BD9470AEFV Datasheet, PDF (27/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
●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.
LATCH _TIME
= 2¹⁶ × ^RRT
1.5 ×10¹⁰
= 65536 × ^{RRT [kΩ][}S]　
1.5 ×10⁷
Here LATCHTIME= A period of time, which the timer is latched
RRT=RT pin connecting resistance
Protection time which described above is applied for LED pin OPEN protection, LED pin SHORT protection, SCP
protection.
The protection of FB overshoot and OVP protection as below:
LATCH
= 2¹⁸ ×
R
RT
=
262144
×
R
RT
[kΩ]
[S]　
TIME
1.5 ×10¹⁰
1.5 ×10⁷
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
LATCH _TIME
=
65536× ^{RRT [kΩ]}
1.5 ×10 ⁷
=
100[kΩ]
65536 ×
1.5 ×10 ⁷
=
0.437[S ]　
And, FB overshoot protection, OVP protection is
LATCH
=
524288 ×
R
RT
[kΩ]
=
262144 ×
100[kΩ]
= 1.75[S]　
TIME
1.5 ×10⁷
1.5 ×10⁷
LED1 SHORT Pro.
detect
12V
LED1_Voltage
0.8V
Oscllator
(internal IC)
CP COUNTER
I_LED1
Current
0V
CP COUNT UP
START
FAIL
LOW
CP COUNT UP
END
LED1
LATCH UP
(OPEN)
FAIL
DET
Figure 31. Timing chart of LSP time latch
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TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003