GRM188R71C105KA12D [NSC]
Evaluation Board Reference Design integrates a boost converter; 评估板参考设计集成了升压转换器
| 型号： | GRM188R71C105KA12D |
| 厂家： | 
National Semiconductor |
| 描述： | Evaluation Board Reference Design integrates a boost converter |
| 文件： | 总8页 (文件大小：601K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
National Semiconductor
Application Note 2056
L.K. Wong
LM3492 Evaluation Board
Reference Design
September 21, 2010
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 tun-
ing, 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 eTSSOP-20 package.
Introduction
The LM3492 integrates a boost converter and a two-channel
current regulator to implement a high efficient and cost effec-
tive 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 Pro-
jected-On-Time control method to give a fast transient re-
sponse 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.
This application note details the design of a LM3492 evalua-
tion board which 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 shown. Typical performance
and operating waveforms are also provided for reference.
To maximize the efficiency, Dynamic Headroom Control
(DHC) automatically adjusts the output voltage to a minimum.
Evaluation Board Schematic and PCB Layout
30123301
FIGURE 1. LM3492 Evaluation Board Schematic
© 2010 National Semiconductor Corporation
301233
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30123302
FIGURE 2. LM3492 Evaluation Board Top Overlay
30123303
FIGURE 3. LM3492 Evaluation Board Top View
30123304
FIGURE 4. LM3492 Evaluation Board Bottom View
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2
Evaluation Board Quick Setup Procedures
Description
Step
Notes
1
2
Connect a power supply to VIN and PGND terminals
VIN range: 9V to 16V
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
3
The EN terminal should be left open for normal operation.
Ground this terminal to shutdown
4
5
Connect DIM1 and DIM2 terminals to a voltage > 2V, apply VIN = 12V Nominal LED current is 150 mA per channel
Ground the EN terminal to check the shutdown function
Evaluation Board Performance Characteristic
Description
Input Voltage
Rail Voltage
LED Current
Symbol
VIN
Condition
Min
Typ
12
Max
Unit
V
9
16
VOUT
ILED
39
V
150
mA
%
LED Current
Regulationt
ALL VIN
conditions
VIN = 9V
-3
+3
ΔILED
Efficiency
85.7
88.2
89.1
%
%
%
VIN = 12V
VIN = 16V
L1 = (VIN(MAX) x ton) / 2IIN(MIN)
(4)
Design Procedure
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 (3), ton is 1.54 µs
when VIN is 9V. Then ILR is
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.
ILR = (VIN x ton) / L1
(5)
Design Parameters:
VIN = 9V to 16V, typical 12V
ILED = 150 mA
From (5), ILR is 0.53A. The steady state peak inductor current
IL1(PEAK) is
IL1(PEAK) = IL1 + ILR / 2
(6)
Step 1: Calculate the output voltage feedback circuit
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.
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
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 (6). In this example, a 100V 3A schottky diode is
selected.
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.
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 volt-
age ripples. Experimentation is usually necessary to deter-
mine their value. The rated DC voltage of capacitors used
should be higher than the maximum DC voltage applied.
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 cur-
rent IL1 should be larger than half of ILR
.
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 ca-
pacitors 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 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) x ILED) / VIN(MAX)
(2)
for COUT
.
Also
CVCC: The capacitor on the VCC pin provides noise filtering
and stabilizes the LDO regulator. It also prevents false trig-
ton = (1 – VIN/VOUT) / fSW
To ensure a CCM operation,
(3)
3
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gering of the VCC UVLO. CVCC is recommended to be a 1 µF
good quality and low ESR ceramic capacitor.
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 con-
necting 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.
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:
(7)
In this example, CCDHC is recommended to be a 0.47 µF good
quality and low ESR ceramic capacitor.
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 cop-
per layer is also a good practice.
RRT and RIREF: The resistors RRT and RIREF set the switching
frequency fSW of the boost converter and the LED current
ILED respectively. From the LM3492 datasheet, RRT is select-
ed to be 274 kΩ if fSW is 500 kHz (Figure 1 of the datasheet),
and RIREF is selected to be 8.25 kΩ if ILED is 150 mA (Figure
4 of the datasheet).
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.
PC Board Layout
The layout of the printed circuit board is critical to optimize the
performance of the LM3492 application circuit. In general,
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Bill of Materials
Item Part Number
Mfg name
muRata
muRata
muRata
muRata
muRata
Vishay
Part Description
Qty Ref Designator(s)
Size
1206
1
2
GRM31CR61E106KA12L
GRM188R71C474KA88D
GRM1885C2A100RA01D
GRM188R71C105KA12D
GRM32ER72A225KA35L
CRCW060352K3FKEA
CRCW0603274KFKEA
CRCW0603402KFKEA
CRCW060316K2FKEA
CRCW06038K25FKEA
CRCW06030000Z0EA
CDRH10D68/ANP-270MC
SK310A-TP
Cap 10 µF 25V X5R
0603/X7R/0.47 µF/16V
0603/COG/10 pF/100V
0603/X7R/1 µF/16V
Cap 2.2uF 100V X7R
2
1
CIN1, CIN2
CCDHC
CFB1
0603
3
1
0603
4
1
CVCC
CO1, CO2
RCOMM
RRT
0603
5
2
1210
6
1
0603
Resistor Chip 52.3 kΩ 1%
Resistor Chip 274 kΩ 1%
Resistor Chip 402 kΩ 1%
Resistor Chip 16.2 kΩ 1%
Resistor Chip 8.25 kΩ 1%
Resistor Chip 0Ω 1%
7
Vishay
1
0603
8
Vishay
1
RFB1
0603
9
Vishay
1
RFB2
0603
10
11
12
13
14
Vishay
1
RIREF
RILIM0
L1
0603
Vishay
1
0603
Sumida
Inductor 27 µH 1.9A
1
10×10×6.8
SMA
Micro Commercial Schottky 100V 3A
1
D1
1502-2k-ND
KEYSTONE
Terminal DBL Turret
0.109”L Brass
11
VIN, GND, PGND,
VLED1, VLED2, IOUT1,
IOUT2, DIM1, DIM2,
COMM, EN
15
16
LM3492EVAL
LM3492MH
NSC
NSC
LM3492 demo board
IC LM3492
1
1
PCB
U1
eTSSOP-20
5
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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)
30123306
30123308
30123310
30123307
Steady State Operation
(VIN = 12V, ILED = 150 mA)
LED 50% Dimming
(VIN = 12V, ILED = 150 mA)
30123309
Power Up
(VIN = 12V, ILED = 150 mA)
Enable Transient
(VIN = 12V, ILED = 150 mA)
30123311
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6
7
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