BD9285F Datasheet, PDF (23/39 Pages) Rohm – Boost 1channel white LED driver For large LCDs

BD9285F
Datasheet
The DCDC input voltage of the power stage = VIN [V] = 24V
The efficiency of DCDC =η[%] = 90%
The averaged input current IIN is calculated as the following.
I
IN
[
A]
=
V_OUT [V ]× I_OUT [ A]
V_IN [V ]×η[%]
　=
40[V ]× 0.48[A]
24[V ]×90[%]
=
0.89
[ A]
And the ripple current of the inductor L (ΔIL[A]) can be calculated if the switching frequency, fsw = 200kHz, the inductor,
L=100µH.
ΔIL
=
(V_OUT [V ] − V_IN [V ]) × V_IN [V ]
L[H ] × V_OUT [V ] × f _SW [Hz ]
　=
100
(40[V ] −
× 10 ^−6 [H ] ×
24[V ]) × 24[V ]
40[V ] × 200 ×10 ³[Hz ]
=
0.48
[ A]
Therefore the inductor peak current, Ipeak is
Ipeak
=
I _IN [ A] +
∆IL[ A] _{[　A] =}
2
0.89[ A] +
0.48[ A]
2
= 1.13
[ A]
The calculation result of the peak current
If Rcs is assume to be 0.3 ohm
VCS _peak = Rcs × Ipeak = 0.3[Ω]×1.13[ A] = 0.339 [V ] << 0.5V
The above condition is met.
And Ipeak_det, the current OCP works is
The Rcs value confirmation
I _{peak _ det}
=
0.5[V ]
0.3[Ω]
= 1.67
[ A]
If the current rating of the used parts is 2A,
I _peak << I _{peak _ det} << ^{The current rating}
= 1.13[ A] << 1.67[ A] << 2.0[ A]
The current rating confirmation of DCDC parts
This inequality meets the above relationship. The parts selection is proper.
And Imin, the bottom of the IL ripple current can be calculated as following.
I _MIN
=
I _IN [ A] −
∆IL[ A][ _{A] = 1.13[ A] − 0.48[ A] = 0.65[ A] >>}
2
0　
This inequality implies the operation is the continuous current mode.
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