BD9470AEFV Datasheet, PDF (22/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
Therefore, in order to solve this problem in this IC, equipped with a mode of “FB current only source 0uA～+100uA”as a
countermeasure to reduce the LED current drop problem.
“FB Source mode”is described as below.
PWM
1,2,3
PWM
4,5,6
②
FB
①
ILED
④
⑥
⑦
③
⑤
VOUT
Figure 27. Timing chart of FB source mode
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.but, FB voltage is larger than feedback voltage, and VOUT setting
voltage also higher.
③when PWM4,5,6=High, LED current increases.
④although LED current is increased but the FB voltage has reached the feedback voltage and will not change at this
time.Therefore, there is no transition and VOUT, LED current will not drop.
⑥LED current and output voltage is operate normally
⑦When PWM1,2,3=Low, LED current reduces.But, FB is only has source ability , FB voltage is maintained continuely
（But, despite the decreasing of LED current, output voltage is increases because FB voltage is not changed.）
According to above operation, the LED undershoot problem cab be prevented by FB source mode.
However, the above description is a simplified explanation for behavior, because the actual behavior of a waveform is
different from the above, please check on the actual system.
When FB source mode is used, care must be taken to the following contents.
Because it can be held at a higher voltage than normal FB voltage, output voltage may be higher. Therefore, please note
that the heat might be higher than PWM = 100% while scanning operation is performed.
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TSZ22111・15・001
22/35
TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003