AN-2056 LM3492 Evaluation Board Reference Design User's Guide 1 Introduction

AN-2056 LM3492 Evaluation Board Reference Design User's Guide 1 Introduction
User's Guide
SNVA438B – September 2010 – Revised April 2013
AN-2056 LM3492 Evaluation Board Reference Design
1
Introduction
The LM3492 integrates a boost converter and a two-channel current regulator to implement a high
efficient and cost effective LED driver for driving two individually dimmable LED strings with a maximum
power of 15W and an output voltage of up to 65V. The boost converter employs a proprietary ProjectedOn-Time control method to give a fast transient response with no compensation required, and a nearly
constant switching frequency programmable from 200 kHz to 1 MHz. The application circuit is stable with
ceramic capacitors and produces no audible noise on dimming. The programmable peak current limit and
soft-start features reduce current surges at startup, and an integrated 190 mΩ, 3.9A N-Channel MOSFET
switch minimizes the solution size. The fast slew rate current regulator allows high frequency and narrow
pulse width dimming signals to achieve a very high contrast ratio of 1000:1 at a dimming frequency of
more than 3 kHz. The LED current is programmable from 50 mA to 200 mA by a single resistor.
To maximize the efficiency, Dynamic Headroom Control (DHC) automatically adjusts the output voltage to
a minimum. DHC also facilitates a single BOM for different number of LED in a string, which is required for
backlight panels of different size, thereby reducing overall development time and cost. The LM3492 comes
with a versatile COMM pin which serves as a bi-directional I/O pin interfacing with an external MCU for the
following functions: power-good, over-temperature, IOUT over- and under-voltage indications, switching
frequency tuning, and channel 1 disabling. Other supervisory functions of the LM3492 include precise
enable, VCC under-voltage lock-out, current regulator Over-Power protection, and thermal shutdown
protection. The LM3492 is available in the thermally enhanced HTSSOP-20 package.
This user's guide details the design of a LM3492 evaluation board that drives 2 LED strings, each of which
consists of 10 LEDs running at 150 mA and the forward voltage of each LED is typically 3.8V. The input
voltage is from 9V to 16V. The evaluation board schematic, PCB layout, Bill of Materials, and circuit
design descriptions are included. Typical performance and operating waveforms are also provided for
reference.
Table 1. Evaluation Board Performance Characteristic
Description
Symbol
Input Voltage
VIN
Rail Voltage
VOUT
LED Current
ILED
LED Current
Regulationt
ΔILED
Condition
Min
Typ
Max
Unit
9
12
16
V
39
V
150
ALL VIN
conditions
Efficiency
-3
mA
+3
%
VIN = 9V
85.7
%
VIN = 12V
88.2
%
VIN = 16V
89.1
%
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SNVA438B – September 2010 – Revised April 2013
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1
Evaluation Board Schematic
2
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Evaluation Board Schematic
Figure 1. LM3492 Evaluation Board Schematic
3
Quick Setup Procedure
Step 1: Connect a power supply to VIN and PGND terminals. VIN range: 9V to 16V.
Step 2: Connect 2 LED strings: from VLED1 to IOUT1 terminals, and VLED2 to IOUT2 terminals. Each
LED string consists of 10 LEDs with a forward voltage of 3.8V per LED at 150 mA.
Step 3: The EN terminal should be left open for normal operation. Ground this terminal to shutdown.
Step 4: Connect DIM1 and DIM2 terminals to a voltage >2V, apply VIN = 12V. Nominal LED current is 150
mA per channel.
Step 5: Ground the EN terminal to check the shutdown function.
4
Design Procedure
The following procedures detail the design of the LM3492 evaluation board driving 2 LED strings consists
of 10 LEDs per string. The forward voltage of each LED is 3.8V, and the LED current is 150 mA. The input
voltage is ranged from 9V to 16V. The switching frequency fSW is designed to be 500 kHz.
Design Parameters:
VIN = 9V to 16V, typical 12V
ILED = 150 mA
Step 1: Calculate the output voltage feedback circuit
The nominal voltage of the LED string with 10 LEDs is 38V, and the minimum voltage of the IOUTn pin
(n = 1, 2) is 0.75V for an ILED of 150 mA. Hence, VOUT(NOM) is 38.75V. Since the dynamic range of VFB under
DHC is from 1.05V to 2V, the nominal voltage on the FB pin VFB(NOM) is designed to be around 1.5V.
Hence, VOUT(MAX) is designed to be 65V. Since:
VOUT(MAX) = 2.5V (1 + RFB1/ RFB2)
(1)
By designing RFB2 to be 16.2 kΩ, RFB1 is calculated to be 405 kΩ, and a standard resistor value of 402 kΩ
is selected. CFB1 is selected to be 10 pF as recommended.
2
AN-2056 LM3492 Evaluation Board Reference Design
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Design Procedure
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Step 2: Determine the inductance
The main parameter affected by the inductor is the peak to peak inductor current ripple (ILR). To maintain a
continuous conduction mode (CCM) operation, the average inductor current IL1 should be larger than half
of ILR.
For a boost converter, IL1 equals to the input current IIN. The minimum IIN occurs when VIN is maximum,
which is 16V in this example, and only 1 LED string is turned on (the 2 LED strings are individually
dimmable). Hence,
IIN(MIN) = (VOUT(NOM) × ILED) / VIN(MAX)
(2)
ton = (1 – VIN/VOUT) / fSW
(3)
Also
To ensure a CCM operation,
L1 = (VIN(MAX) × ton) / 2IIN(MIN)
(4)
It can be calculated that IIN(MIN), ton, and L1 are 0.363A, 1.17 µs, and 25.8 µH. On the other hand, IIN is
maximum when VIN is minimum, which is 9V in this example, and 2 LED strings are turned on. Hence
IIN(MAX) is 1.29A. From Equation 3, ton is 1.54 µs when VIN is 9V. Then ILR is
ILR = (VIN × ton) / L1
(5)
From Equation 5, ILR is 0.53A. The steady state peak inductor current IL1(PEAK) is
IL1(PEAK) = IL1 + ILR / 2
(6)
As a result, IL1(PEAK) is 1.56A. A standard value of 27 µH is selected for L1, and the saturation current of L1
should be larger than 1.56A.
Step 3: Determine the diode
The selection of the boost diode D1 depends on two factors. The first factor is the reverse voltage, which
equals to VOUT in a boost converter. The second factor is the peak diode current at the steady state, which
equals to the peak inductor current as shown in Equation 6. In this example, a 100V 3A Schottky diode is
selected.
Step 4: Determine the value of other components
CIN and COUT: The function of the input capacitor CIN and the output capacitor COUT is to reduce the input
and output voltage ripples. Experimentation is usually necessary to determine their value. The rated DC
voltage of capacitors used should be higher than the maximum DC voltage applied.
Owing to the concern of product lifetime, ceramic capacitors are recommended. But ceramic capacitors
with high rated DC voltage and high capacitance are rare in general. Multiple capacitors connecting in
parallel can be used for CIN and COUT. In this example, two 10 µF 25V ceramic capacitor are used for CIN,
and two 2.2 µF 100V ceramic capacitor are used for COUT.
CVCC: The capacitor on the VCC pin provides noise filtering and stabilizes the LDO regulator. It also
prevents false triggering of the VCC UVLO. CVCC is recommended to be a 1 µF good quality and low ESR
ceramic capacitor.
CCDHC: The capacitor at the CDHC pin mainly determines the soft-start time tSS, i.e. the time for the output
voltage to reach its maximum. tSS is determined from the following equation:
tSS =
CCDHC x 2.25V
120 PA
(7)
In this example, CCDHC is recommended to be a 0.47 µF good quality and low ESR ceramic capacitor.
RRT and RIREF: The resistors RRT and RIREF set the switching frequency fSW of the boost converter and the
LED current ILED respectively. From LM3492/LM3492Q Two-Channel Individual Dimmable LED Driver with
Boost Converter and Fast Current Regulator (SNVS656), RRT is selected to be 274 kΩ if fSW is 500 kHz
(Figure 1 of the datasheet (SNVS656)), and RIREF is selected to be 8.25 kΩ if ILED is 150 mA (Figure 4 of
the datasheet (SNVS656)).
RCOMM: Since the COMM pin is open drain, a resistor RCOMM of 52.3 kΩ is used to connect the VCC and
COMM pins to implement a pull-up function.
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PC Board Layout
5
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PC Board Layout
The layout of the printed circuit board is critical to optimize the performance of the LM3492 application
circuit. In general, external components should be placed as close to the LM3492 and each other as
possible in order to make copper traces short and direct. In particular, components of the boost converter
CIN, L1, D1, COUT, and the LM3492 should be closed. Also, the output feedback capacitor CFB1 should be
closed to the output capacitor COUT. The ground plane connecting the GND, PGND, and LGND pins and
the exposed pad of the LM3492 and the ground connection of the CIN and COUT should be placed on the
same copper layer.
Good heat dissipation helps optimize the performance of the LM3492. The ground plane should be used
to connect the exposed pad of the LM3492, which is internally connected to the LM3492 die substrate.
The area of the ground plane should be extended as much as possible on the same copper layer around
the LM3492. Using numerous vias beneath the exposed pad to dissipate heat of the LM3492 to another
copper layer is also a good practice.
6
Bill of Materials
Item
4
Ref Designator(s)
Size
1
Part Number
GRM31CR61E106KA12L
Mfg name
muRata
Cap 10 µF 25V X5R
2
CIN1, CIN2
1206
2
GRM188R71C474KA88D
muRata
0603/X7R/0.47 µF/16V
1
CCDHC
0603
3
GRM1885C2A100RA01D
muRata
0603/COG/10 pF/100V
1
CFB1
0603
4
GRM188R71C105KA12D
muRata
0603/X7R/1 µF/16V
1
CVCC
0603
5
GRM32ER72A225KA35L
muRata
Cap 2.2uF 100V X7R
2
CO1, CO2
1210
6
CRCW060352K3FKEA
Vishay
Resistor Chip 52.3 kΩ
1%
1
RCOMM
0603
7
CRCW0603274KFKEA
Vishay
Resistor Chip 274 kΩ
1%
1
RRT
0603
8
CRCW0603402KFKEA
Vishay
Resistor Chip 402 kΩ
1%
1
RFB1
0603
9
CRCW060316K2FKEA
Vishay
Resistor Chip 16.2 kΩ
1%
1
RFB2
0603
10
CRCW06038K25FKEA
Vishay
Resistor Chip 8.25 kΩ
1%
1
RIREF
0603
11
CRCW06030000Z0EA
Vishay
Resistor Chip 0Ω 1%
1
RILIM0
0603
12
CDRH10D68/ANP-270MC
Sumida
Inductor 27 µH 1.9A
1
L1
10×10×6.8
13
SK310A-TP
Micro Commercial
Schottky 100V 3A
1
D1
SMA
14
1502-2k-ND
KEYSTONE
Terminal DBL Turret
0.109”L Brass
11
VIN, GND, PGND,
VLED1, VLED2,
IOUT1, IOUT2, DIM1,
DIM2, COMM, EN
15
LM3492EVAL
Texas Instruments
LM3492 demo board
1
PCB
16
LM3492
Texas Instruments
IC LM3492
1
U1
AN-2056 LM3492 Evaluation Board Reference Design
Part Description
Qty
HTSSOP20
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Typical Performance and Waveforms
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7
Typical Performance and Waveforms
All curves and waveforms taken at VIN = 12V with the evaluation board and TA = 25°C unless otherwise
specified.
Efficiency vs Input Voltage
(ILED = 150 mA)
ILED Regulation vs Input Voltage
(ILED = 150 mA)
Steady State Operation
(VIN = 12V, ILED = 150 mA)
LED 50% Dimming
(VIN = 12V, ILED = 150 mA)
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Typical Performance and Waveforms
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Power Up
(VIN = 12V, ILED = 150 mA)
6
AN-2056 LM3492 Evaluation Board Reference Design
Enable Transient
(VIN = 12V, ILED = 150 mA)
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PCB Layout
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8
PCB Layout
Figure 2. LM3492 Evaluation Board Top Overlay
Figure 3. LM3492 Evaluation Board Top View
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PCB Layout
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Figure 4. LM3492 Evaluation Board Bottom View
8
AN-2056 LM3492 Evaluation Board Reference Design
SNVA438B – September 2010 – Revised April 2013
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Copyright © 2010–2013, Texas Instruments Incorporated
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