BD9420F Datasheet, PDF (15/31 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter Type)

BD9420F
Datasheet
●Timer latch function
This IC has a built-in timer latch counter to make setting of timer latch time by counting a clock frequency set with the RT
pin.
○Timer latch time
The timer latch counter begins counting from the timing when any abnormal state is detected. The timer will be latched
after a lapse of a period of time given by the following equation.
If the abnormal state continues even when PWM is set to Low level, the counter will not reset counting.
Here LATCH_TIME= A period of time, which the timer is
latched
R_RT=RT pin connecting resistance
Protection time which described above is applied for LED OPEN protection, LED SHORT protection, SCP protection.
OVP protection as below:
Clock oscillation of timer latch uses DCDC clock. So timer latch time depend on unevenness of DCDC oscillation. In
150kHz, timer latch time is ±5% unevenness.
【Setting Example】
In LED_OPEN protection, LED_SHORT protection, SCP protection,
When RT resistance=100kohm, the timer latch time is
And OVP protection is
Figure 13-1. Timing chart of LSP time latch
To prevent the miss detection there is 4 count interval of mask before starting the timer count at LED OPEN, SHORT and
GND SHORT protection.
If PWM=H time is
PWM=H time < 4count・・・Not detect protection because it is in interval time
PWM=H time > 4count・・・Detect protection because it is out of interval time
Please verify enough to operate narrow PWM.
Figure 13-2. Timing chart of Timer count
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TSZ02201-0F1F0C100300-1-2
31.Jan.2014 Rev.002