BD9420F Datasheet, PDF (21/31 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter Type)

BD9420F
Datasheet
●The setting of REG75 capacity and shutdown procedure
VOUT discharge function is built-in this IC when IC is shutdowned, the below decribes the operation sequence.
Figure 14.Timing chart of shutdown
○Explanation of shutdown sequence
①Set STB pin to “OFF” will stops DC/DC converter and REG75, but LED driver will remain operation.
②Discharge the REG75 pin voltage from 7.5V to 4.0V with 1MΩ.
③The VOUT voltage will be discharged with ILED current and the discharged VOUT voltage is no flow ILED current.
④When REG75 pin voltage will reach 4.0V (Typ.) or less to shut down all systems
○REG75 capacitance setting procedure
The shutdown time “T_OFF” can be calaulated by the following equation.
T [sec]  C [F]  R []  In ^{REG75t0[V]}  C [F] 1[M]  In ^7.5[V]  628.6 10³  C [sec]
OFF
REG
REG
REG75 [V] ^REG
4.0[V]
REG
UVLO
The longest VOUT discharge time will be obtained when the PWM duty cycle is set to the minimum VOUT.
Make REG capacitance setting with an adequate margin so that systems will be shut off after VOUT voltage is fully
discharged.
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31.Jan.2014 Rev.002