BD9470AEFV Datasheet, PDF (29/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
○Setting example
Output voltage=VOUT [V]=40V
LED total current=IOUT [A]=120mA×6ch=0.72A
DCDC input voltage=VIN [V]=24V
DCDC efficiency=η[%]=90%
mean input current IIN required for the whole system is given by the following equation
I
IN
[
A]
=
V_OUT [V ]× I _OUT [ A]
V_IN [V ]×η[%]
　=
40[V ]× 0.72[ A]
24[V ]× 90[%]
=
1.33
[ A]
DCDC switching frequency=fsw[Hz]=200kHz
Inductor L[H]=47μH
The Inductor ripple currentΔIL[A] is:
ΔIL
=
(V_OUT [V ] −V_IN [V ]) ×V_IN [V ]
L[H ]×V_OUT [V ]× f _SW [Hz]
　=
(40[V ] − 24[V ]) × 24[V ]
47 ×10^−6[H ]× 40[V ]× 200 ×10³[Hz]
= 1.02
[ A]
As a result, the IL peak current Ipeak is:
∆IL[ A]
1.02[ A]
Ipeak = I _IN [ A] + ₂ [　A] = 1.33[ A] + ₂ = 1.84
[ A]
… Result of peak current
calculation
When RCS resistance is set to 0.15ohm, the VCS peak voltage will be given by the following equation
VCS _peak
= Rcs × Ipeak = 0.15[Ω]×1.84[ A] = 0.276[V ] < 0.5V
… Result of review of
RCS resistance
Consequently, the result meets the condition.
Furthermore, IOCP current at which OCP is detected is given by the following equation
I _ocp
=
0.4[V ]
0.15[Ω]
=
2.67
[ A]
If the current tolerance for components to be used (e.g. FETs, inductors, diodes) is smaller than 2.5A,
I _peak < I_OCP < ^{Max. Current tolerance for component} = 1.84[ A] < 2.67[ A] < 3.0[ A]
… Result of review of current
tolerance for DC/DC components
As a result, since the condition above is met, the selection of components is accepted.
And, the lower limit of IL ripple current Imin is:
Im
in
=
I
IN
[
A]
−
∆IL[ A]
2
[
A]
=
1.33[
A]
−
1.02[
2
A]
=
0.82[
A]
>
0　
The system will not be put into discontinuous mode.
To select DC/DC components, please consider IC variations as well as individual component variations, andthen conduct
thorough verification on practical systems.
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TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003