BD9470AEFV Datasheet, PDF (21/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
●FB current Source mode・Sink/Source mode
The output of GMAMP is constant current control in normal operation ans output anout±100uA(typ.) in this IC.
But, when PWM scanning operation and local dimming is performed, total LED current and output voltage will different by
each timming and FB feedback voltage.The below describes the this operation.
PWM
1,2,3
PWM
4,5,6
②
FB
①
ILED
④
⑥
③
⑤
VOUT
CFB
FB
RFB
PWM1
PWM2
PWM3
PWM4
PWM5
PWM6
N
CS
RCS
PGND
OVP
LED1
LED2
LED3
LED4
LED5
LED6
Figure 26. Timing chart of FB sink/source mode
As above shown,short PWM1,2,3 ans PWM4,5,6, assumed that scanning operation is performed.
At this time, the sequence is described as below.
①When PWM4,5,6=High→Low, FB voltage, VOUT（OVP）voltage is copied
②Copied voltage is hold.
③When PWM1,2,3=High again, normal DCDC operation
④When PWM4,5,6=High again, LED current increase.
⑤Because LED current increase resulting in FB voltage change.it take a long transition time because FB source current is
100uA at this time, therefore FB voltage is not insufficient and output voltage and LED current will drop.
⑥FB voltage reaches the feedback voltage and LED current and output voltage will operate normally.
In other words, ILED current drops at the point ⑤, This may be due to the transition time of the behavior that FB current
sink first and then charge again.
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TSZ22111・15・001
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TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003