LG -A230 Service Manual page 47

3. TECHNICAL BRIEF
LED Backlight Current
RT8966AGQW communicates with a host (master) Using the standard I2C 2-wire interface.
The two bus lines of SCL and SDA must be pulled high when the bus is not in use. Internal pull-up resistors are
installed. After the START condition, the I2C master sends a chip address. This address is eight bits long,
consisting of seven address bits and a following data direction bit (R/W). The RT8966AGQW address is
10101000 (A8h) and is a receive-only (slave) device. The second word selects the register to which the data
will be written. The third word contains data to write to the selected register. Figure 2 shows the writing
information for the four LDOs as well as for each LED current.
In the second word, the sub-address of the four LDOs is "001" and the sub-address of the LED Driver for
different dimming modes are respectively "010", "011" and "100". For the LDO output voltage setting, bits B1
to B4 represent each LDO channel respectively where a "1" indicates selected and a "0" means not selected.
The B0 bit controls on/off (1/0) mode for the selected LDO channel(s).
Then, in the third word, bite C0 to C3 control a 16-step setting of LDO1 to LDO4.The voltage values are listed
in Table 1. For LED dimming, there are three operating modes (Backlight I, Backlight II and Backlight III) to
select from by writing respectively "010", "011" and "100" into the First three bits of the second word, It
should be noticed that no matter which mode is selected, LED1 to LED3 must be turned on, else LED4 to
LED6 can not be Turned on.
When Backlight I is selected, all six LEDs have the same behavior. Their 64-step dimming currents are set by
bits C0 to C5, which are listed in Table 2. The bits C6 and C7 determine the fade in/out time of each step as
shown in Figure 2. For Backlight II and Backlight III, two sets of LEDs, called Main and Sub, can work separately.
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.
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