BD9470AEFV Datasheet, PDF (19/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
●Explanation of VOUT（OVP） voltage holding function when PWM=Low
VIN
VCC
OVP COPY
OVP_IN
Holding OVP
Vol. BLOCK
OVP
LED_OK
PWM signal
VOUT
-
H : DCDC ON
OVP
COMP
L : DCDC OFF
+
FB
+
CONTROL _DRIVER
N
SS
+ ^ICOMP
LOGIC
SLOPE +
-
CS
DCDC_GND
LED1-6
Figure 23. Block diagram of KEEP_OVP
OVP holding function means VOUT(OVP) voltage will be copy to IC internally at a time of PWM from High to Low, voltage
will be maintained even in the period of PWM=Low.
In addition to measures of the above problems, by applying this function, the high-speed start-up can be achieved without
depending on the PWM.
Because VOUT voltage resolution is the same as FB holding function which is split by 40 from 4V,so the voltage can be
copied to IC internally in 0.1V Step.
The description of OVP holding function is divided into narrow PWM operation and start-up operation.
○Explanation of OVP holding function at start-up
PWM
②
①
⑤
OVP
OVP COPY
③
N
④
Figure 24. Timing chart 1 of KEEP_OVP
In order to launch high speed start-up without depending on the PWM DUTY, OVP holding function will behave like the
following descriptions.
①PWM=High, normal boost operation.
②OVP voltage is copied into IC when PWM is from High to Low.OVP voltage will be copied upper 1BIT at this time. For
example: if OVP=2.43V, the copied voltage is 2.5V in IC.
③The copied OVP voltage will be compared with OVP pin voltage internally, if OVP_COPY>OVP, DCDC is operated.In
other words, it is possible to achieve fast start-up by letting the voltage on the 1BIT boosted up in the interval of PWM =
Low.
④When OVP_COPY<OVP pin voltage, DCDC is stopped.
⑤Even if in the period of PWM=Low and VOUT is discharged, output voltage will be hold by performing DCDC operation
in order to let OVP_COPY＜OVP pin voltage.
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TSZ22111・15・001
19/35
TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003