LM3432BSQ National Semiconductor, LM3432BSQ Datasheet - Page 12

• Manufacturer Part Number: LM3432BSQ
• Description: LM3432/LM3432B 6-Channel Current Regulator for LED Backlight Application; Package: LLP; No of Pins: 24; Qty per Container: 1000; Container: Reel
• Manufacturer: National Semiconductor

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OVER-TEMPERATURE MONITOR
If the LM3432/LM3432B junction temperature exceeds ap-
proximately 125°C, the OTMb pin will be pulled to ground but
the part will continue to function. Action must be taken to lower
the temperature at this point. The PWM duty factor may be
lowered as an example to reduce the amount of heat gener-
ated, which will in turn lower the die temperature. If the
junction temperature is allowed to rise beyond approximately
165°C, the part will shut down. When the device is cooled
down to about 145°C, device operation will resume.
Note that this thermal shutdown protection is only intended as
a fault mode protection feature. Device operation above rated
maximum operating junction temperature is neither recom-
mended nor guaranteed.
FIGURE 3. Fault Detect Functional Block Diagram
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DYNAMIC HEADROOM CONTROL
To use the DHC function, connect a gain setting resistor from
the VDHC pin of the LM3432/LM3432B to the VDHC pin of
the LM3430 (National Semiconductor’s Boost Controller for
LED Backlighting) that is supplying power to the LED rails.
The LM3432/LM3432B’s DHC function will regulate the volt-
age between the IOUT pins and ground to a minimum of
0.625V typical (IOUT = 20 mA) to optimize overall efficiency.
A large DHC time constant needs to be set in order not to
interfere with the loop response of the DC-DC converter. This
can be implemented by connecting a capacitor from the
LM3432/LM3432B’s CDHC pin to ground. If the DHC function
is not needed, leave the VDHC pin floating. When operated
in this manner, if the lowest voltage between any of the IOUT
pins and ground is greater than 2V, the LED short fault de-
tection function will be disabled. For the LM3432B, if any open
condition occurs on any channels, the DHC function will be
inactive and the LED supply rail voltage from the LM3430 will
stay at its preset level. If the excess headroom voltage is
greater than the short fault detect threshold, 7.3V(min), the
device may latch off the LED string(s) due to short fault pro-
tection. In order to insure the proper operation of good LED
strings, designers must design for sufficient excess head-
room voltage below the short fault detect threshold.
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