BD9470AEFV Datasheet, PDF (17/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
●The necessity for holding output voltage and FB voltage while PWM=Low
In conventional control method, DCDC will be stopped and FB voltage become high impendence while PWM=Low.
However, if PWM=0% is continued to inputted to system, output voltage and FB voltage is reduced because of discharge
phenomenon.eventually output voltage is equal to VIN, and FB voltage drop to 0V.There are several problems such as the
following listed if PWM dimming signal is tried to light-up a system.
①Slow start cannot be controlled resulting in the FB voltage overshoot and rush current flow to Inductor.
②Flash phenomenon occur due to start-up control does not work.
③Because there is a need to re-boost, take a long time to light up.
In this IC, the problems as above mentioned is resolved by coping output voltage and FB voltage to IC internally at a time
of PWM from High to Low.
The below describes FB and VOUT voltage holding function in detail.
○Explanation of FB voltage holding function while PWM=Low
PWM signal
FB
100pF～2200pF
H
L
-
-
GMAMP _-
L
LED1
H
LED2
L
H
H
+
L
FB IN
FB COPY
Holding FB Vol.
BLOCK
Figure 20. Block diagram of KEEP_FB
FB holding function means FB voltage will be copy to IC internally at a time of PWM from High to Low, FB voltage will be
maintained even in the period of PWM=Low.
Because FB voltage resolution is split by 40 from 4V, so the voltage can be copied to IC internally in 0.1V Step.
In addition, FB pin voltage will be influenced by DCDC operation, the copied have ±0.1V difference problem. But because
FB voltage is returned as feedback voltage immediately and will not cause an operational problem while PWM=H, it is
recommended to add about 100pF～2200pF to FB pin for noise reduction.
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TSZ22111・15・001
17/35
TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003