BD9486F Datasheet, PDF (20/37 Pages) Rohm – 1ch Boost up type White LED Driver for large LCD

BD9486F
Datasheet
[setting example]
Output voltage = VOUT [V] = 40V
LED total current = IOUT [A] = 0.48V
DCDC input voltage of the power stage = VIN [V] = 24V
Efficiency of DCDC =η[%] = 90%
Averaged input current IIN is calculated as follows.
I_IN[A]

V_OUT[V]  I_OUT[A]
V_IN[V]  [%]
　
40[V]  0.48[A]
24[V]  90[%]

0.89
[A]
If the switching frequency, f_SW = 200kHz, and the inductor, L=100μH, the ripple current of the inductor L (ΔIL[A]) can be
calculated as follows.
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_IN[A] 
IL[A] _　[A]
2

0.89[A] 
0.48[ A ]
2
 1.13
[A ] _{…calculation result of the peak current}
If Rcs is assumed to be 0.3Ω
VCS _peak  Rcs  Ipeak  0.3[] 1.13[A]  0.339 [V]  0.4V
…Rcs value confirmation
The above condition is met.
And Ipeak_det, the current OCP works, is
I_{peak _ det}

0.4[ V ]
0.3[]
 1.33
[A]
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 ] …current rating confirmation
of DCDC parts
This inequality meets the above relationship. The parts selection is proper.
And I_MIN, the bottom of the IL ripple current, can be calculated as follows.
I_MIN

I_IN[A] 
IL[A] _[A]
2

1.13[A] 
0.48[ A ]

0.65[ A ]

0
This inequality implies that the operation is continuous current mode.
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TSZ02201-0F1F0C100240-1-2
13.Feb.2014 Rev.004