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BD9483F Datasheet, PDF (13/32 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type)
◁
BD9483F,FV
Datasheet
●3.4 External components selection
●3.4.1 The start up operation and the setting of Soft Start external capacitance
The below explanations are the start up sequency of BD9483F,FV.
①
STB
OSC
SS
FB
PWM
GATE ②
VOUT
SS=0.4V
ILED ③
LED_OK
④
⑤
⑥
SS
SLOPE
SS
Css
FB
5V
SS-FB OSC
Circuit
COMP
DRIVER
0.7V
LED_OK
N
DIMOUT
ISENSE
PWM
VOUT
Figure 19.
○The explanation of start up sequency
①The internal bias voltage of REG90 turns on by VCCUVLO. And as STB is H, the reset signal is released.
②With the first PWM=H, BD9483F,FV enables to output the boost pulse, and the SS start to charge to the external
capacitance. At this moment, the voltage of FB will be clamped to SS+0.7V voltage regardless of the PWM logic.
③The boost of VOUT (GATE pulse) is started as SS=0.4V(typ), because the internal ramp reaches the bottom voltage of
saw-toothed wave and the DC/DC start to output the pulse signal.
④VOUT is boosted to a certain level, and the LED current is rising.
⑤When the LED current reached to a certain level, FB is removed from SS+0.7V internally. And the start up operation
completed. By this SS-FB clamped circuit, turning on can be completed quickly in spite of small PWM duty.
⑥IC start the normal operation by sensing the voltage of ISENSE pin. FBMAX detection starts monitoring.
○The setting method of SS external capacitance
As above described, SS continues to be charged in spite of PWM logic or VOUT level, and FB level is clamped by
SS+0.7V.
TFB is defined as the time for the SS voltage to reach to the FB feedback voltage.
When the FB voltage during LED turns on is expressed VFB, the equality on TFB is the following.
TFB

Css[F] VFB[V]
3[A]
[Sec]
●3.4.2 Shutdown method and the setting of REG90 capacitance
When this IC shuts down, VOUT discharge function works. Indicate the sequence.
Figure 20.
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Figure 21.
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TSZ02201-0F1F0C100100-1-2
28.Nov.2013 Rev.003
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