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LM3492HC Datasheet, PDF (13/17 Pages) Texas Instruments – Two-Channel Individual Dimmable LED Driver with Boost Converter and Fast Current Regulator
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ready presented, no command can be written to the
LM3492HC.
CHANNEL 1 DISABLE
After power good, channel 1 of the current regulator can be
disabled by writing a command (refer to table 2) to the COMM
pin. If LED string 1 is malfunctioning, channel 1 can be dis-
abled and the signal applied on the DIM1 pin can serve as
only a clock signal for the data flow of the COMM pin. Channel
1 is by default enabled after reset. If the over-temperature in-
dication or any COMM bit pattern has already presented, no
command can be written to the LM3492HC.
TABLE 2. Commands
Command
Command Bit Pattern
fSW resume normal
fSW tune down by 20%
fSW tune down by 40%
Channel 1 disable
1111011101110111
1111000100010001
1111001100110011
1111010101010101
Application Information
EXTERNAL COMPONENTS
The following procedures are to design an LED driver using
the LM3492HC with an input voltage ranged from 10V to 24V
and two LED strings consists of 10 LEDs each with a forward
voltage of 3V for each LED when running at 250 mA. The
output power is 15W. The switching frequency fSW is designed
to be 300 kHz.
RFB1, RFB2, and CFB: The nominal voltage of the LED string
with 10 LEDs is 30V, and the minimum voltage of the IOUTn
pin (n = 1, 2) is 1.25V when ILED is 250 mA. As a result, VOUT
(NOM) is 31.25V. Design VOUT(MAX) to be 50V. From (5), VFB
(NOM) is about 1.8V, which falls in the recommended operation
range from 1.05V to 2V. Also, design RFB2 to be 16.2 kΩ. From
(3), RFB1 is calculated to be 265.1 kΩ, and a standard resistor
value of 261 kΩ is selected. CFB is selected to be 10 pF as
recommended.
L1: 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. For a boost converter,
IL1 equals to the input current IIN. Hence,
IIN = (VOUT(NOM) x 2ILED ) / VIN
(6)
Also,
ton = (1 – VIN/VOUT) / fSW
(7)
L1 = (VIN x ton) / 2IIN
(8)
If VIN is maximum, which is 24V in this example, and only one
LED string is turned on (since the two channels of the
LM3492HC are individually dimmable), IIN is minimum. From
(6)-(8), it can be calculated that IIN(MIN), ton, and L1 are 0.326A,
0.77 µs, and 28.5µH. On the other hand, from (6), IIN is max-
imum when VIN is minimum, which is 10V in this example, and
the two LED strings are turned on together. Hence IIN(MAX) is
1.56A. Then, ILR is
ILR = (VIN x ton) / L1
(9)
From (7), ton is 2.27 µs. From (9), ILR is 0.80A. The steady
state peak inductor current IL1(PEAK) is
IL1(PEAK) = IL1 + ILR / 2
(10)
As a result, IL1(PEAK) is 1.96A. A standard value of 27 µH is
selected for L1, and its saturation current is larger than 1.96A.
D1: The selection of the boost diode D1 depends on two fac-
tors. The first factor is the reverse voltage, which equals to
VOUT for 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 (10). In this example, a 100V 3A
schottky diode is selected.
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. Ow-
ing 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 ceramic capacitor are used
for CIN, and two 2.2 µF 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 trig-
gering 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 not only affects the
sensitivity of the DHC but also determines the soft-start time
tSS, i.e. the time for the output voltage to rise until power good.
tSS is determined from the following equation:
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 Figure 1, if fSW is 300 kHz, RRT is se-
lected to be 499 kΩ. From Figure 4, if ILED is 250 mA, RIREF
is selected to be 4.99 kΩ.
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 act as a pull-up function.
PC Board Layout
The layout of the printed circuit board is critical in order to
optimize the performance of the LM3492HC application cir-
cuit. In general, external components should be placed as
close to the LM3492HC and each other as possible in order
to make copper traces short and direct. In particular, compo-
nents of the boost converter CIN, L1, D1, COUT, and the
LM3492HC should be closed. Also, the output feedback ca-
pacitor CFB should be closed to the output capacitor COUT.
The ground plane connecting the GND, PGND, and LGND
pins and the exposed pad of the LM3492HC 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
LM3492HC. The ground plane should be used to connect the
exposed pad of the LM3492HC, which is internally connected
to the LM3492HC die substrate. The area of the ground plane
should be extended as much as possible on the same copper
layer around the LM3492HC. Using numerous vias beneath
the exposed pad to dissipate heat of the LM3492HC to an-
other copper layer is also a good practice.
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