LM3492HC Datasheet, PDF (10/17 Pages) Texas Instruments – Two-Channel Individual Dimmable LED Driver with Boost Converter and Fast Current Regulator

the above functions with nearly no effect on the operation of
the boost converter and the current regulator.
Current Limit
The current limit I_CL of the integrated MOSFET of the
LM3492HC provides a cycle-by-cycle current limit for protec-
tion. It can be decreased by injecting a small signal current
I_ILIM into the ILIM pin, and the relationship between I_CL and
I_ILIM is
I_CL = I_CL(MAX) – 4290 I_ILIM
(2)
where I_CL(MAX) is the maximum current limit. Its typical value
is 3.9A. As shown in Figure 3, I_ILIM can be provided by con-
necting a resistor R_ILIM from the VCC pin to the ILIM pin. The
typical voltage on the ILIM pin is 0.7V. To obtain the maximum
current limit, connect the ILIM pin to the ground.
30170505
FIGURE 3. Programmable Current Limit
Thermal Protection
Thermal protection is implemented by an internal thermal
shutdown circuit, which activates at 165°C (typically) to dis-
able the LM3492HC. In this case, the LDO regulator is turned
off and the CDHC pin is internally grounded. Thermal protec-
tion helps prevent catastrophic failures from accidental device
overheating. When the junction temperature of the
LM3492HC falls back below 145°C (typical hysteresis = 20°
C), the LM3492HC resumes normal operation.
Dynamic Headroom Control, Over-
ridding, and Soft-start
Dynamic headroom control (DHC) is implemented in the
LM3492HC to adjust the output voltage V_OUT of the boost
converter in order to reduce the power loss of the current reg-
ulator to maximize efficiency. Let V_LED,n and V_IOUT,n be the
forward voltage of an LED string connecting to the IOUTn pin
and the voltage of the IOUTn pin, where n is 1, 2 for channels
1, 2 of the current regulator. Since V_LED,n is normally decreas-
ing gradually (in terms of minutes) owing to the rise of LED
die temperature during operation, DHC adjust V_OUT by ad-
justing a threshold which is reflected on the voltage of the FB
pin with reference to V_IOUT,n, which is the difference between
V_OUT and V_LED,n. The sensitivity of DHC, which is the re-
sponse time on adjusting V_OUT, is set by C_CDHC. If C_CDHC is
small, V_OUT is more sensitive to the variation of V_LED,n.
DHC over-ridding can be implemented by internal pull-up or
external pull-up (by connecting the CDHC and VCC pins with
a resistor, e.g. 10 MΩ). In this case, the voltage of the CDHC
pin will rise over 2.5V, and the voltage of the FB pin will rise
until over-voltage protection is hit. Since the pull-up is weak,
DHC over-ridding will occur only at low contrast ratio (e.g.
<1%).
During startup, the voltage of the CDHC pin is rised from 0V
to 2.25V at a speed depends on C_CDHC. This makes the volt-
age of the FB pin as well as the output voltage ramps up in a
controlled manner, and effectively a soft-start function is im-
plemented. The soft-start can be programmable by the
C_CDHC.
An internal switch grounds the CDHC pin if any of the follow-
ing cases happens: (i) V_CC is below the VCC UVLO threshold;
(ii) a thermal shutdown occurs; or (iii) the EN pin is pulled low.
The CDHC pin cannot be connected to the ground externally.
Current Regulator
The LM3492HC integrates a two-channel current regulator for
controlling the current of two LED strings. The two LED strings
are dimmable individually by dimming signals applied to the
DIM1 and DIM2 pins for LED strings 1 and 2, which are con-
nected from the VOUT pin to the IOUT1 and IOUT2 pins. The
DIM1 and DIM2 pins are internally pulled low. The lowest
contrast ratio is 10000:1. The finest pulse width of the dim-
ming signal for the DIM1 and DIM2 pins is 300 ns.
The current of an LED string (I_LED) is programmable from 50
mA to 250 mA by an external resistor R_IREF connecting from
the IREF pin to the ground. The relationship between I_LED and
R_IREF is shown in Figure 4. The two channels of the current
regulator can work in parallel for only one LED string by con-
necting the IOUT1 and IOUT2 pins together to provide an LED
current of up to 500 mA. In this case, the DIM1 and DIM2 pins
should also be connected together.
250
200
150
100
50
0
5
10
15
20
RIREF (kΩ)
FIGURE 4. I_LED vs R_IREF
25
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