LG GX300 Service Manual page 53

3. TECHNICAL BRIEF
LED Backlight Current Sinks
The backlight current is set via the SemWire interface. The current is regulated to one of 32 values
between 0.5mA and 25mA. The step size varies depending upon the current setting. Between 0.5mA
and 12mA, the step size is 0.5mA. The step size increases to 1mA for settings between 12mA and 15mA
and 2mA for settings greater than 15mA. This feature allows fi ner adjustment for dimming functions in
the low current setting range and coarse adjustment at higher current settings where small current
changes are not visibly noticeable in LED brightness.
All backlight current sinks have matched currents, even when there is variation in the forward voltages
( VF ) of the LEDs. A VF of 1.2V is supported when the input voltage is at 3.0V. Higher VF LED mis-
match is supported when VIN is higher than 3.0V. All current sink outputs are compared and the lowest
output is used for setting the voltage regulation at the VOUT pin. This is done to ensure that suffi cient
bias exists for all LEDs.
The backlight LEDs default to the off state upon powerup. For backlight applications using less than
four LEDs, any unused output must be left open and the unused LED driver must remain disabled.
When writing to the Backlight Enable Control register, a zero
bit of any unused output.
Backlight Quiescent Current
The quiescent current required to operate all four backlights is reduced by 1.5mA when backlight
current is set to 4.0mA or less. This feature results in higher efficiency under light-load conditions.
Further reduction in quiescent current will result from using fewer than four LEDs.
LGE Internal Use Only
(0) must be written to the corresponding
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3. TECHNICAL BRIEF
Only for training and service purposes