BD9470AEFV Datasheet, PDF (14/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
However, if SS is set too short, inductor rush current will occur during start-up.In addition, if SS time is set too long, will
result in the brighter in stages.SS capacity will veries with various factors, such as voltagestep-up ratio, DCDC driver
frequency, LED current and output output condencer, so it is recommended to test and confirm on the actual system.
（SS capacity is often set at about 0.047uF～0.47uF approximately as a reference value）
○Setting example
SS time when the start-up is complete and Css = 0.1uF, Iss = 2uA, Vss = 3.7V will be calculated as follows.
T_ss
=
0.1
E^−6 [F]
2 E^−6
× 3.7
[A]
[
V]
=
0.185
[Sec]
In addition, when FB output is operated in Sink/Source mode（refer to “FB pin output current setting for detailed
explanation.）, SS voltage can be set to be in the range of 3.9V～4.4V at the SS pin voltage resistor divider.Soft-start time
will be set in that case is as follows.
T
ss
=
− ¹ ln1 −
A_
A ×Vss[V ] _
B_
[Sec]
A = ^{R1 [ohm] + R2 [ohm]}
Css [F ] × R1 [ohm] × R2 [ohm]
B
=

VREG58 [V
R1 [ohm]
]
+
Iss[A]_{}
÷ Css[F ]
REG58
3.9V<VSS<4.4V
FB=Sin/Source Mode
4.9V
V
4.4V
Iss[A]
V
Css[F]
3.9V＜Vss＜4.4V
SS ⇒FB Output Current
=Sink & Source mode
3.9V
Finished Start Up
SS=FB
Time
Figure 17. SS setting procedure in FB sink/ source mode
○Setting example
When R1=200kohm, R2=470kohm, Css=1.0uF, VREG58=5.8V, Iss=2uA, Vss=3.7V, SS time is set as below
T_ss
=
− ¹ ln1−
7.12 _
7.12 × 3.7 _{ =}
31 _
0.266
[Sec]
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TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003