BD9470AEFV Datasheet, PDF (16/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
● VCC series resistance setting procedure
VIN
By inserting a series resistor to VCC will has the following affection.
①Reduce the voltage VCC, and it is possible to suppress the heat generation of IC.
（ICC×VIN is power consumption of IC）
②Possible to Raise the surge ability to VCC.
However, if resistance is set too large, it is needed to consider that will
RVCC
ΔV
VCC
result in VCC become VCC<9V（ Minimum operation voltage）.So the
I_IN
appropriate series resistance setting is needed.
ICC
The current influx of IC I_IN as shown on the right is
・Circuit current of IC…ICC
・Current to load is connected toREG58…IREG
・Current which used to drive DCDC FET…IDCDC
There are 3 paths within IC and the ΔV of RVCC can be decided.
VCC voltage generated by the relation as above described at that time
can be represented as below.
+
-
IC
BLOCK
IREG ^REG58
IDCDC
RREG
VCC[V ] = VIN[V ] − (ICC[A] + IDCDC[A] + IREG[A] )× RVCC[Ω] [　V ] > 9[V ]
DCDC
DRIVER
N
I_N
The Criterion of 9V is the minimum operating limit of the IC.
When a series resistance is considered, please set with a sufficient
margin.
Figure 19. ICC paths diagram
○Setting example
Above equation can be transformed as below.
RVCC[Ω] <
VIN[V ] − 9[V ]
ICC[A] + IDCDC[A] + IREG[A]
In typical operation, VIN=24V, ICC=5.5mA, RREG=10kΩ, IDCDC=2mA can be assumed and the VCC voltage is
RVCC[Ω] <
24[V ] − 9[V ]
= 1.86[kΩ]
0.0055[ A] + 0.002[ A] + 5.8[V ] 10000[Ω]
However, the result is in typical operation and the variability and margin is not considered.
If the variability of VIN=24V×(-20%）,ICC=8.5ｍA,RREG=10k×(-5%）,REG58=5.8V×(+5%),IDCDC=2mA×(+100%),VCC
operation limit voltage9V×(+20%) are assumed:
RVCC[Ω] <
24 × 0.8[V ] − 9 ×1.2[V ]
= 640[Ω]
0.0085[ A] + 0.002 × 2[ A] + 5.8×1.015[V ] (10000[Ω]× 0.95)
According to above result, set RVCC = 640Ω or less is adequate on actual application.
When a series resistance is considered, please set with a sufficient margin.
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TSZ22111・15・001
16/35
TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003