BD9420F Datasheet, PDF (12/31 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter Type)

BD9420F
Datasheet
●Maximum DCDC output Voltage
In this IC, the voltage of BSx pin is depending on VREF pin voltage. The maximum voltage of VREF pin（VREF＝3.0V）,
the voltage of BSx pin will become 2.0V（2/3 of VREF voltage ）.
The maximum voltage of DCDC output will have be vary with 1.6V while Analog dimming is performed (2/3 of 3.0V -
0.6V).
●Soft Start Time setting
This IC have a built in soft start time setting, there is no need to change from the outside. Timer time can be set by
counting the clock frequency with RT pin. Starts counting when shutdown function terminal STB etc is released, and start
time are considered in the count (in soft-start). Therefore, LED OPEN protection ・SHORT protection are not detected
during this time.
The soft start time is set inside the IC, as the following equation.
(Please note that soft-start time set here is the mask of the running time and not the time until the output stabilizes of the
DCDC. Time to stabilize the output or load is greater than the boost DCDC dependent.)
Soft-start time "TSS", RT pin connection resistor "RRT" :
T_SS
[sec]

12480

R_RT []
1.5 10¹⁰
Start-up sequence
The following describes the start-up sequence of this IC.
①
STB
OSC
Internal SS
FB ^SLOPE
PWM
N②
VOUT
③
ILED
LED_OK
④
⑥
⑤
○Description of start-up sequence
①Set STB from Low to High
②System will be activated while PWM=H. SS counting start.
At this time, a circuit in which internal SS voltage for slow start becomes equal to FB pin voltage operates to
equalize the FB pin and internal SS voltages regardless of whether the PWM pin is set to Low or High level.
③Since the FB pin and internal SS reach the lower limit of the internal sawtooth wave of the IC, the DC/DC converter
operates to start VOUT voltage rising.
④The Vout voltage continues rising to reach a voltage at which LED current starts flowing.
⑤When the LED current reaches the amount of current, isolate the FB circuit from the SS circuit. With this, the startup
operation is completed.
⑥After that, normal operation is controlled by following the feedback voltage of LED pins.
If the SS pin voltage reaches 4V or higher, the LED protection function will be activated to forcedly end the SS and FB
equalizing circuit.
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TSZ02201-0F1F0C100300-1-2
31.Jan.2014 Rev.002