LM3492HC Datasheet, PDF (9/17 Pages) Texas Instruments – Two-Channel Individual Dimmable LED Driver with Boost Converter and Fast Current Regulator

Overview
The LM3492HC integrates a boost converter and a two-chan-
nel 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 provides power for the LED
strings, and the current regulator controls the dimming of the
LED strings individually. The LM3492HC integrates an N-
channel MOSFET switch and a two-channel current regulator
in order to minimize the component count and solution size.
The boost converter of the LM3492HC employs a Projected
On-Time (POT) control method to determine the on-time of
the MOSFET with respect to the input and output voltages
and an external resistor R_RT. During the on-period, the boost
inductor is charged up, and the output capacitor is discharged
to provide power to the output. A cycle-by-cycle current limit
(which is 3.9A typically and programmable by an external re-
sistor) is imposed on the MOSFET for protection. After the on-
period, the MOSFET is turned off such that the boost inductor
is discharged. The next on-period is started when the voltage
of the FB pin is dropped below a threshold which is deter-
mined by Dynamic Headroom Control (DHC) and is ranged
from 1.05V to 2.0V (DHC affects the threshold only when the
DIM1 and/or DIM2 pins are high). The boost converter under
POT control can maintain the switching frequency nearly con-
stant so that the switching frequency depends on only R_RT
Figure 1. Also, POT control requires no compensation circuit
and gives a fast transient response of the output voltage.
(MCUs). The LM3492HC will not latch off and continue to op-
erate in the presence of the indications. Other useful features
include thermal shutdown, VCC under-voltage lock-out, and
precision enable. The LM3492HC is available in the thermally
enhanced eTSSOP-20 package.
LDO Regulator
A 5.5V LDO regulator is integrated in the LM3492HC. For
stability, an external capacitor C_VCC of more than 0.47 µF
should be connected between the VCC and GND pins. The
current limit of the LDO is typically 30 mA. It can be used to
pull-up the open-drain COMM pin with an external resistor,
and inject current to the ILIM pin to adjust the current limit of
the integrated MOSFET. When the voltage on the VCC pin
(V_CC) is higher than the under-voltage lock-out (UVLO)
threshold of 3.78V, the LM3492HC is enabled and the CDHC
pin sources a current to charge up an external capacitor
C_CDHC to provide a soft-start function.
Enable and Disable
To enable the LM3492HC, the voltage on the EN pin (V_EN)
must be higher than an enable threshold of typically 1.63V. If
V_EN is lower than 1.43V, the LM3492HC is shutdown. In this
case, the LDO regulator is turned off and the CDHC pin is
internally grounded. The EN pin is internally pulled up. After
enable, the EN pin is pulled up by a 40 µA current source. If
the EN pin is connected to low such that the LM3492HC is
shutdown, the pull-up current is reduced to 2 µA. These take
the advantages that the LM3492HC can effectively avoid
false disabling by noise during operation, and minimize power
consumption during shutdown. The enable threshold is pre-
cise such that it can be used to implement an UVLO function
for the input voltage as shown in Figure 2. The input voltage
can be connected to the EN pin through a resistor divider
consists of R_EN1 and R_EN2. This can ensure that the
LM3492HC is operated after the input voltage reaches a min-
imum require value V_IN(EN), which can be calculated by the
following equation:
V_IN(EN) = 1.63V(1 + R_EN1/ R_EN2)
(1)
A zener diode D_EN should be placed between the EN and
GND pins to keep V_EN below its absolute maximum caused
by the increase of the input voltage.
30170525
FIGURE 1. Switching Frequency
The two-channel current regulator of the LM3492HC is fast
response so that it can allow very high contrast ratio of
10000:1. The two channels are dimmable individually. Chan-
nel 1 of the current regulator can be disabled by a digital
command send through the COMM pin. In this case, the DIM1
pin can serve only as a clock signal for the data flow of the
COMM pin. The power dissipated by the current regulator is
adaptively minimized by Dynamic Headroom Control in order
to maximize efficiency.
The LM3492HC can be applied in numerous applications like
automotive LCD backlight panels. It can operate efficiently for
inputs as high as 65V. Diagnostic functions including power
good indication, over-temperature indication, IOUT over- and
under-voltage indications facilitate the interface of the
LM3492HC application circuit with external micro-processors
30170504
FIGURE 2. Input Voltage UVLO Implemented by Precision
Enable
Once the EN pin is pulled low, the LM3492HC will perform the
following functions: reset IOUT over- and under-voltage indi-
cations and the corresponding COMM bit pattern, resume the
switching frequency tuning to the normal frequency, and re-
sume channel 1 of the current regulator if it is disabled. Pulling
the EN pin low for a short period of about 200 ns can achieve
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