BD9483F Datasheet, PDF (17/32 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type)

BD9483F,FV
Datasheet
(the selection method of Rcs)
Ipeak flows into Rcs and that cause the voltage signal to CS pin. (Please refer the right timing chart)
That peak voltage VCSpeak is as following.
VCS _peak  Rcs  Ipeak [V]
As this VCSpeak reaches to 0.4V, the DCDC output stops the switching.
Therefore, Rcs value is necessary to meet the under condition.
Rcs  Ipeak[V]  0.4[V]
(the current rating of the external DCDC parts)
The peak current as the CS voltage reaches to OCP level (0.4V) is defined as Ipeak_det.
I  ^0.4[V] [A]
Rcs[ ] ^peak_det
… (2)
The relation among Ipeak (equality (1)), Ipeak_det (equality (2)) and the current rating of parts is required to meet the
following
I _peak  I _{peak _ det}  ^{The current rating of parts}
Please make the selection of the external parts to meet the above condition such as FET, Inductor, diode.
[setting example]
The output voltage = VOUT [V] = 40V
LED total current = IOUT [A] = 0.48V
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]  24[V])  24[V]
10 ^6[H]  40[V]  200 10³[Hz]

0.48
[A]
Therefore the inductor peak current, Ipeak is
Ipeak
 I [A] 
IN
ΔIL[A] _{　 0.89[A]}
2

0.48[A]
2
 1.13[A]
…The calculation result of the peak current
If Rcs is assume to be 0.3 ohm
VCS  Rcs  Ipeak  0.3[]  1.13[A]  0.339[V]  0.4V
peak
…The Rcs value confirmation
The above condition is met.
And Ipeak_det, the current OCP works is
I  ^0.4[V]  1.33[A]
0.3[ ] ^peak_det
If the current rating of the used parts is 2A,
I _peak  I _{peak _ det}  ^{The current rating}
 1.13[ A]  1.33[ 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] 
ΔIL[A] _[A]
2
 1.13[A]
 0.48[A]

0.65[A]

0　
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