LM3492MH/NOPB National Semiconductor, LM3492MH/NOPB Datasheet - Page 10

• Manufacturer Part Number: LM3492MH/NOPB
• Description: IC LED DVR DIM 2CH 20-TSSOP
• Manufacturer: National Semiconductor

Specifications of LM3492MH/NOPB

• Internal Driver: Yes
• Type - Primary: Automotive, Backlight
• Type - Secondary: High Brightness LED (HBLED)
• Mounting Type: Surface Mount
• Topology: Step-Up (Boost)
• Number Of Outputs: 2
• Frequency: 200kHz ~ 1MHz
• Voltage - Supply: 4.5 V ~ 65 V
• Package / Case: 20-TSSOP (0.173", 4.40mm Width) Exposed Pad
• Operating Temperature: -40°C ~ 125°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -

www.national.com
the above functions with nearly no effect on the operation of
the boost converter and the current regulator.
Current Limit
The current limit I
provides a cycle-by-cycle current limit for protection. It can be
decreased by injecting a small signal current I
pin, and the relationship between I
where I
is 3.9A. As shown in Figure 3, I
necting a resistor R
typical voltage on the ILIM pin is 0.7V. To obtain the maximum
current limit, connect the ILIM pin to the ground.
Thermal Protection
Thermal protection is implemented by an internal thermal
shutdown circuit, which activates at 165°C (typically) to dis-
able the LM3492. In this case, the LDO regulator is turned off
and the CDHC pin is internally grounded. Thermal protection
helps prevent catastrophic failures from accidental device
overheating. When the junction temperature of the LM3492
falls back below 145°C (typical hysteresis = 20°C), the
LM3492 resumes normal operation.
Dynamic Headroom Control and
Soft-start
Dynamic headroom control (DHC) is implemented in the
LM3492 to adjust the output voltage V
verter in order to reduce the power loss of the current regu-
lator to maximize efficiency. Let V
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
ing gradually (in terms of minutes) owing to the rise of LED
die temperature during operation, DHC adjust V
justing a threshold which is reflected on the voltage of the FB
pin with reference to V
V
sponse time on adjusting V
small, V
During startup, the voltage of the CDHC pin is rised from 0V
to 2.25V at a speed depends on C
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
An internal switch grounds the CDHC pin if any of the follow-
ing cases happens: (i) V
OUT
CDHC
and V
.
CL(MAX)
OUT
FIGURE 3. Programmable Current Limit
is more sensitive to the variation of V
LED,n
I
is the maximum current limit. Its typical value
CL
. The sensitivity of DHC, which is the re-
CL
= I
ILIM
of the integrated MOSFET of the LM3492
CL(MAX)
IOUT,n
from the VCC pin to the ILIM pin. The
CC
is below the VCC UVLO threshold;
, which is the difference between
OUT
– 4290 I
, is set by C
ILIM
CDHC
CL
LED,n
LED,n
can be provided by con-
ILIM
and I
30116105
OUT
. This makes the volt-
is normally decreas-
and V
ILIM
of the boost con-
CDHC
ILIM
is
IOUT,n
. If C
into the ILIM
LED,n
OUT
CDHC
be the
.
by ad-
(2)
is
10
(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 LM3492 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 1000:1. The finest pulse width of the dimming
signal for the DIM1 and DIM2 pins is 300 ns, which implies
that a contrast ratio of 1000:1 can be achieved at a dimming
frequency of more than 3 kHz.
The current of an LED string (I
mA to 200 mA by an external resistor R
the IREF pin to the ground. The relationship between I
R
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 400 mA. In this case, the DIM1 and DIM2 pins
should also be connected together.
IREF
is shown in Figure 4. The two channels of the current
FIGURE 5. Over-power Protection
FIGURE 4. I
LED
LED
and R
) is programmable from 50
IREF
IREF
connecting from
30116126
30116127
LED
and