BD9420F Datasheet, PDF (11/31 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter Type)

BD9420F
Datasheet
●LED current setting (VREF pin, CLx pin)
Please decide VREF pin input voltage first. When Analog dimming is performed,
please be noted that VREF pin input voltage range is (0.6V ～ 3.0V), and decide the
input voltage in normal operation. Basically, if VREF pin voltage is high, it will cause
power dissipation of external PNP Tr become high, so it is preferred to set the VREF pin
voltage lower.
Later, VREF=0.9V will be set as basic. For example if you create 0.9V from REG75, it
is possible to use resistive divider by setting 88kohm and 12kohm.
The LED current detection is performed by CLx pin. CLx pin is controlled so that the
voltage of 1/3V(typ.) of VREF voltage. If VREF=0.9V, CLx=0.3V to control external PNP
Tr. Therefore, if 「R_CL」 is set as a resistance which between CLx pin and GND, and
VREF pin voltage is set as 「V_VREF」, LED current 「I_LED」can be calculated as below.
R_CL
[ohm] 
V_VREF [V ]
I _LED [ A] 3
For current setting, set at each channel. For this reason, in 1ch～3ch and 4ch～6ch, it
is possible to change current by setting 「R_CL」value.
●DCDC operation frequency setting (RT Pin)
The operation frequency of DCDC output is set by resistance which connected to RT pin.
○The relationship between operation and RT resistance (ideal)
R_RT

15000
f_SW [kHz]
[k]　
Here, fsw＝DCDC converter oscillation frequency[kHz]
Above is an ideal equation which do not putted with correction terms.
For accurate frequency setting, please confirm on the real system.
But the frequency setting range is 100kHz～800kHz.
【Setting Example】
If DCDC frequency is set to be 200kHz, RRT as below:
R_RT

15000 _ 15000 _{ 75}
f _sw[kHz] 200[kHz]
[k]
　
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TSZ02201-0F1F0C100300-1-2
31.Jan.2014 Rev.002