BD9285F Datasheet, PDF (16/39 Pages) Rohm – Boost 1channel white LED driver For large LCDs

BD9285F
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 BD9285.
①
STB
OSC
SS
SLOPE
FB
PWM
GATE
②
VOUT
③
ILED
④
LED_ OK
⑥
⑤
Figure 21. the turn-on waveform
SS
SLOPE
SS
Css
5V
QD
PWM
COMP
SS=FB
Circuit
OSC
DRIVER
GATE
LED_OK
FB
DIMOUT
ISENSE
VOUT
PWM
Figure 22. the turn-on circuit
○The explanation of start up sequency
①When STB is H, the internal bias voltage of TC54 rising.
②With the first PWM=H, BD9285 enables output the boost pulse, and the SS start to charge to the external capacitance.
At this moment, the voltage of FB will be the same as SS voltage internally regardless of the PWM logic.
③The FB=SS voltage reach the bottom voltage of saw-toothed wave and the DC/DC start to output the pulse signal.
Therefore the boost of VOUT is started.
④VOUT is boosted to fixed level, and the LED current is rising.
⑤When the LED current reached to fixed level, FB is removed from SS internally. The start up operation completed.
⑥IC start the normal operation by sensing the voltage of ISENSE pin. When SS is more than 3.0V, even if the LED
current does not flows, the clamped circuit of SS and FB is off, and the protect detection of SCP and OPEN starts.
○The setting method of SS external capacitance
As above desribed, DC/DC stops when the PWM1=L. It means the boost operation only enabled within PWM1=H
duration and SS time will be extented while boost with samll PWM duty. Also the SS time is affected by the output
capacitance, the LED current and application conditions.
Tss is defined as the time for the SS voltage to reach to the FB feedback voltage. Please set the Tss longer than
Trise_min, which is the start up time of the minimum PWM duty.
When the FB voltage during LED turns on is expressed VFB, the equality on Tss is the following.
T_ss
=
C_ss[F]×VFB[V ]
3[µA]
[Sec]
So please set the external capacitance to meet the Tss>>Trise_min.
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TSZ22111・15・001
16/36
TSZ02201-0F1F0C100030-1-2
11.Jul.2012 Rev.001