circuit is designed to drive an array of 5 to 8 seriesconnected 3W LEDs from an input of 12 VDC/12 VAC for
existing QR111, PAR30/38 luminaire form factor.
L1
VIN
U1
COUT
LM3421
• Since the total forward voltage of the LED string is higher
than the input voltage, the step-up (boost) topology with
LM3421 is employed.
VIN
CIN
IS
QSW
GATE
RT
CT
Enable
CCOMP
RHSP
HSP
RCT
RHSN
HSN
LED1
EN
VCC
nDIM
DDRV
COMP
PGND
AGND
EP
RCOMP
RCSH
CSH
CBYP
LED5
RSENSE
5 LEDs in
Series
QDIM
Test Data:
1: Output Voltage & Current
Parameter
Reading
VIN
Loading
VOUT
ILED
12 VDC
5 LEDs
19.98V
0.72A
2: Efficiency
Reading
Input Voltage
VIN
IIN
VOUT
ILED
Efficiency
12V
12V
1.27A
19.98V
0.72A
94.50%
BOM (Main Component)
Item
Designation
Description
Part No.
Vendor
1
U1
Low-side controller for constant-current LED drivers
LM3421 (eTSSOP−16)
National
2
L1
15 µH
7447709150
Coilcraft
3
CIN
150 µF/50V
Aluminum eletrolytic capacitor, EEEFK1H151P
Panasonic
4
COUT
150 µF/50V
Aluminum eletrolytic capacitor, EEEFK1H151P
Murata
14
48V Bus Street Lamp
Key Benefits of LM3402/4/6HV in 48V
(or higher) Bus LED Street Lamp System
• Integrated FET LED driver and no compensation is required
° Easy to use
Temp Sensor
LM94022
LM95071
LM73
PWM Dimming
Signal
MCU
3.3V
48V
• High operating VIN buck LED driver
° Maximize the number of LEDs per string (~10 to 12 LEDs
in series for 1 LED driver)
° Lower total system solution cost
• Ultra-high-efficiency LED driving solution
° 96% + efficiency with 10 LEDs connected in series
° Enhanced thermal performance in the harsh street lamp
working environment
AC/DC
LED Driver
LM3402HV
LM3404HV
LED Driver
LM3402HV
LM3404HV
LED Driver
LM3402HV
LM3404HV
LM3424 with Integrated Temperature Management
• Temperature foldback
° Eliminates the need for external thermal
management circuitry
LED Street Lamp Architecture
° Allows LEDs to last longer in high temperatures for
system reliability
° WEBENCH® LED Designer online tool with thermal
management feature available to implement
temperature foldback
-Ease of design on a system level
The Concept:
The thermal foldback feature lowers regulated current as the
temperature increases to optimize the LED lifetime. The feature
includes two parameters: A temperature corner (Tcorner)
after which the nominal operating current is reduced and the
slope corresponding to the amount of LED current decrease
per temperature. The LM3424 allows the user to program both
the breakpoint and slope of the thermal foldback profile using
external resistors.
INOM
LED Current
TCORNER
national.com/LED
Temperature
15
High-Brightness LED Applications
General Illumination
Street Lamp
Design 1: 1W LED String Using LM3402HV
Description:
• This circuit is designed to drive an array of 10 to 12 seriesconnected 1W LEDs from the source of 48V output
AC/DC SMPS.
• Multiple LM3402HV LED drivers are used in the LED street
lamp system, depending on the street lamp’s output wattage.
• Each LM3402HV LED driver provides constant current for a
single LED string. This enables consistent brightness of each
LED in the LED street lamp.
Test Data:
1: Output Voltage & Current
Parameter
Reading
VIN
Loading
VOUT
ILED
48 VDC
12 LEDs
38.20V
0.33A
2: Efficiency
Reading
Input Voltage
VIN
IIN
VOUT
ILED
Efficiency
48V
47.91V
0.27A
38.20V
0.33A
98.04%
BOM (Main Component)
Item
Designation
Description
Part No.
Vendor
1
U1
75V, 0.5A LED driver
LM3402HV (SOIC-8 or PSOP-8)
National
2
L1
18.5 x 15.4 x 7.1 mm 330 μH, 1.9A, 0.56Ω
DO5022P-334
Coilcraft
3
Cin
2.2 μF/100V/1812
C4532X7R2A225M
TDK
4
Co
0.15 μF, 100V, 1206
C3216X7R2A154M
TDK
16
Design 2: 3W LED String Using LM3404HV
Description:
• This circuit is designed to drive an array of 10 to 12 seriesconnected 3W LEDs from a 48V output AC-DC SMPS.
• Multiple LM3404HV LED drivers are used in the LED street
lamp system, depending on the street lamp’s output wattage.
• Each LM3404HV LED driver provides constant current for a
single LED string. This guarantees consistent brightness of
each LED in the LED street lamp.
Test Data:
1: Output Voltage & Current
Parameter
Reading
VIN
Loading
VOUT
ILED
52 VDC
12 LEDs
41.975V
1.071A
2: Efficiency
Reading
Input Voltage
VIN
IIN
VOUT
ILED
Efficiency
52V
51.97V
1.017A
41.975V
1.017A
96.97%
BOM (Main Component)
Item
Designation
Description
Part No.
Vendor
1
U1
75V, 1.2A LED driver
LM3404HV (SOIC-8 or PSOP-8)
National
2
L1
Inductor 220 μH, 0.229Ω, 2.2A
MSS1278-184KL
Coilcraft
3
Cin
2.2 μF/100V/1812
C4532X7R2A225M
TDK
4
Co
0.15 μF, 100V, 1206
C3216X7R2A154M
TDK
national.com/LED
17
High-Brightness LED Applications
LED Light Source for Office Equipment
Portable Projector
• The LM3433 is a high-power constant-current LED driver
controller which employs a negative synchronous buck
topology, perfect for applications where a common-anode
LED system is used for high current output.
• An example power architecture of a portable projector
using the LM3433 is shown below. The -12 VDC isolated
AC/DC SMPS is used for powering LM3433 LED drivers while
the LM3481 floating buck-boost is used to generate positive
outputs for other logic and interface.
Common Anode
Window
Die Junction
TB
Window Frame
LM3433
Negative Buck
HB LED Driver
Thermistor
THS
-12 VDC Output
TREF
Heat Sink
TJ
LM3433
Negative Buck
LED Driver
LM3433
HB LED Driver
Negative Buck
Copper core-board
Isolated AC/DC
SMPS
Chassis
+12V/4A
LM3481 Floating
for other
Buck-boost Converter electronics
Heat Sink
85-264 VAC
r
0.1
µF
DIM
DIM
GND
-12V
VEE
CGND
LED
HS
0.47µF
BST
VCC
VIN
6 µH
Q2
LO
0.01
µF
Q3
Q1
HO
LS
EN
4.7 µF
18
LM3433
EN
+3.3V
DMO
DIMR
TON
ADJ
ADJ
SS
-12V
COMP
270 pF
LED ANODE
BST2
CSP
CSN
44.2k
2.2 µF
22 µF
0.01
LED CATHODE
LED Projector Using LM3433
Description:
• This circuit is designed to drive a high-brightness commonanode LED module from a -12 VDC source derived from an
isolated AC-DC SMPS for portable projector applications.
• In LED-based portable projector systems, green, blue, and
red high-brightness common-anode LED modules are used as
light sources. Each color requires one LM3433 driver.
Typical Application Circuit
0.1
µF
LM3433
EN
DIM
DIM
VEE
CGND
4.7 µF
Q1
LED
HS
0.47µF
BST
6 µH
VCC
VIN
GND
Q3
HO
0.01
LED CATHODE
Q2
LO
LS
EN
+3.3V
DMO
DIMR
TON
ADJ
ADJ
SS
-12V
COMP
270 pF
LED ANODE
BST2
CSP
CSN
44.2k
22 µF
2.2 µF
0.01
µF
-12V
Test Data:
1: Output Voltage and Current
2: Efficiency
Parameter
Reading
Reading
VIN
Loading
VOUT
ILED
Input Voltage
VIN
IIN
VOUT
ILED
Efficiency
-12 VDC
1 LED
4.60V
6A
-12V
-12V
2.47A
-4.60V
6A
93%
BOM (Main Component)
Item
Designation
Description
Part No.
Vendor
1
U1
Common-anode-capable high-brightness LED driver with high-frequency dimming
LM3433 (LLP-24)
National
2
L2
12 μH, 14A
GA3252-AL
Coilcraft
3
C3
150 μF, 16V
16SA150M
MULTICAP
4
C4
1210 22 μF x 2, 16V
GRM32ER61C226KE20L
Murata
5
C6
1210 47 μF, 16V
GRM32ER61C476ME15L
Murata
6
MOSFET (Q1,Q2,Q3,Q4) PowerPAK 30V, 9.5 mΩ
Si7386DP
Vishay
7
LED
PT39
Luminus
national.com/LED
6A
19
High-Brightness LED Applications
LED Light Source for Office Equipment
Scanner/Multi-Function Printer (MFP)
• Traditionally, CCFLs have been widely used as a light source
for multi-function printers even though it requires longer
warm-up time before the image scanning process can begin,
especially when the MFP has just powered up or is in energysaving mode.
• CCFL is not an environmentally friendly light source due to the
mercury inside. This is why high-brightness LEDs are used as
the light source for new model MFPs.
• A comparison between a CCFL and an LED in MFP light
source applications is shown below.
Conventional Light Source vs LED
CCFL
LED
Efficiency
Good (50 lm/W~)
Good (50 lm/W~)
Brightness
Low
Depends on current and number of LEDs
Start up
Slow
Fast
Dimming
NA
Available
More Advantages of LEDs
• Longer life time
• No hazardous materials
• Easier to drive
Side-Lit LEDs Scanning Light Bar
General scanning action on MFP, copier, or scanner
To move forward & backward
Light Guide
OPTICS (LIGHT SOURCE, LENS, ETC...)
Cable
Control Board
CCD/CIS
POSITION SENSORS
Side-Lit LEDs
• 1 side or both sides
• 1 or 2 LEDs at each side
Back-Lit LEDs Scanning Light Bar
STEPPER
MOTOR
Light Guide
LED Array
The brightness variation of each LED is a problem
20
Scanner/MFP Using LM3402
Description:
CS
• This circuit is designed to drive multiple strings of 6 series-connected LEDs from 24V AC-DC
adapter for scanner/MFP application to replace the conventional CCFL lamp.
Vin
GND
Vcc
487 KΩ
DIM
Rsource
U1
LM3402
Ron
Ron
Cb 10 nF
Boot
L1 47 uH
If (total) = 330 mA
Switch
Vin
100 nF
Cin
2.2 µF
C1
D1
4.7 uF
Co
Rsns
0.56 Ω
* For the above configuration of LED array, binning of VF may be required to balance the current of each LED.
Test Data:
1: Output Voltage & Current
Parameter
Reading
VIN
Loading
VOUT
ILED
24 VDC
Multiple strings of 6 series-connected white LEDs
20.4V
330 mA
2: Efficiency
Reading
Input Voltage
VIN
IIN
VOUT
ILED
Efficiency
24 VDC
24V
0.293A
20.4V
330 mA
95.8%
BOM (Main Component)
Item
Designation
Description
Part No.
Vendor
1
U1
42V, 0.5A LED driver
LM3402 (mini SOIC-8)
National
2
Cin
2.2 μF
C3225X7R1H225M
TDK
3
L1
47 μH, 0.15Ω
SLF7045T-470MR75
TDK
4
Co
4.7 μF
C3225X7R1E475M
TDK
national.com/LED
21
High-Brightness LED Applications
Automotive Lighting
From headlights to LCD backlighting in infotainment systems,
LEDs are an integral part of the driving experience. National’s
portfolio of LED drivers offer key features like PWM dimming,
accurate Under Voltage Lockout (UVLO), and high-side
current sensing.
Plus, low LED ripple current and external oscillator sync
capabilities allow designers to reduce issues with EMI. These
LED drivers provide maximum efficiency and effectiveness in any
automotive lighting system.
Features
Benefits
High efficiency
Better thermal management
High-side current sensing
LEDs grounded to chassis for better thermal dissipation
Accurate current control
Extends LED lifetime
PWM and analog dimming
Easily reduces current when battery is low to avoid excessive battery drain
Wide voltage range
Stable under instant on, low and high battery, high voltage transients
External oscillator sync capability
External spread spectrum for low EMI
Interior Lighting
Infotainment Backlighting
Signaling
Headlights
22
Daytime Running and Fog Lights
Design 1: Driving Daytime Running Lamp (DRL) with LM3423 Boost LED Driver
Description:
• This circuit is designed to drive a single string of 12 seriesconnected 1W LEDs from the battery input for daytime running
lamps (DRL) in passenger cars.
• Since the total forward voltage of the LED string is higher
than the battery input voltage, a boost (step-up) LED driver
is required.
VO
JA
TP12
L1
VIN
CB
JB
C11
R14
VIN
TP15
R17
R21
R13
DRL
Module
C3
CSH
IS
RCT
VCC
FLT
R16 TP9
RPD
C12
TP7
C13
J2
TP13
R12
OVP
J7
Q5
C2
R11
J3
J4A
J14
LED
(+)
TP1
RPD
DPOL
J6
R9
C18
TIMR LRDY
TP11
RPD
R7
nDIM DDRV
VIN
TP6
HSP
OVP PGND
TP8
R25
U1
R8
AGND GATE
OVP
R26
C4
C16
TP3
HSN
COMP RPD
TP14
C5
R30
U1
VIN
J1
C7
LM3423
C1
R27
EN
LM3423
Boost
LED Driver
D1
C17
PGND
VOUT
Vbat: 9-16 VDC
PWM Dimming
DEMO Board
J4B
R6
TP5
C6
LED
(-)
R15
Q9
TP2
VIN
BNC
R31
TP10
Test Data:
Q6
J13
1: Output Voltage & Current
Parameter
Reading
VIN
Loading
VOUT
ILED
12 VDC
12 series-connected 1W LEDs
46V
0.40A
2: Efficiency
Reading
Input Voltage
VIN
IIN
VOUT
ILED
Efficiency
12V
12V
1.65A
46V
0.40A
92.93%
BOM (Main Component)
Item Designation Description
Part No.
1
U1
Low-side controller for constant-current LED drivers
LM3423 (eTSSOP-20) National
2
L1
22 μH
DO5040H
Coilcraft
3
C8 (Cin)
330 μF, 35V 5 mm
ECA-1VM331
Panasonic
4
C7 (Cout1)
330 μF, 35V 5 mm
ECA-1VM331
Panasonic
5
C11 (Cout2)
1210 10 μF, 25V
ECJ-4YB1E106M
Panasonic
national.com/LED
Vendor
23
High-Brightness LED Applications
Automotive Lighting
Design 2: Headlamp Using LM3423 Buck-Boost LED Driver
Description:
• This circuit is designed to drive a single string of 6 seriesconnected 3W LEDs from both a 12V and a 24V bus battery
input for automotive headlamp applications.
• Since the total forward voltage drop of the LED string can be
either higher or lower than the input voltage, a buck-boost
LED driver is required.
Test Data:
1: Output Voltage & Current
Parameter
Reading
VIN
Loading
VOUT
ILED
6-32 VDC
20V at 1A
20V
1A
2: Efficiency
Reading
Input Voltage
VIN
IIN
VOUT
ILED
Efficiency
12.00V
12V
1.87A
20V
1A
88.98%
24.00V
24V
0.93A
20V
1A
89.51%
BOM (Main Component)
Item
Designation
Description
Part No.
Vendor
1
U1
Buck-boost controller for constant-current LED drivers
LM3423 (eTSSOP-20)
National
2
L1
22 μH
DO5040H
Coilcraft
3
C8 (Cin)
330 μF/35V 5 mm Lead
ECA-1VM331
Panasonic
4
C7 (Cout1)
330 μF/35V 5 mm Lead
ECA-1VM331
Panasonic
5
C11 (Cout2)
1210 10 μF, 25V
ECJ-4YB1E106M
Panasonic
24
Design 3: LED Backlighting Applications Using LM3431
LED Backlighting for TFT Displays
Description:
• This circuit is designed to drive four channels of 8 series-connected 140 mA LEDs
from a 12V bus battery input for automotive LED backlighting in a TFT display.
VIN: 8V to 18V, 4 Strings of 8 LEDs, 140 mA per String
L1
VIN
C2
C1
VCC
LED Backlighting for Dashboards
VIN
R2
RMODE
C5
EN
DIM
R8
R7
R9
LG
EN
CS
LEDOFF
DIM
MODE/F
ILIM
LM3431
REF
Rff
C7
VCC
R17
VC1
D2-5
D6-9
VC2
VC3
VC4
DLY
SGND
Q2
SNS2
EP
REFIN
Op1
NDRV2
SS/SH
C6
Rhys THM
SNS1
RT
R6
GND
NDRV1
VCC
C3
THM
CFB
FF
Rth
R18
R3
SC
COMP
C13
C9
THM
PGND
AFB
REFIN
C4
C8
R4
VA
External
Thermistor
R19
Q1
R1
R16
External LED Array
D1
R15
Q3
NDRV3
SNS3
R10
R11
R12
LEDOFF
Q6
Q4
R13
R14
RRESTART
VCC
C15
Test Data:
1: Output Voltage & Current
Parameter
Reading
VIN
Loading
VOUT
ILED
8 to 18 VDC
4 strings of 8 LEDs, Vf: 3.2V
25.60V
0.14A
2: Efficiency
Reading
Input Voltage
VIN
IIN
VOUT
ILED
Efficiency
12V
12V
0.34A
25.60V
0.14A
88%
BOM (Main Component)
Item
Designation
Description
Part No.
Vendor
1
U1
Boost controller for multi-channel constant-current LED drivers
LM3431 (eTSSOP-28)
National
2
L1
7 µH 3.1A inductor
MSS1038-702NL
Coilcraft
3
C2 (Cin_1)
10 µF 50V electrolytic
UUD1H100MCL
Nichicon
4
C1 (Cin_2)
1 µF 50V B ceramic
GRM32RB11H105KA01
Murata
5
C3 & C8 (Cout)
2 x 4.7 µF 50V X7R ceramic
GRM32ER71H475KA88L
Murata
6
Q1
60V 200 mA N-channel MOSFET
2N7002K
Vishay
national.com/LED
25
High-Brightness LED Applications
TRIAC Dimming
TRIAC Dimmable LED Lamp Using LM3445
TRIAC Dimmable LED Lamp with LM3445
• The TRIAC phase-control dimmer is today’s most popular
and common dimming method, but it is designed to interface
to a purely resistive load, such as incandescent or halogen
light bulbs. Since an LED does not appear as a resistive load
to the TRIAC dimmer, dimming an LED using a conventional
TRIAC wall dimmer does not achieve good dimming
performance.
• National’s LM3445 TRIAC dimmable offline LED driver
overcomes the issue and enables LEDs to be used as a
direct replacement for incandescent or halogen lamp
systems which are currently interfaced to a TRIAC wall
dimmer. The LM3445 is an offline solution that offers 100:1
full-range, uniform dimming capability, is free of flicker at
100/120 Hz, and supports master/slave operation.
Description:
• This design is configured to support 90 VAC to 135 VAC inputs
to drive 7 or 8 series-connected LEDs at an average current
of 350 mA for TRIAC dimmable LED lamp applications.
Demo Board
VBUCK
D3
V+
D7
BR1
R2
D4
TRIAC
Dimmer
Q1
VAC
D1
D9
D8
C10
C12
C9
+
VLED
-
VLED-
D2
D10
R5
Q3
C5
L2
TRIAC Wall
Dimmer
VAC
LM3445
TRIAC Dimmable
LED Driver
LM3445
ASNS
U1
ICOLL
FLTR1
C3
Test Data:
1: Output Voltage & Current
Parameter
Loading
VOUT
ILED
110 VAC
12 LEDs
46 VDC
0.35A
VCC
DIM
GATE
COFF
ISNS
FLTR2
GND
Reading
110 VAC
VIN
—
IIN
—
VOUT
ILED
Efficiency
46.0V
0.35A
84.20%
BOM (Main Component)
Item
Designation
Description
Part No.
Vendor
1
U1
IC, LED driver controller, MSOP-10
LM3445MM (mini SOIC-10)
National
2
BR1
Bridge rectifier, SMT, 400V, 800 mA
HD04-T
Diode
3
L2
Inductor. SHLD. SMT, 1A, 470 µH
MSS1260-474KLB
Coilcraft
4
C7, C9
Cap, AL, 200V, 105C, 20%, 33 µF
EKXG201ELL330MK15L
UCC
5
D4, D9
Diode, FR, SOD123, 200V, 1A
RF071M2S
Rohm
6
D10
Diode, FR, SMB, 400V, 1A
MURS140T3G
On Semi
7
Q1, Q2
XSTR, NFET, DPAK, 300V, 4A
FQD7N30TF
Fairchild
26
R3
C11
2: Efficiency
Input Voltage
Q2
C4
Reading
VIN
BLDR
R1
Universal AC
Inputs
R4
Designer’s Corner
Light Matters Part 1: The ABC’s of LEDs
Anatomy
The physical anatomy of LEDs resembles p-n junction diodes.
As in p-n junctions, the electrons and the holes flow towards the
junction when a positive differential voltage is applied between
the anode (p-side) and cathode (n-side). Once an electron is
recombined with a hole, it releases energy. Depending on the
physical properties of the p-n junction materials, the released
energy can be non-radiative, as in normal diode materials, or
may produce optical emissions in the form of photons with
LED materials. For an LED, the wavelength of the emitted light
(its color) depends on the band-gap characteristics of its p-n
junction material. LED materials have relatively low reverse
breakdown voltages since they have relatively low band gaps.
Wavelength
(nm)
940
635
570
430
8000K
Color
Name
Infrared
High Eff. Red
Super Lime
Green
Ultra Blue
Cool White
Anode
+
Introduction
Light and lighting represent basic and crucial elements in
the life of humankind. The pursuit of new lighting sources
has been a trend of our civilization. This pursuit is generally
driven by technological advancements, needs, challenges,
and, sometimes, by luxury. Now that we are waking up to
realize the consequences of abusing our world’s limited
resources, the push towards energy conservation has come
to be a mandate, not a choice. Therefore, our world’s current
challenge is how to balance between the needs of our modern,
possibly spoiled, lifestyle and the necessity to "go green." When
it comes to lighting, it is quite easy to imagine the impact of
globally improving the efficiency of lighting sources by 10%.
But what if it could be improved by 1000%? The use of newly
enhanced Light-Emitting Diodes (LEDs) as lighting sources
has the potential to achieve these efficiency improvements
while maintaining outstanding performance and reliability that
supersede many of the currently used sources. Part One of this
two-part series sheds some light on the basics of LEDs physical
structure, colors, efficiency, applications, and drivers.
-
Cathode
Figure 1: LED Symbol
Colors
Red LEDs were the first to become commercially available in
the late 1960s, and, as one acquaintance put it, "A dark cave
was needed to see the light." Despite the low light output, they
were commonly used in seven-segment displays. Thanks to
the advancements in material science, nowadays LEDs are
commercially available in a variety of colors with some of them
having light outputs that would blind you if you stared directly at
them. (Please do not try that at home!)
Blue became widely available a few years ago. Mixing blue
LEDs with red and green LEDs produces white light (think
triad pixel). This technique of generating white light provides
a large color gamut, dynamic light tuning, and excellent color
rendering (CRI), which is well suited for high-end backlighting
applications. A simpler and more economical way of producing
white light is to use blue LEDs and a phosphor coating that
converts some of the blue light to yellow. The yellow light
stimulates the red and green receptors of the eye; therefore,
mixing the blue and yellow lights gives the appearance of
white. This scheme can provide good CRI but the LED’s light
output may suffer from inconsistent color temperatures due to
manufacturing discrepancies and varying thicknesses in the
phosphor coating layer.
Fwd Voltage
(Vf @ 20 mA)
1.5
2
LED Dye Material
GaAIAs/GaAs – Gallium Aluminum Arsenide/Gallium Arsenide
GaAsP/GaP – Gallium Arsenic Phosphide/ Gallium Phosphide
2
InGaAIP – Indium Gallium Aluminum Phosphide
3.8
3.6
SiC/GaN – Silicon Carbide/ Gallium Nitride
SiC/GaN – Silicon Carbide/ Gallium Nitride
Figure 2: LEDs Color Chart for the Basic Colors
national.com/LED
27
Designer’s Corner
Efficiency
High efficiency has been the buzz word for LED-based light
sources. When it comes to lighting, efficiency is defined as
the light output per unit power. Thus, in the metric system, it
is measured as lumens (lm) per watt (W). Recently some LED
manufacturers introduced LEDs with promised efficiencies
hitting the 150 lm/W mark. In comparison, incandescent comes
at 15 lm/W, and fluorescent provides 70 lm/W.
So could LEDs put incandescent and fluorescent out of
business any time soon? Maybe, but, unfortunately some of
these LED’s efficiency numbers are subject to "specmanship."
Here is the problem: the LED inefficiency has to do more with
the fact that a considerable portion of the produced light is
reflected at the surface of the packing material back into the
LED die. This reflected light is likely to be absorbed by the
semiconductor material and turned into heat. Utilizing antireflection coating and minimizing the reflection angles by
using a half-sphere package with the LED placed at the center
reduce the amount of reflected light and improve efficiency.
However, these techniques are subject to manufacturing
variations and may require high premiums to ensure a
consistent performance. Otherwise, you can always opt out
to specmanship to show off! In a nut shell, there is a rapidly
increasing adoption of LEDs by the electronics industry, but the
change is far from complete.
Applications
There are many factors which make LEDs eye-catching for
high-performance modern electronics. For example, their higher
light output per watt is well-suited for portable applications as
it extends the battery life. On the other hand, LEDs’ fast turn-on/
turn-off characteristics fit perfectly with automotive tail lights
needs, especially the brake lights, since it improves safety
by providing drivers more response time. Using RGB LEDs in
backlighting complies with ROHS standards, since LEDs do not
contain lead or mercury. LED lighting facilitates a full-spectrum
light source with larger color gamut. LEDs have an exceptionally
long life span which enables their use in applications where
long term reliability is highly desirable, such as traffic lights.
Machine vision systems require a focused, bright, and
homogeneous light source – LEDs are a great match. LEDs, with
their simple-to-implement dynamic light-tuning, would also allow
you to set the light in your living room to green when you need to
relax and to red when it’s time for bullfighting.
essential to achieve the desired color and brightness level. An
LED driver scheme can be as simple as a voltage source and a
ballast resistor (Figure 3A). This solution works best for narrowinput range, low-current applications in which the LED’s
forward voltage drop is slightly below the input supply voltage.
Variations in the input supply voltage or the LED forward
voltage drop will increase the LED current and, therefore, the
light intensity and the color will shift. Linear regulators can be
used to provide tighter LED current control in small step-down
ratio applications (Figure 3B). In the case of low-current stepup requirements, switching capacitor circuits can be utilized
(Figure 3C).
For wide input range, high-current applications, simple driver
schemes such as those mentioned above will yield high power
dissipation and poor efficiency. For example, a linear regulatorbased LED driver yields 70% efficiency when supplying 1A
from a 5V input source to a typical white InGaN LED (VF= 3.5V).
Under the same operating conditions, the driver’s efficiency will
drop to approximately 30% when the input voltage increases
to 12V. In addition to degrading the overall performance of an
LED-based application, such poor efficiencies would require
impractical thermal management schemes.
Consequently, more efficient and relatively more complex solutions
such as switching regulators would be needed (Figure 3D).
Switching regulators process power by interrupting the power
flow and controlling the conversion duty cycle which results in
pulsating current and voltage. They can be configured in isolated
and non-isolated configurations to realize voltage or current stepdown (buck), step-up (boost), or both (buck-boost) functions.
In general, a switching-regulator topology is selected based
on a tradeoff between cost and desired performance at a given
power conversion requirement. On the other hand, in order
to properly drive LEDs, the switching regulators should be
configured as constant current sources.
VIN
28
RB
+
VIN
-
A. Voltage Source and
Ballast Resistor
+
+
-
FB
RFB
Control
Block
B. Linear Regulator
L
VIN
Drivers
LEDs are inherently current-driven devices since their brightness
varies with their forward current, If. Depending on the color as
well as the forward current, the LEDs’ forward voltage drop,
VF, varies as well. Thus, driving LEDs with a constant current is
+
-
+
-
+
Control
Block
C. Switched Capacitors
FB
RFB
VIN
+
-
+
Control
Block
D. Switching Regulators
Figure 3: Simplified LED Drivers Schemes
FB
RFB
Switching Regulators
To improve the conversion efficiency, switching regulators interrupt
the power flow while controlling the conversion duty cycle to
program the desired output voltage or output current. Interrupting
the power flow results in pulsating current and voltage, and
therefore, it necessitates the use of energy storage elements
(inductors and/or capacitors) to filter these pulsating waveforms.
Contrary to linear regulators, switching regulators can be
configured in different arrangements to realize voltage or current
step-down (buck), step-up (boost), or both (buck-boost) functions.
They are also capable of achieving high conversion efficiencies
across wide input/output range. Replacing the linear regulator with
a buck-based LED driver in the previous example yields 95% to 98%
efficiency across 5V to 12V input range.
The configuration flexibility and the efficiency improvements
of switching regulators come at the expense of higher noise
generation caused by the periodic switching events, as well as
higher premiums and reduced reliability due to their perceived
complexity. Utilizing switching regulators to drive constant-current
LEDs favors regulator topologies that can be simply configured
as a constant-current source. The selected topology should also
combine high performance with minimum component count to
increase the driver’s reliability and reduce cost. It should also
facilitate the use of various dimming techniques to take advantage
of the LED's dynamic light tuning characteristic. Fortunately, the
most basic step-down (buck) switching topology enjoys all these
characteristics, making it the regulator of choice to drive LEDs
whenever possible.
Constant-Current Power Stage
Switching regulators are most commonly known as voltage
regulators. Figure 4A illustrates a basic constant-voltage buck
regulator. The buck controller maintains a constant output
voltage as the line voltage changes by varying the operating
duty cycle (D) or the switching frequency. The desired output
voltage set point is programmed using the following equation:
VO = VFB
RFB1 + RFB2
RFB1
Eq1
The inductor, L, is selected to set the peak-to-peak current
ripple, IPP, while the capacitor, Co, is selected to program a
desired output-voltage ripple and to provide output-voltage
hold-up under load transients. The average inductor current in
a buck converter is equal to the load current, and, therefore,
the load current can be programmed by controlling the peakto-peak inductor-current ripple. This significantly simplifies the
conversion of a constant-voltage source into a constant-current
source. Figure 4B illustrates a basic constant-current buck
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regulator. Similarly, constant-current buck regulators provide
line regulation by adjusting the conversion duty cycle or the
switching frequency, and the LED current, IF, is programmed
using the following equation:
IF =
VFB
RFB
Eq2
Setting the LED current, IF, requires the proper sensing of the
inductor current. Theoretically, multiple current sense schemes
such as MOSFET RDSON sensing and inductor DCR sensing can
be used. However, practically, the current sense precision of
some of these would not meet the required LED current set point
accuracy (5% to 15% for high-brightness LED). Directly sensing
IF through an inline resistor, RFB, yields the needed precision, but
may lead to excessive power dissipation in the current sense
resistor. Lowering the feedback voltage, VFB, allows the use of
lower resistance value for the same IF (Eq2), which minimizes
losses. Newly released dedicated LED drivers generally offer
reference voltages (feedback voltages) within the range of 50 mV
to 200 mV.
Uniquely, constant-current buck-driven regulators can be
configured without output capacitance. The use of the output
capacitor, Co, in these regulators is limited to AC current filtering
since they inherently do not experience load transients and have
continuous output currents. Configuring a constant-current buck
regulator without output capacitance substantially increases the
converter’s output impendence and, in turn, boosts the converter’s
ability to rapidly change its output voltage so that it can maintain
a constant current. As a result, the dimming speed and dimming
range of the converter improve significantly. Wide dimming range is
a highly demanded feature in applications such as backlighting and
machine visions.
On the other hand, lacking the output capacitance AC-currentripple filtering necessitates the use of higher inductance values in
order to meet the LED manufacturers recommended ripple current
( IF = ±5% to ±20% of the DC forward current). At the same current
rating, higher inductance values would increase the size and cost
of the LED driver. Consequently, the use of output capacitors in
constant-current buck-based LED drivers is governed by a tradeoff
between cost and size on one hand versus dimming speed and
dimming range on the other hand. For example, to drive a single
white LED (VF ≈ 3.5V) at 1A with a ripple current, iF, of ±5% from
an input of 12V at 500 kHz would require a 50 μH inductor with a
current rating of 1.1A. However if the inductor ripple-current is
allowed to increase to ±30%, then the inductance required is less
than 10 μH. For the same core material and at approximately the
same current rating, a 10 μH inductor will be typically offered at
roughly half the size and the cost of a 50 μH inductor.
29
Designer’s Corner
To attain the desired iF (±5%) using the 10 μH inductor, the
output capacitance required is calculated based on the dynamic
resistance, rD, of the LED, the sense resistance, RFB, and the
impedance of the capacitor at the switching frequency, using the
following expressions:
Hysteretic control is well-suited for applications such as
light bulbs and traffic lights, in which variable switching
frequencies are tolerated or where narrow input voltage
range supplies are used. Hysteretic control does not
experience control-loop bandwidth restrictions, which
eliminates the need for loop compensation because of its
inherent stability. Utilizing hysteretic control to drive a buckbased LED driver (Figure 5A) greatly simplifies the design as
well as reducing the component count and the cost of the
driver. This configuration also yields superior PWM dimming
ranges that outperform other buck-based schemes.
Eq3
Where:
Zc =
ΔIF
� (RFB + rD)
ΔIL – ΔIF
Eq4
L
VIN
VIN
NGATE
GND
Using hysteretic buck-based LED drivers with the shuntdimming approach is well-suited for applications that require
ultra-wide dimming ranges at high dimming frequencies and
that can tolerate variable switching frequencies.
VO
CO
RFB2
Basic Buck
Controller
FB
RFB1
Figure 4A: Basic Step-Down (Buck) Voltage Regulator Figure
L
VIN
VIN
NGATE
VO
LED
IF
Basic Buck
Controller
GND
CO
FB
RFB
Figure 4B: Basic Step-Down (Buck) Current Regulator
30
Control-Loop Schemes
Buck-based power stages are well-matched to several
control-loop schemes and free of stability limitations such
as right-half-plane zeros. They uniquely facilitate the shunt
PWM dimming approach in addition to being compatible with
other dimming methods. This provides the system designer
with configuration flexibility when designing an LED driver
for specific requirements.
Quasi-hysteretic buck-based LED drivers offer a good
compromise between fixed-frequency operation and
hysteretic control for applications in which variable
switching frequencies may not be desired. The controlled
on-time (quasi-hysteretic) buck-based LED driver (Figure 5B)
employs a control scheme based on a hysteretic comparator
and a one-shot on-timer which is used to set a controlled
on-time. This controlled on-time is programmed so that it is
inversely proportional to the input voltage, and, therefore,
it minimizes the switching frequency variations as the line
voltage changes. Using this scheme also eliminates the
need for control-loop bandwidth limitations, enabling it to
achieve wide dimming ranges when used with different
dimming configurations. In some cases, as in a number of
automotive applications, synchronizing the LED driver(s)
to an external clock or to each other may be required to
minimize noise interference. Implementing the frequency
synchronization feature with the non-clock-based hysteretic
and quasi-hysteretic scheme can be challenging. In contrast,
this feature can be simply realized in clock-based regulators
such as the fixed-frequency buck LED driver shown in
Figure 5C. Fixed-frequency control generally yields a more
complex solution, and it limits the dimming range of the
driver regardless of the dimming approach due to its dynamic
response limitations.
L
VIN
LED
CO
VIN
L
VO
VIN
Logic
RON
IF
CO
RON
On -Time
Generator
LED
IF
Logic
+
-
+
-
HYS
SW
VIN
VO
GND
Vref
FB
LM3402
Figure 5A: Basic Hysteretic Buck-Based LED Driver
FB
RFB
Figure 5B: Basic Controlled On-Time Buck-Based LED Driver
L
SW
VIN
V IN
Vref
GND
RFB
LM3401
Clock
VO
CO
Logic
IF
+
-
LOOP
COMP
LED
GND
Vref
LM3405
FB
RFB
Figure 5C: Basic Fixed-Frequency Buck-Based LED Driver
Conclusion
There are many characteristics which make buck-based
regulators attractive LED drivers. They are simple to configure
as a current source and can be realized with minimum
component counts, which simplifies the design process,
improves the drivers’ reliability, and reduces cost. Buck-based
LED drivers also provide configuration flexibility since they are
compatible with multiple control schemes. They also allow for
high-speed dimming as well as wide dimming ranges since
they can be configured without output capacitance and are
well-matched to various dimming approaches including shunt
dimming. All these features make buck-based (step-down) LED
drivers the topology of choice whenever the application permits.
national.com/LED
Now, the question is: what if the application does not permit?
Applications such as residential and commercial lighting require
thousands of lumens, creating a need to drive LED strings. The
total forward voltage drop of an LED string is equal to the sum of
the forward voltage drops of all the LEDs in the string. In some
cases, the input voltage range of the system can be lower than
the forward voltage drop of the LED string, or it can vary so that
sometimes it’s lower and sometimes it’s higher. These scenarios
would require either boost or buck-boost switching regulators.
The next article will discuss, in detail, the challenges of using
boost and buck-boost topologies to drive LEDs as well as LED
dimming with these schemes.
31
Designer’s Corner
Light Matters Part 2: Boosting, Buck-Boosting and Dimming
Common rails are 12V and 24V, and in some cases 48V. Rarely
are these intermediate bus rails higher than 60V, which is the
cutoff for DC voltages under U.L. Class 2.
Introduction
Although the buck is preferred, the boost regulator is finding
more use as a direct drive for LEDs as the number of LEDs
used in LED lighting applications increases. Designers are
targeting large-scale general illumination and systems that
require thousands of lumens. Examples include street lighting,
residential and commercial lighting, stadium lighting, and
decorative or architectural lighting of spaces both interior and
exterior.
The Challenge of Boost
Boost regulators are more difficult to design and control than
buck regulators, regardless of whether the output voltage or
the output current is being controlled. Boost regulators require
design review at the limits of input voltage to ensure correct
design of the inductor, especially the peak current rating. A
boost LED driver adds a variable output voltage that influences
duty cycle and therefore the inductance and current rating of
the main inductor. To prevent inductor saturation, the maximum
average and peak currents must be evaluated at both VIN-MIN
and VO-MAX. The more LEDs that are placed in series, the greater
the gap between VO-MIN and VO-MAX.
Ideally every LED in every light source would be placed
in a single series chain, ensuring that the same current
flows through each device. Even though most general
lighting is powered from AC line voltage, in many cases an
intermediate DC bus voltage is used, derived from an ACDC regulator that takes a universal AC input and provides
PFC, isolation, and filtering. Safety standards and electrical
codes such as U.L. and C.E. limit the output voltage of the
AC-DC power supply that forms the input to an LED driver.
VO
VIN
VIN
NGATE
Basic Buck
Controller
VO
LED 1
LEDn
VIN
LED 1
Basic Boost
LEDn
Controllerchallenge for boost converters is the control
Another serious
loop. Most boost converters use peak current-mode control,
GND
FB
GND
FB
where the impedance of the load has a strong effect on both
the DC gain and the low-frequency system pole. For voltage
regulators, the load impedance is determined by dividing output
voltage by output current.
VO = n x VF
VO = n x VF
LEDs are diodes
VO > with
VIN a dynamic resistance. This dynamic
VO < VIN
resistance can only be determined by plotting the VF vs. IF curve
and then taking the tangent line to find the slope at the desired
VO
Figure 1: Buck and
Boost LED Drivers
withcurrent.
VO Calculation
forward
As shown in Figure 1, the current regulator
uses the load itself as a feedback divider to close the control
LED 1
loop. This reduces the DC gain by a factor of (RSNS / (RSNS + rD.)).
VIN
NGATE
It is tempting to compensate a boost LED driver with a simple
integrator, sacrificing bandwidth for stability. The reality is that
Basic Boost
LEDn
Controller
many, if not most, LED driver applications require dimming.
Whether dimming is done by linear adjustment of IF (analog
GND
FB
dimming) or by turning the output on and off at high frequency,
(digital, or PWM dimming) the system requires high bandwidth
and fast transient response just a voltage regulator does.
LEDn
VO = n x VF
VO > VIN
Figure 1: Buck and Boost LED Drivers with VO Calculation
Buck and Boost LED Drivers with VO Calculation
32
VIN
Unlike the buck regulator with its output inductor, the boost
converter has a discontinuous output current. An output
capacitor is required to keep the output
VO voltage and output
current continuous. In a current regulator the output capacitor
functions purely as an AC current filter and
capacitance is made
LED 1
as lowVINas possibleNGATE
while still maintaining the desired LED ripple
current.
Further Challenge: Buck-Boost
LEDs for lighting are being adopted much faster than the standards
for solid-state illumination have developed, and one result is that
the input voltages for LED lighting systems often overlap the output
voltage (remember, VO = n x VF). Every buck-boost topology stores
the entire energy delivered to the load during each cycle in an
inductor, transformer, or a capacitor, which results in higher peak
currents, higher peak voltages, or both in the power switches. In
particular, evaluation of the converter at the corners of both input
voltage and output voltage is necessary because peak switch
current occurs at VIN-MIN and VO-MAX, but peak switch voltage
occurs at VIN-MAX and VO-MAX.
VO
VIN
VIN
The single inductor buck-boost can be built with the same
parts count as a buck regulator or boost regulator, making it
attractive from a system cost standpoint. One disadvantage
of this topology is that the polarity of VO is inverted (Figure 2a)
or regulated with respect to VIN (Figure 2b). Level-shifting or
polarity inverting circuitry must be employed. Like the boost
converter, they have a discontinuous output current and require
an output capacitor to maintain a continuous LED current. The
power MOSFET suffers a peak current of IIN plus IF and a peak
voltage of VIN plus VO.
LED1
SW
LM3410 as
SEPIC
GND
LEDn
FB
Figure
SEPIC
LED
Driver
Figure3:3:
SEPIC
LED
Driver
-VO
VIN
VIN
VO
VIN
SW
VIN
LM3405
Becomes
Buck-Boost
LED1
GND
LED1
VIN
LM3410
Becomes
Buck-Boost
GND
FB
LED1
SW
LM3410
Becomes
Buck-Boost
LEDn
GND
LEDn
FB
LEDn
FB
FB
Differential
Current
Sense/Level
Shift
Negative
Feedback
Figure 2b: Low-side Buck-Boost
VO
VIN
LEDn
Figure 4:4:Cuk
Figure
CukRegulator
Regulator
V IN
-VO
Figure 2a:
2a: High-side
High-side Buck-Boost
Figure
Buck-Boost
VIN
SW
LM3410 as Cuk
-VO
GND
LED1
LED1
SW
VIN
LEDn
Control LED Light: Dimming
Two main choices for LED light control exist: adjust the LED
current linearly (analog dimming) or turning the current on and
off at a frequency high enough for the eye to average the light
output (digital dimming). Using PWM to set the period and duty
cycle is the traditional way to accomplish digital dimming.
Differential
Current
Sense/Level
Shift
V IN
VO
st
The SEPIC converter and Cuk converters both use low-side
regulators and have the advantage of a continuous input
current due to the input inductor. Their disadvantage lies in
needing two inductors (they can be coupled inductors) and an
additional capacitor. The SEPIC requires an output capacitor to
maintain a smooth LED current but has a positive VO, where the
Cuk can eliminate the output capacitor but has a negative VO.
Figure
2b: Low-side
Buck-Boost
Figure
2b: Low-side
Buck-Boost
national.com/LED
33
Designer’s Corner
VIN
CB
BOOT
VIN
CIN
RON
4.7 µF
68.1 kΩ
IF = 1A
10 nF
L1
SW
VDIM
33 µH
5V/DIV
D1
RON
LM3404
CS
VDIM
RSNS
DIM
GND
0.5A/DIV
0.22Ω
VCC
IF
CF
0.1 µF
4 µs/DIV
400 kHz
Figure 5: Figure
LED Driver
with Waveforms
5: LEDUsing
DriverPWM
UsingDimming
PWM Dimming
with Waveforms
Analog dimming also presents a challenge to the output current
accuracy. Almost every LED driver uses a resistor in series with
the output to sense current. The current sense voltage, VSNS, is
selected as a compromise between low power dissipation and
high signal-to-noise ratio, SNR. To reduce output current in a
closed-loop system, VSNS must be lowered, reducing the output
current accuracy in proportion.
PWM Dimming Preferred
Analog dimming is often simpler to implement; however, PWM
dimming is used in many designs due to a fundamental property
of LEDs: correlated color temperature (white LEDs) or dominant
wavelength shifts in proportion to the average drive current.
To make white LEDs, a blue LED is coated with a broad range
phosphor. At low current the light looks more yellow (warm white),
but at high current, the blue emission dominates and the light
becomes more bluish, or cool white. LED manufacturers specify a
certain drive current in the electrical characteristics tables of their
products where they guarantee the dominant wavelength or CCT.
Dimming with PWM ensures that the LEDs emit the color that the
lighting designer needs regardless of the intensity.
Dimming Frequency vs. Contrast Ratio
Every LED driver has a finite response time when responding
to a PWM dimming signal. Three types of delay are shown
in Figure 6, and the longer these delays are, the lower the
achievable contrast ratio (a measure of control over lighting
intensity).
T
T
T
VDIM
DMIN
D
tD
tSU
tSD
tD
DMAX
tSU tSD
tD
tSU
tSD
IF
T=
1
f DIM
D MIN
t D + t SU
T
Figure6:6:Dimming
Dimming Delays
Delays
Figure
34
D MAX =
T – t SD
T
In Figure 6, the quantity tD represents the propagation delay
from when VDIM goes high to when LED current begins to rise.
The quantity tSU represents the LED current slew-up time, and
the quantity tSD represents the slew-down time. In general,
the lower the dimming frequency, fDIM, the higher contrast
ratio, as these fixed delays consume a smaller portion of the
dimming period, TDIM. The lower limit for fDIM is approximately
120 Hz, below which the eye no longer blends the pulses into
a perceived continuous light. The upper limit is determined by
the minimum contrast ratio that is required. Contrast ratio is
typically expressed as the inverse of the minimum on-time:
CR = 1 / tON-MIN : 1
tON-MIN = tD + tSU
Dimming with a Switching Regulator
Switching regulators designed for standard power supplies
often have an enable or shutdown pin to which a logic-level
PWM signal can be applied, but the associated delays are often
quite long. This is because the silicon design emphasizes low
shutdown current over response time. Dedicated switching
regulators for driving LEDs will do the opposite, keeping their
internal control circuits active while the enable pin is logic low
to minimize delay.
Optimizing light control with PWM requires minimum slewup and slew-down delays not only for best contrast ratio, but
to minimize the time that the LED spends between 0 and the
target level. Where a standard switching regulator will have
a soft-start and often a soft-shutdown, dedicated LED drivers
do everything within their control to reduce these slew rates.
Reducing tSU and tSD involves both the silicon design and the
topology of switching regulator that is used.
Buck regulators are superior to all other switching topologies
with respect to fast slew rates for two distinct reasons. First,
the buck regulator is the only switching converter that delivers
VIN
CB
VIN
CIN
4.7 µF
BOOT
10 nF
SW
RON
68.1 kΩ
power to the output while the control switch is on. This makes
the control loops of buck regulators with voltage-mode or
current-mode PWM (not to be confused with the dimming via
PWM) faster than the boost regulator or the various buck-boost
topologies. Power delivery during the control switch on-time
also adapts easily to hysteretic control, which is even faster
than the best voltage-mode or current-mode control loops.
Second, the buck regulator’s inductor is connected to the output
during the entire switching cycle. This ensures a continuous
output current and means that the output capacitor can be
eliminated. Without an output capacitor, the buck regulator
becomes a true, high-impedance current source, capable of
slewing the output voltage very quickly. Cuk and zeta converters
can claim continuous output inductors, but fall behind when
their slower control loops (and lower efficiency) are factored in.
Faster than the Enable Pin
Some applications need high PWM dimming frequency and
high contrast ratio, which requires faster slew rates and
shorter delay times than even a hysteretic buck without output
capacitance can provide. The PWM dimming frequency
must often be pushed to beyond the audio band, to 25 kHz or
more. Total rise and fall times for the LED current, including
propagation delays, must be reduced to the nanosecond range.
Starting with a fast buck regulator with no output capacitor, the
delays in turning the output current on and off come from the
IC’s propagation delay and the physical properties of the output
inductor. The best way to bypass both is by using a power
switch in parallel to the LED chain, shown in Figure 7. To turn the
LEDs off, the drive current is shunted through the switch, which
is typically an N-MOSFET. The IC continues to operate and
the inductor current continues to flow. Some power is wasted
while the LEDs are off, but the output voltage drops to equal the
current sense voltage during this time.
IF = 1A
L1
VDIM
33 µH
5V/DIV
D1
RON
LM3404
VDIM
CS
RSNS
DIM
GND
VCC
0.5A/DIV
0.22Ω
CF
0.1 µF
IF
200 ns/DIV
400 kHz
Figure 7: Shunt FET Circuit with Waveforms
Figure 7: Shunt FET Circuit with Waveforms
national.com/LED
35
Designer’s Corner
Dimming with a shunt FET causes rapid shifts in the output
voltage, to which the IC’s control loop must respond in an
attempt to keep the output current constant. As with logic-pin
dimming, the faster the control loop, the better the response,
and again buck regulators with hysteretic control provide the
best response.
VO
VIN
VIN
IS
VCC
GATE
PGND
VO
RCT
EN
PWM DIM
SIGNAL
NDIM
HSP
LM3421
Fast PWM with Boost and Buck-Boost
Neither the boost regulator nor any of the buck-boost topologies
are well-suited to PWM dimming. Their slower control loops
and mandatory output capacitors (except for Cuk) make logic
pin dimming much slower than bucks. Trying to dim the output of
a boost with parallel FETs will cause an input short circuit and
will cause runaway input inductor current in SEPIC or Cuk. A
two-stage system that uses boost and then a buck regulator as
the second LED driving stage is one possibility. When space and
cost do not permit this approach, the next best choice is a series
switch, shown in Figure 8. Series FET dimming is difficult to
achieve without a dedicated LED driver IC because interruption
of the LED current also disconnects the feedback to the control
loop, which causes the output voltage to rise uncontrollably.
HSN
IF
CSH
DDRV
COMP
OVP
AGND
RPD
Figure 8: Boost Regulator with Series DIM Switch
Figure 8: Boost Regulator with Series DIM Switch
36
VO
Designer’s Corner
EMI Design for LED Street Lamp Application
EMI Design for LED Street Lamp Application
The LM3402HV circuitry shown in Figure 1 is based on a street
lamp application. The input is 48 VDC and the output is 12 seriesconnected 1W LEDs. To address the EMI concerns, the schematic
and PCB layout were modified. As a result of the modifications, as
shown in Figure 2 below, better EMI performance was achieved
and the design passed the EN55022 standard.
8
Ron
2M
6
3
SW
LED+
50R
1
D1
SS110
COUT
1 µf
LM3402HVMR
RON
DIM
4
DIM1
U1
VIN
BOOT
CIN
2.2 µF/63V
GND
VIN
Radiated Emission Measurement
L1
220 µH
RZ
10 nF
2
CB
The modifications are:
1: One resistor Rz (50 Ω) is added between the SW pin and Cb
pin. This changes the SW node waveform from Figure 3 to
Figure 4. The criterion of Rz selection is dependent on the SW
turn-on slew rate and its ringing. The smaller the ringing,
the better.
2: 1 µF output cap is added across LED connection port.
3: Input loop area should be kept as small as possible, which
is shown in the blue-dashed area of Figure 5. CIN should be
connected with the anode of catch diode directly.
4: The SW node should be kept as short as possible.
CS
VCC
5
LED-
7
CF
100 nF
RSNS
0.68R
Figure 1
Figure 2
Pulsating Current
ST
ESRIN
VIN
+
-
CIN
CHF
SW
High dv/dt node
LF
I
PGND
Figure 3
national.com/LED
Figure 4
Figure 5
37
Designer’s Corner
Two-Wire Dimming
Two-Wire Dimming
LM3406 Two-Wire Dimming
Adding an external input diode and using the internal VINS
comparator allows the LM3406/06HV to sense and provide
PWM dimming of the LED by chopping of the input voltage. This
method is also referred to as "two-wire dimming," and a typical
application circuit is shown below.
If the VINS pin voltage falls 70% below the VIN pin voltage, the
LM3406/06HV disables the internal power FET and shuts off the
current to the LED array. The support circuitry (driver, bandgap,
VCC) remains active in order to minimize the time needed to turn
the LED back on when the VINS pin voltage rises and exceeds
70% of VIN. This minimizes the response time for turning the LED
array back on.
The benefit of two-wire dimming:
One wire less than traditional PWM dimming, further reducing the wiring cost.
38
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39
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