LT3497 - Dual Full Function White LED Driver

LT3497 - Dual Full Function White LED Driver
LT3497
Dual Full Function White
LED Driver with Integrated
Schottky Diodes
DESCRIPTION
FEATURES
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Drives Up to 12 White LEDs (6 in Series per
Converter) from a 3V Supply
Two Independent Boost Converters Capable of
Driving Asymmetric LED Strings
Independent Dimming and Shutdown Control of the
Two LED Strings
High Side Sense Allows “One Wire Current Source”
per Converter
Internal Schottky Diodes
Open LED Protection (32V)
2.3MHz Switching Frequency
±5% Reference Accuracy
VIN Range: 2.5V to 10V
Dual Wide 250:1 True Color PWMTM Dimming
Requires Only 1µF Output Capacitor per Converter
Available in a 3mm × 2mm 10-Pin DFN Package
APPLICATIONS
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The LT®3497 is a dual full function step-up DC/DC converter specifically designed to drive up to 12 white LEDs
(6 white LEDs in series per converter) from a Li-Ion cell. Series connection of the LEDs provides identical LED currents
resulting in uniform brightness and eliminating the need
for ballast resistors and expensive factory calibration.
The two independent converters are capable of driving
asymmetric LED strings. Accurate LED dimming and
shutdown of the two LED strings can also be controlled
independently. The LT3497 features a unique high side LED
current sense that enables the part to function as a “one
wire current source;” one side of the LED string can be
returned to ground anywhere, allowing a simpler 1-wire
LED connection. Traditional LED drivers use a grounded
resistor to sense LED current, requiring a 2-wire connection to the LED string.
The 2.3MHz switching frequency allows the use of tiny
inductors and capacitors. Few external components are
needed for the dual white LED Driver: open-LED protection
and the Schottky diodes are all contained inside the 3mm
× 2mm DFN package. With such a high level of integration, the LT3497 provides a high efficiency dual white LED
driver solution in the smallest of spaces.
Cellular Phones
PDAs, Handheld Computers
Digital Cameras
MP3 Players
GPS Receivers
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
True Color PWM is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
Li-Ion Power Driver for 4/4 White LEDs
Efficiency
VIN
3V TO 5V
80
VIN = 3.6V
4/4LEDs
75
1µF
15µH
VIN
CAP1
SW2
CAP2
LT3497
10Ω
EFFICIENCY (%)
SW1
15µH
10Ω
1µF
70
65
60
1µF
LED1
LED2
CTRL1 GND CTRL2
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
55
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 2
3497 TA01a
50
0
5
10
15
LED CURRENT (mA)
20
3497 TA01b
3497f
1
LT3497
ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
(Note 1)
Input Voltage (VIN) ...................................................10V
SW1, SW2 Voltages ..................................................35V
CAP1, CAP2 Voltages ................................................35V
CTRL1, CTRL2 Voltages ............................................10V
LED1, LED2 Voltages ................................................35V
Operating Temperature Range ................. –40°C to 85°C
Maximum Junction Temperature .......................... 125°C
Storage Temperature Range................... –65°C to 125°C
TOP VIEW
LED1 1
10 CAP1
CTRL1 2
GND 3
9
11
SW1
8
VIN
CTRL2 4
7
SW2
LED2 5
6
CAP2
DDB PACKAGE
10-LEAD (3mm × 2mm) PLASTIC DFN
TJMAX = 125°C, θJA = 76°C/W, θJC = 13.5°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB
ORDER PART NUMBER
DDB PART MARKING
LT3497EDDB
LCGT
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3V, VCTRL1 = VCTRL2 = 3V.
PARAMETER
CONDITIONS
MIN
Minimum Operating Voltage
TYP
MAX
2.5
UNITS
V
LED Current Sense Voltage (VCAP1 – VLED1)
VCAP1 = 16V
●
190
200
210
mV
LED Current Sense Voltage (VCAP2 – VLED2)
VCAP2 = 16V
●
190
200
210
mV
Offset Voltage (VOS) Between
(VCAP1 – VLED1) – (VCAP2 – VLED2) Voltages
VOS = |(VCAP1 – VLED1) – (VCAP2 – VLED2)|
0
2
8
mV
CAP1, LED1 Pin Bias Current
VCAP1 = 16V, VLED1 = 16V
20
40
µA
CAP2, LED2 Pin Bias Current
VCAP2 = 16V, VLED2 = 16V
20
40
µA
VCAP1, VLED1 Common Mode Minimum Voltage
2.5
V
VCAP2, VLED2 Common Mode Minimum Voltage
2.5
V
Supply Current
VCAP1 = VCAP2 = 16V, VLED1 = VLED2 = 15V,
VCTRL1 = VCTRL2 = 3V
6
8.5
mA
VCTRL1 = VCTRL2 = 0V
12
18
µA
1.8
2.3
2.8
MHz
Switching Frequency
Maximum Duty Cycle
88
92
%
Converter 1 Switch Current Limit SW1
●
300
400
mA
Converter 2 Switch Current Limit SW2
●
300
400
mA
Converter 1 VCESAT
ISW1 = 200mA
200
mV
Converter 2 VCESAT
ISW2 = 200mA
200
mV
Switch 1 Leakage Current
VSW1 = 16V
0.1
5
µA
Switch 2 Leakage Current
VSW2 = 16V
0.1
5
µA
3497f
2
LT3497
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3V, VCTRL1 = VCTRL2 = 3V.
PARAMETER
CONDITIONS
VCTRL1 Voltage for Full LED Current
VCAP1 = 16V
●
1.5
V
VCTRL2 Voltage for Full LED Current
VCAP2 = 16V
●
1.5
V
●
100
mV
VCTRL1 or VCTRL2 Voltage to Turn On the IC
MIN
TYP
VCTRL1 and VCTRL2 Voltages to Shut Down the IC
MAX
50
CTRL1, CTRL2 Pin Bias Current
100
UNITS
mV
nA
CAP1 Pin Overvoltage Protection
●
30
32
34
V
CAP2 Pin Overvoltage Protection
●
30
32
34
V
Schottky 1 Forward Drop
ISCHOTTKY1 = 100mA
0.8
V
Schottky 2 Forward Drop
ISCHOTTKY2 = 100mA
0.8
V
Schottky 1 Reverse Leakage Current
VR1 = 25V
4
µA
Schottky 2 Reverse Leakage Current
VR2 = 25V
4
µA
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LT3497E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
3497f
3
LT3497
TYPICAL PERFORMANCE CHARACTERISTICS
Switch Saturation Voltage
(VCESAT)
15
400
400
350
–50°C
300
125°C
25°C
250
200
150
100
50
0
50
350
300
250
125°C
200
25°C
150
–50°C
100
12
25°C
125°C
9
6
3
50
0
100 150 200 250 300 350 400
SWITCH CURRENT (mA)
–50°C
SHUTDOWN CURRENT (µA)
SCHOTTKY FORWARD CURRENT (mA)
SWITCH SATURATION VOLTAGE (mV)
Shutdown Current
(VCTRL1 = VCTRL2 = 0V)
Schottky Forward Voltage Drop
450
0
(TA = 25°C unless otherwise specified)
0
0
200
400
800
600
SCOTTKY FORWARD DROP (mV)
3497 G01
1000
0
2
6
4
8
3497 G03
3497 G02
Sense Voltage (VCAP – VLED)
vs VCTRL
Open-Circuit Output Clamp
Voltage
Input Current in Output Open
Circuit
34
240
10
VIN (V)
30
25°C
160
–50°C
125°C
120
80
40
0
25
33
–50°C
32
500
1000
VCTRL (mV)
1500
2000
125°C
25°C
31
150°C
20
15
25°C
10
–50°C
5
30
0
INPUT CURRENT (mA)
OUTPUT CLAMP VOLTAGE (V)
SENSE VOLTAGE (mV)
200
2
0
6
4
8
10
0
2
VIN (V)
6
VIN (V)
3497 G05
3497 G04
Switching Waveform
8
10
3497 G06
Transient Response
VSW
10V/DIV
VCAP
5V/DIV
VCAP
50mV/DIV
VCTRL
5V/DIV
IL
100mA/DIV
200ms/DIV
VIN = 3.6V
FRONT PAGE
APPLICATION CIRCUIT
4
3497 G07
IL
200mA/DIV
1ms/DIV
VIN = 3.6V
FRONT PAGE
APPLICATION CIRCUIT
3497 G08
3497f
4
LT3497
TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Current
(TA = 25°C unless otherwise specified)
Schottky Leakage Current vs
Temperature (–50°C to 125°C)
Current Limit vs Temperature
7
3
500
25°C
5
CURRENT LIMIT (mA)
QUIESCENT CURRENT (mA)
6
SCHOTTKY LEAKAGE CURRENT (µA)
125°C
–50°C
4
3
2
450
400
350
1
0
2
0
4
6
300
–50
10
8
–25
VIN (V)
75
0
25
50
TEMPERATURE (°C)
100
30
75
0
25
50
TEMPERATURE (°C)
100
75
0
25
50
TEMPERATURE (°C)
2.60
VIN = 3V
125
VIN = 3.6V
2.50
20
15
10
2.40
2.30
2.20
2.10
2.00
1.90
0
–50 –25
125
100
3497 G12
SWITCHING FREQUENCY (MHz)
INPUT CURRENT (mA)
50
25
75
0
TEMPERATURE (°C)
100
3497 G13
125
1.80
–50 –25
0
50
75
25
TEMPERATURE (°C)
Sense Voltage (VCAP – VLED)
vs VCAP
100
125
3497 G15
3497 G14
Sense Voltage vs Temperature
208
206
204
SENSE VOLTAGE (mV)
–25
–25
Switching Frequency vs
Temperature
5
28
–50
16V
0
–50
125
25
SENSE VOLTAGE (mV)
OUTPUT CLAMP VOLTAGE (V)
36
30
1
Input Current in Output Open
Circuit vs Temperature
(–50°C to 125°C)
Open-Circuit Output Clamp Voltage
vs Temperature (–50°C to 125°C)
32
24V
3497 G11
3497 G09
34
2
125°C
200
25°C
196
–50°C
202
198
194
192
188
5
10
20
15
VCAP (V)
25
30
3497 G16
190
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
3497 G17
3497f
5
LT3497
PIN FUNCTIONS
LED1 (Pin 1): Connection point for the anode of the first
LED of the first set of LEDs and the sense resistor (RSENSE1).
The LED current can be programmed by:
ILED1 =
200mV
RSENSE1
CTRL1 (Pin 2): Dimming and Shutdown Pin. Connect
CTRL1 below 50mV to disable converter 1. As the pin voltage is ramped from 0V to 1.5V, the LED current ramps from
0 to (ILED1 = 200mV/RSENSE1). The CTRL1 pin must not
be left floating.
GND (Pin 3): Connect the GND pin to the PCB system
ground plane.
CTRL2 (Pin 4): Dimming and Shutdown Pin. Connect
CTRL2 below 50mV to disable converter 2. As the pin voltage is ramped from 0V to 1.5V, the LED current ramps from
0 to (ILED2 = 200mV/RSENSE2). The CTRL2 pin must not
be left floating.
CAP2 (Pin 6): Output of Converter 2. This pin is connected
to the cathode of internal Schottky diode 2. Connect the
output capacitor to this pin and the sense resistor (RSENSE2)
from this pin to LED2 pin.
SW2 (Pin 7): Switch Pin. Minimize trace area at this pin
to minimize EMI. Connect the inductor at this pin.
VIN (Pin 8): Input Supply Pin. This pin must be locally
bypassed.
SW1 (Pin 9): Switch Pin. Minimize trace area at this pin
to minimize EMI. Connect the inductor at this pin.
CAP1 (Pin 10): Output of Converter 1. This pin is connected
to the cathode of internal Schottky diode 1. Connect the
output capacitor to this pin and the sense resistor (RSENSE1)
from this pin to LED1 pin.
Exposed Pad (Pin 11): Ground. Must be soldered to
PCB.
LED2 (Pin 5): Connection point for the anode of the first
LED of the second set of LEDs and the sense resistor
(RSENSE2). The LED current can be programmed by:
ILED2 =
200mV
RSENSE2
3497f
6
COUT1
1µF
RSENSE1
10Ω
1
10
LED1
CAP1
OVERVOLTAGE
PROTECT
R
Q1
A = 6.25
A3
R
Q R
S
2
CTRL1
1.25V
CONVERTER 1
START-UP
–
+
–
+
DRIVER
–
+
+
A1
RC
RAMP
GENERATOR
CC
GND
3
2.3MHz
OSCILLATOR
RC
CC
A2
gm AMP
Figure 1. LT3497 Block Diagram
gm AMP
A2
8
VIN
+
SW1
+
–
9
CIN
1µF
–
L1
15µH
A1
–
+
+
R
R Q
S
4
CTRL2
1.25V
CONVERTER 2
L2
15µH
+
–
+
START-UP
–
A = 6.25
A3
DRIVER
R
LED2
CAP2
OVERVOLTAGE
PROTECT
SW2
Q2
7
5
6
3497 F01
RSENSE2
10Ω
COUT2
1µF
LT3497
BLOCK DIAGRAM
3497f
7
LT3497
OPERATION
Main Control Loop
The LT3497 uses a constant frequency, current mode control scheme to provide excellent line and load regulation.
It incorporates two identical, but fully independent PWM
converters. Operation can be best understood by referring
to the Block Diagram in Figure 1. The oscillator, start-up
bias and the band gap reference are shared between the two
converters. The control circuitry, power switch, Schottky
diode etc., are identical for both the converters.
At power up, the capacitors at CAP1 and CAP2 pins are
charged up to VIN (input supply voltage) via their respective
inductor and the internal Schottky diode. If either CTRL1
and CTRL2 or both are pulled higher than 100mV, the
bandgap reference, the start-up bias and the oscillator
are turned on.
The main control loop can be understood by following the
operation of converter 1. At the start of each oscillator cycle,
the power switch, Q1, is turned on. A voltage proportional
to the switch current is added to a stabilizing ramp and the
resulting sum is fed into the positive terminal of the PWM
comparator, A2. When this voltage exceeds the level at the
negative input of A2, the PWM logic turns off the power
switch. The level at the negative input of A2 is set by the
error amplifier, A1, and is simply an amplified version of
the difference between the VCAP1 and VLED1 voltage and
the bandgap reference. In this manner the error amplifier,
A1, sets the correct peak current level in inductor L1 to
keep the output in regulation. The CTRL1 pin is used to
adjust the LED current.
If only one of the converters is turned on, the other converter
will stay off and its output will remain charged up to VIN
(input supply voltage). The LT3497 enters into shutdown
when both CTRL1 and CTRL2 pins are pulled lower than
50mV. The CTRL1 and CTRL2 pins perform independent
dimming and shutdown control for the two converters.
Minimum Output Current
The LT3497 can drive a 4-LED string at 2mA LED current
without pulse skipping. As current is further reduced, the
device may begin skipping pulses.
This will result in some low frequency ripple, although the
average LED current remains regulated down to zero. The
photo in Figure 2 details circuit operation driving 4 white
LEDs at 2mA. Peak inductor current is less than 50mA and
the regulator operates in discontinuous mode, meaning
the inductor current reaches zero during the discharge
phase. After the inductor current reaches zero, the SW
pin exhibits ringing due to the LC tank circuit formed
by the inductor in combination with the switch and the
diode capacitance. This ringing is not harmful; far less
spectral energy is contained in the ringing than in the
switch transitions.
IL
50mA/DIV
VSW
10V/DIV
VIN = 4.2V
ILED = 2mA
4 LEDs
200ns/DIV
3497 F02
Figure 2. Switching Waveforms
3497f
8
LT3497
APPLICATIONS INFORMATION
DUTY CYCLE
15µH MURATA LQH32CN150K53
15µH MURATA LQH2MCN150K02
15µH COOPER SD3112-150
15µH TOKO 1001AS-150M TYPE D312C
15µH SUMIDA CDRH2D11/HP
The duty cycle for a step-up converter is given by:
D=
VOUT + VD – VIN
VOUT + VD – VCESAT
80
75
70
EFFICIENCY (%)
where:
VOUT = Output voltage
VD = Schottky forward voltage drop
VCESAT = Saturation voltage of the switch
VIN = Input voltage
A 15µH inductor is recommended for most LT3497 applications. Although small size and high efficiency are
major concerns, the inductor should have low core losses
at 2.3MHz and low DCR (copper wire resistance). Some
inductors in this category with small size are listed in
Table 1. The efficiency comparison of different inductors
is shown in Figure 3.
Table 1: Recommended Inductors
PART
LQH32CN150K53
LQH2MCN150K02
LQH32CN100K53
LQH2MCN100K02
SD3112-150
1001AS-150M
(TYPE D312C)
CDRH2D11/HP
L
(µH)
15
15
10
10
15
MAX
DCR
(Ω)
0.58
1.6
0.3
1.2
0.654
CURRENT
RATING
(mA)
300
200
450
225
440
15
0.80
360
15
0.739
410
VENDOR
Murata
www.murata.com
Cooper
www.cooperet.com
Toko
www.toko.com
Sumida
www.sumida.com
CAPACITOR SELECTION
The small size of ceramic capacitors make them ideal for
LT3497 applications. Use only X5R and X7R types because
they retain their capacitance over wider temperature ranges
than other types such as Y5V or Z5U. A 1µF input capacitor
60
55
50
The maximum duty cycle achievable for LT3497 is 88%
when running at 2.3MHz switching frequency. Always
ensure that the converter is not duty-cycle limited when
powering the LEDs at a given frequency.
INDUCTOR SELECTION
65
45
0
5
10
15
LED CURRENT (mA)
20
3497 F03
Figure 3. Efficiency Comparison of Different Inductors
and a 1µF output capacitor are sufficient for most applications. Table 2 shows a list of several ceramic capacitor
manufacturers. Consult the manufacturers for detailed
information on their entire selection of ceramic parts.
Table 2: Recommended Ceramic Capacitor Manufacturers
Taiyo Yuden
(800) 368-2496
www.t-yuden.com
AVX
(803) 448-9411
www.avxcorp.com
Murata
(714) 852-2001
www.murata.com
OVERVOLTAGE PROTECTION
The LT3497 has an internal open-circuit protection
circuit for both converters. In the cases of output open
circuit, when the LEDs are disconnected from the circuit
or the LEDs fail open circuit, the converter VCAP voltage
is clamped at 32V (typ). Figure 4a shows the transient
response of the front page application step-up converter
with LED1 disconnected. With LED1 disconnected, the
converter starts switching at the peak inductor current
limit. The converter output starts ramping up and finally
gets clamped at 32V (typ). The converter will then switch
at low inductor current to regulate the converter output
at the clamp voltage. The VCAP and input current during
output open circuit are shown in the Typical Performance
Characteristics.
3497f
9
LT3497
APPLICATIONS INFORMATION
For low DCR inductors, which are usually the case for this
application, the peak inrush current can be simplified as
follows:
VCAP
10V/DIV
α=
ISW
200mA/DIV
VIN = 3.6V
FRONT PAGE
APPLICATION CIRCUIT
500µs/DIV
r
2 •L
ω=
1
r2
–
L • C 4 • L2
IPK =
VIN – 0.6
⎛ α π⎞
• exp ⎜ – • ⎟
⎝ ω 2⎠
L •ω
3497 F04a
LEDs DISCONNECTED
AT THIS INSTANT
Figure 4a. Transient Response of Switcher 1 with LED1
Disconnected from the Output
IL1
50mA/DIV
where L is the inductance, r is the DCR of the inductor
and C is the output capacitance.
Table 3 gives inrush peak currents for some component
selections.
VSW1
20V/DIV
Table 3: Inrush Peak Currents
IL2
50mA/DIV
VSW2
20V/DIV
VIN = 3.6V
4 LEDs
LED 2 DISCONNECTED
200ms/DIV
3497 F04b
VIN (V)
r (Ω)
L (µH)
COUT (µF)
IP (A)
4.2
0.58
15
1
0.828
4.2
1.6
15
1
0.682
4.2
0.8
15
1
0.794
4.2
0.739
15
1
0.803
Figure 4b. Switching Waveforms with Output 1 Open Circuit
In the event one of the converters has an output open
circuit, its output voltage will be clamped at 32V. However,
the other converter will continue functioning properly.
The photo in Figure 4b shows circuit operation with
converter 2 output open circuit and converter 1 driving
4 LEDs at 20mA. Converter 2 starts switching at a lower
peak inductor current and begins skipping pulses, thereby
reducing its input current.
INRUSH CURRENT
The LT3497 has built-in Schottky diodes. When supply
voltage is applied to the VIN pin, an inrush current flows
through the inductor and the Schottky diode and charges
up the CAP voltage. Both the Schottky diodes in the LT3497
can sustain a maximum current of 1A. The selection of
inductor and capacitor value should ensure the peak of
the inrush current to be below 1A.
PROGRAMMING LED CURRENT
The LED current of each LED string can be set independently by the choice of resistors RSENSE1 and RSENSE2,
respectively. For each LED string, the feedback resistor
(RSENSE) and the sense voltage (VCAP – VLED) control the
LED current.
For each independent LED string, the CTRL pin controls
the sense reference voltage as shown in the Typical
Performance Characteristics. For CTRL higher than 1.5V,
the sense reference is 200mV, which results in full LED
current. In order to have accurate LED current, precision
resistors are preferred (1% is recommended). The formula
and Table 4 for RSENSE selection are shown below.
RSENSE =
200mV
ILED
3497f
10
LT3497
APPLICATIONS INFORMATION
Table 4: RSENSE Value Selection for 200mV Sense
ILED (mA)
RSENSE (Ω)
5
40
10
20
15
13.3
20
10
PWM
10kHz TYP
LT3497
R1
100k
C1
0.1µF
CTRL1,2
3497 F05
Figure 5. Dimming Control Using a Filtered PWM Signal
DIMMING CONTROL
Direct PWM Dimming
There are three different types of dimming control circuits.
The LED current can be set by modulating the CTRL pin
with a DC voltage, a filtered PWM signal or directly with
a PWM signal.
Changing the forward current flowing in the LEDs not only
changes the intensity of the LEDs, it also changes the color.
The chromaticity of the LEDs changes with the change in
forward current. Many applications cannot tolerate any
shift in the color of the LEDs. Controlling the intensity of
the LEDs with a direct PWM signal allows dimming of the
LEDs without changing the color. In addition, direct PWM
dimming offers a wider dimming range to the user.
Using a DC Voltage
For some applications, the preferred method of brightness
control is a variable DC voltage to adjust the LED current.
The CTRL pin voltage can be modulated to set the dimming of the LED string. As the voltage on the CTRL pin
increases from 0V to 1.5V, the LED current increases from
0 to ILED. As the CTRL pin voltage increases beyond 1.5V,
it has no effect on the LED current.
The LED current can be set by:
ILED ≈
200mV
when VCTRL > 1.5V
RSENSE
ILED ≈
VCTRL
when VCTRL < 1.25V
6.25 • RSENSE
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics.
Using a Filtered PWM Signal
A filtered PWM can be used to control the brightness of
the LED string. The PWM signal is filtered (Figure 5) by a
RC network and fed to the CTRL1, CTRL2 pins.
The corner frequency of R1, C1 should be much lower
than the frequency of the PWM signal. R1 needs to be
much smaller than the internal impedance in the CTRL
pins which is 10MΩ (typ).
Dimming the LEDs via a PWM signal essentially involves
turning the LEDs on and off at the PWM frequency. The
typical human eye has a limit of ~60 frames per second.
By increasing the PWM frequency to ~80Hz or higher,
the eye will interpret that the pulsed light source is continuously on. Additionally, by modulating the duty cycle
(amount of “on time”) the intensity of the LEDs can be
controlled. The color of the LEDs remains unchanged in
this scheme since the LED current value is either zero or
a constant value.
Figure 6 shows a Li-ion powered 4/4 white LED driver. Direct
PWM dimming method requires an external NMOS tied
between the cathode of the lowest LED in the string and
ground as shown in Figure 6. Si2318DS MOSFETs can be
used since its sources are connected to ground. The PWM
signal is applied to the (CTRL1 and CTRL2) control pins of
the LT3497 and the gate of the MOSFET. The PWM signal
should traverse between 0V to 5V to ensure proper turn
on and off of the converters and the NMOS transistors (Q1
and Q2). When the PWM signal goes high, LEDs are connected to ground and a current of ILED = (200mV/RSENSE)
flows through the LEDs. When the PWM signal goes low,
the LEDs are disconnected and turn off. The low PWM
input applied to the LT3497 ensures that the respective
3497f
11
LT3497
APPLICATIONS INFORMATION
Example:
converter turns off. The MOSFETs ensure that the LEDs
quickly turn off without discharging the output capacitors
which in turn allows the LEDs to turn on faster. Figures 7
and 8 show the PWM dimming waveforms and efficiency
for the Figure 6 circuit.
ƒ = 100Hz, tSETTLE = 40μs
tPERIOD = 1/ƒ = 1/100 = 0.01s
Dim Range = tPERIOD/tSETTLE = 0.01s/40μs = 250:1
Min Duty Cycle = tSETTLE/tPERIOD • 100
= 40μs/0.01s = 0.4%
Duty Cycle Range = 100%→0.4% at 100Hz
The time it takes for the LEDs current to reach its programmed value sets the achievable dimming range for a
given PWM frequency. For example, the settling time of
the LEDs current in Figure 7 is approximately 40μs for a
3V input voltage. The achievable dimming range for this
application and 100Hz PWM frequency can be determined
using the following method.
The calculations show that for a 100Hz signal the dimming
range is 250 to 1. In addition, the minimum PWM duty
cycle of 0.4% ensures that the LEDs current has enough
3V TO 5V
1µF
L1
15µH
SW1
L2
15µH
VIN
CAP1
RSENSE1
10Ω
1µF
SW2
CAP2
RSENSE2
10Ω
LT3497
1µF
LED1
LED2
CTRL1 GND CTRL2
Q1
Si2318DS
Q2
Si2318DS
100k
5V
5V
0V
0V
PWM
FREQ
100k
PWM
FREQ
3497 F06
Figure 6. Li-Ion to 4/4 White LEDs with Direct PWM Dimming
80
VIN = 3.6V
4/4 LEDs
ILED
20mA/DIV
EFFICIENCY (%)
78
IL
200mA/DIV
PWM
5V/DIV
VIN = 3.6V
4 LEDs
2ms/DIV
3497 F07
Figure 7. Direct PWM Dimming Waveforms
76
74
72
70
0
5
10
15
LED CURRENT (mA)
20
3497 F08
Figure 8. Efficiency
3497f
12
LT3497
APPLICATIONS INFORMATION
time to settle to its final value. Figure 9 shows the available dimming range for different PWM frequencies with
a settling time of 40μs.
3V TO 5V
1µF
L2
15µH
L1
15µH
10000
SW1
VIN
SW2
PWM DIMMING RANGE
CAP1
1µF
PULSING MAY BE VISIBLE
1000
RSENSE1
10Ω
CAP2
RSENSE2
10Ω
LT3497
1µF
LED1
LED2
CTRL1 GND CTRL2
100
5V
5V
0V
10
0V
PWM
FREQ
PWM
FREQ
Q1
Si2318DS
1
100k
10
100
1000
PWM FREQUENCY (Hz)
100k
Q2
Si2318DS
10000
3497 F10
3497 F09
Figure 9. Dimming Ratio vs Frequency
Figure 10. Li-Ion to 4/4 White LEDs with Both PWM Dimming
and Analog Dimming
The dimming range can be further extended by changing
the amplitude of the PWM signal. The height of the PWM
signal sets the commanded sense voltage across the sense
resistor through the CTRL pin. In this manner both analog
dimming and direct PWM dimming extend the dimming
range for a given application. The color of the LEDs no
longer remains constant because the forward current of
the LED changes with the height of the CTRL signal. For
the 4-LED application described above, the LEDs can be
dimmed first, modulating the duty cycle of the PWM signal.
Once the minimum duty cycle is reached, the height of the
PWM signal can be decreased below 1.5V down to 100mV.
The use of both techniques together allows the average LED
current for the 4-LED application to be varied from 20mA
down to less than 20µA. Figure 10 shows the application
for dimming using both analog dimming and PWM dimming. A potentiometer must be added to ensure that the
gate of the NMOS receives a logic-level signal, while the
CTRL signal can be adjusted to lower amplitudes.
lower battery voltage. This technique allows the LEDs to
be powered off two alkaline cells. Most portable devices
have a 3.3V supply voltage which can be used to power
the LT3497. The LEDs can be driven straight from the
battery, resulting in higher efficiency.
LOW INPUT VOLTAGE APPLICATIONS
The LT3497 can be used in low input voltage applications. The input supply voltage to the LT3497 must be
2.5V or higher. However, the inductors can be run off a
Figure 11 shows 3/3 LEDs powered by two AA cells.
The battery is connected to the inductors and the chip is
powered off a 3.3V logic supply voltage.
3.3V
2 AA CELLS
2V TO 3.2V
C2
1µF
C1
1µF
L1
15µH
L2
15µH
SW1
VIN
SW2
CAP1
RSENSE1
10Ω
C3
1µF
CAP2
RSENSE2
10Ω
LT3497
LED1
LED2
CTRL1 GND CTRL2
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
C4
1µF
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 2
C1, C2: TAIYO YUDEN LMK212BJ105MG
C3, C4: TAIYO YUDEN GMK212BJ105KG
L1, L2: MURATA LQH32CN150K53
3497 F11
Figure 11. 2 AA Cells to 3/3 White LEDs
3497f
13
LT3497
APPLICATIONS INFORMATION
BOARD LAYOUT CONSIDERATIONS
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To prevent electromagnetic interference (EMI) problems,
proper layout of high frequency switching paths is essential.
Minimize the length and area of all traces connected to
the switching node pins (SW1 and SW2). Keep the sense
voltage pins (CAP1, CAP2, LED1 and LED2) away from
the switching node. Place the output capacitors (COUT1
and COUT2) next to the output pins (CAP1 and CAP2).
The placement of a bypass capacitor on VIN needs to be
in close proximity to the IC to filter EMI noise from SW1
and SW2. Always use a ground plane under the switching
regulator to minimize interplane coupling. Recommended
component placement is shown in Figure 12.
VIA TO
GROUND PLANE
COUT2
SW2
L2
CAP2
CTRL2
CIN
VIA TO
GROUND
PLANE
LED2
VIN
L1
10
5
9
4
8
3
7
2
6
1
GND
CTRL1
CAP1
SW1
LED1
COUT1
3497 F12
VIAS TO
GROUND PLANE
Figure 12. Recommended Component Placement
TYPICAL APPLICATIONS
Li-Ion to 1/2 White LEDs
Conversion Efficiency
VIN
3V TO 5V
70
L1
10µH
L2
10µH
SW1
VIN
CAP1
RSENSE1
10Ω
C2
1µF
60
SW2
CAP2
RSENSE2
10Ω
LT3497
LED1
LED2
CTRL1 GND CTRL2
VIN = 3.6V
65 1/2LEDs
EFFICIENCY (%)
C1
1µF
C3
1µF
3497 TA02a
55
50
45
40
35
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 2
C1, C2: TAIYO YUDEN GMK212BJ105KG
C3: TAIYO YUDEN LMK212BJ105MG
L1, L2: MURATA LQH32CN100K53
30
0
5
10
15
20
LED CURRENT (mA)
3497 TA02b
3497f
14
LT3497
TYPICAL APPLICATIONS
Li-Ion to 2/2 White LEDs
Conversion Efficiency
VIN
3V TO 5V
70
SW1
VIN
CAP1
RSENSE1
10Ω
C2
1µF
L2
10µH
L1
10µH
SW2
CAP2
RSENSE2
10Ω
LT3497
LED1
LED2
CTRL1 GND CTRL2
60
55
50
3497 TA12a
45
OFF ON
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
VIN = 3.6V
2/2 LEDs
65
EFFICIENCY (%)
C1
1µF
C3
1µF
40
SHUTDOWN
AND DIMMING
CONTROL 2
0
5
10
15
LED CURRENT (mA)
20
3497 TA12b
C1, C2: TAIYO YUDEN GMK212BJ105KG
C3: TAIYO YUDEN LMK212BJ105MG
L1, L2: MURATA LQH32CN100K53
Li-Ion to 2/2 White LEDs
Conversion Efficiency
3V TO 5V
80 V
IN = 3.6V
2/2LEDs
75
C3
1µF
L2
10µH
L1
10µH
VIN
CAP1
C1
1µF
RSENSE1
10Ω
SW2
CAP2
C2
1µF
RSENSE2
10Ω
LT3497
LED1
LED2
CTRL1 GND CTRL2
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
EFFICIENCY (%)
70
SW1
65
60
55
50
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 2
C1, C2: TAIYO YUDEN GMK212BJ105KG
C3: TAIYO YUDEN LMK212BJ105MG
L1, L2: MURATA LQH32CN100K53
45
3497TA13a
40
0
5
10
15
20
LED CURRENT (mA)
3497 TA13b
3497f
15
LT3497
TYPICAL APPLICATIONS
Li-Ion to 2/4 White LEDs
Conversion Efficiency
VIN
3V TO 5V
C3
1µF
SW1
VIN
CAP1
RSENSE1
10Ω
70
L2
15µH
L1
10µH
SW2
CAP2
RSENSE2
10Ω
LT3497
C2
1µF
65
60
55
LED1
LED2
CTRL1 GND CTRL2
50
OFF ON
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
VIN = 3.6V
2/4LEDs
75
EFFICIENCY (%)
C1
1µF
80
3497 TA03a
45
0
SHUTDOWN
AND DIMMING
CONTROL 2
5
10
15
20
LED CURRENT (mA)
3497 TA03b
C1, C2: TAIYO YUDEN GMK212BJ105KG
C3: TAIYO YUDEN LMK212BJ105MG
L1: MURATA LQH32CN100K53
L2: MURATA LQH32CN150K53
Li-Ion to 3/3 White LEDs
Conversion Efficiency
VIN
3V TO 5V
80
C3
1µF
L1
15µH
L2
15µH
VIN
CAP1
RSENSE1
10Ω
70
SW2
CAP2
RSENSE2
10Ω
LT3497
65
60
55
LED1
LED2
CTRL1 GND CTRL2
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
C2
1µF
EFFICIENCY (%)
SW1
C1
1µF
VIN = 3.6V
3/3LEDs
75
3497 TA04a
50
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 2
C1, C2: TAIYO YUDEN GMK212BJ105KG
C3: TAIYO YUDEN LMK212BJ105MG
L1, L2: MURATA LQH32CN150K53
45
0
5
10
15
20
LED CURRENT (mA)
3497 TA04b
3497f
16
LT3497
TYPICAL APPLICATIONS
Li-Ion to 4/6 White LEDs
Conversion Efficiency
VIN
3V TO 5V
80
C3
1µF
L1
15µH
C1
1µF
VIN
CAP1
RSENSE1
10Ω
75
L2
15µH
SW2
CAP2
RSENSE2
10Ω
LT3497
LED1
LED2
CTRL1 GND CTRL2
C2
1µF
EFFICIENCY (%)
SW1
VIN = 3.6V
4/6LEDs
70
65
60
55
OFF ON
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
SHUTDOWN
AND DIMMING
CONTROL 2
50
0
3497 TA05a
5
10
15
LED CURRENT (mA)
C1, C2: TAIYO YUDEN GMK212BJ105KG
C3: TAIYO YUDEN LMK212BJ105MG
L1, L2: MURATA LQH32CN150K53
20
3497 TA05b
Li-Ion to 5/5 White LEDs
Conversion Efficiency
VIN
3V TO 5V
80
C3
1µF
L1
15µH
C1
1µF
VIN
RSENSE1
10Ω
SW2
CAP2
RSENSE2
10Ω
LT3497
C2
1µF
LED1
LED2
CTRL1 GND CTRL2
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
C1, C2: TAIYO YUDEN GMK212BJ105KG
C3: TAIYO YUDEN LMK212BJ105MG
L1, L2: MURATA LQH32CN150K53
70
65
60
55
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 2
EFFICIENCY (%)
SW1
75
L2
15µH
CAP1
VIN = 3.6V
5/5LEDs
3497 TA06a
50
0
5
10
15
LED CURRENT (mA)
20
3497 TA06b
3497f
17
LT3497
TYPICAL APPLICATIONS
Li-Ion to 6/6 White LEDs
VIN
3V TO 5V
Conversion Efficiency
80
C3
1µF
L1
15µH
L2
15µH
VIN
RSENSE1
10Ω
C1
1µF
75
SW2
EFFICIENCY (%)
SW1
CAP1
CAP2
RSENSE2
10Ω
LT3497
C2
1µF
LED1
LED2
CTRL1 GND CTRL2
70
65
60
55
OFF ON
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
VIN = 3.6V
6/6LEDs
SHUTDOWN
AND DIMMING
CONTROL 2
50
3497 TA07a
0
5
C1, C2: TAIYO YUDEN GMK212BJ105KG
C3: TAIYO YUDEN LMK212BJ105MG
L1, L2: MURATA LQH32CN150K53
10
15
LED CURRENT (mA)
20
3497 TA07b
2-Cell Li-Ion Movie and Flash Mode/6 White LEDs Control
VIN
6V TO 9V
Conversion Efficiency
C3
1µF
80
CAP1 VIN
SW2
LED1
CAP2
RSENSE2
10Ω
LT3497
SW1
LED2
CTRL1 GND CTRL2
FLASH
VCTRL1 680mV
MOVIE
MODE
MOVIE
FLASH
75
70
1.5V
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 2
ILED
100mA
200mA
C2
1µF
EFFICIENCY (%)
D1 L1
15µH
1-100mA LED/6 LEDs
L2
15µH
RSENSE1
1Ω
C1
4.7µF
85
C1: TAIYO YUDEN LMK212BJ475KD
C2: TAIYO YUDEN GMK212BJ105KG
C3: TAIYO YUDEN LMK212BJ105MG
D1: AOT-2015 HPW1751B
L1, L2: MURATA LQH32CN150K53
3497 TA08a
65
6
6.5
7
7.5
VIN (V)
8
8.5
9
3497 TA08b
3497f
18
LT3497
PACKAGE DESCRIPTION
DDB Package
10-Lead Plastic DFN (3mm × 2mm)
(Reference LTC DWG # 05-08-1722 Rev Ø)
0.64 ±0.05
(2 SIDES)
0.70 ±0.05
2.55 ±0.05
1.15 ±0.05
PACKAGE
OUTLINE
0.25 ± 0.05
0.50 BSC
2.39 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
3.00 ±0.10
(2 SIDES)
R = 0.05
TYP
R = 0.115
TYP
6
0.40 ± 0.10
10
2.00 ±0.10
(2 SIDES)
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.200 REF
0.75 ±0.05
0.64 ± 0.05
(2 SIDES)
5
0.25 ± 0.05
0 – 0.05
PIN 1
R = 0.20 OR
0.25 × 45°
CHAMFER
1
(DDB10) DFN 0905 REV Ø
0.50 BSC
2.39 ±0.05
(2 SIDES)
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING CONFORMS TO VERSION (WECD-1) IN JEDEC PACKAGE OUTLINE M0-229
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
3497f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
19
LT3497
TYPICAL APPLICATION
2 Li-Ion to 8/8 White LEDs
Conversion Efficiency
VIN
6V TO 9V
85
C3
1µF
L1
15µH
L2
15µH
VIN
RSENSE1
10Ω
75
SW2
CAP2
RSENSE2
10Ω
LT3497
LED1
LED2
CTRL1 GND CTRL2
C1
1µF
EFFICIENCY (%)
SW1
CAP1
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 1
VIN = 7.2V
8/8LEDs
80
C2
1µF
65
60
55
OFF ON
SHUTDOWN
AND DIMMING
CONTROL 2
C1, C2: TAIYO YUDEN GMK212BJ105KG
C3: TAIYO YUDEN LMK212BJ105MG
L1, L2: MURATA LQH32CN150K53
70
50
0
5
10
15
20
LED CURRENT (mA)
3497 TA11b
3497 TA11a
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1937
Constant Current, 1.2MHz, High Efficiency White LED
Boost Regulator
Up to 4 White LEDs, VIN: 2.5V to 10V, VOUT(MAX) = 34V,
IQ = 1.9mA, ISD < 1µA, ThinSOTTM/SC70 Packages
LTC3200-5
Low Noise, 2MHz Regulated Charge Pump White LED Driver
Up to 6 White LEDs, VIN: 2.7V to 4.5V, IQ = 8mA, ISD < 1µA,
ThinSOT Package
LTC3201
Low Noise, 1.7MHz Regulated Charge Pump White LED Driver
Up to 6 White LEDs, VIN: 2.7V to 4.5V, IQ = 6.5mA,
ISD < 1µA, MS Package
LTC3202
Low Noise, 1.5MHz Regulated Charge Pump White LED Driver
Up to 8 White LEDs, VIN: 2.7V to 4.5V, IQ = 5mA, ISD < 1µA,
MS Package
LTC3205
High Efficiency, Multidisplay LED Controller
Up to 4 (Main), 2 (Sub) and RGB, VIN: 2.8V to 4.5V,
IQ = 50µA, ISD < 1µA, 24-Lead QFN Package
LT3465/LT3465A
Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED
Boost Regulator with Integrated Schottky Diode
Up to 6 White LEDs, VIN: 2.7V to 16V, VOUT(MAX) = 34V,
IQ = 1.9mA, ISD < 1µA, ThinSOT Package
LT3466/LT3466-1
Dual Full Function, 2MHz Diodes White LED Step-Up Converter Up to 20 White LEDs, VIN: 2.7V to 24V, VOUT(MAX) = 39V,
with Built-In Schottkys
DFN, TSSOP-16 Packages
LT3486
Dual 1.3A White LED Converter with 1000:1 True Color PWM
Dimming
Drives Up to 16 100mA White LEDs. VIN: 2.5V to 24V,
VOUT(MAX) = 36V, DFN, TSSOP Packages
LT3491
White LED Driver in SC70 with Integrated Schottky
Drives Up to 6 20mA White LEDs, VIN: 2.5V to 12V,
VOUT(MAX) = 27V, 8-Lead SC70 Package
ThinSOT is a trademark of Linear Technology Corporation.
3497f
20 Linear Technology Corporation
LT 1206 • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
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