BD9470AEFV Datasheet, PDF (18/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
PWM
② ③④⑤
①
FB
FB COPY
Figure 21. Timing chart of KEEP_FB
①PWM=High, normal feedback operation by LED pin
②FB voltage is copied to IC at a time of PWM from High to Low. FB voltage will be copied by less than 1Bit.
For Example：when FB=2.16V, FB COPY voltage is 2.1V.
③GMAMP is works as Buffer with while PWM=Low, FB voltage is discharged to FB COPY voltage.
④FB COPY=FB voltage.
⑤FB COPY=FB voltage and maintain.
If PWM=0% and because follow the state⑤ continuously, FB voltage will not dropped by natural discharge.
※Notice
FB voltage holding function is performed at 0.1V STEP. If PWM signal is in low duty, FB voltage is not able to rise
sufficiently when FB series resistance is small causing to RFB×IFB(typ.100uA)＜0.1V(typ.), The output voltage may not be
boosted up to the set voltage.
Therefore, it is recommended to set RFB> 2kohm so that ΔV = RFB × IFB> 0.2V.
IFB(100uA typ)
FB
RFB ⊿V=RFB×IFB>0.2V
CFB
Figure 22. Voltage to FB resistor
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TSZ22111・15・001
18/35
TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003