BD9421F_15 Datasheet, PDF (17/31 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter Type)

BD9421F
Datasheet
○Selection of inductor L
ΔIL
The value of inductor has significant influence on the input ripple current. As
shown by Equation (1), the larger the inductor and the higher the switching
frequency, the inductor ripple current ∆IL becomes increasingly lower.
V_IN
I_L
L
R_CS
V_OUT
C_OUT
ΔIL  ^{(VOUT  VIN )  VIN} [　A]　　　　・・・・・　 　(1)
L  V_OUT  f _SW
Expressing efficiency as shown by Equation (2), peak input current is given as
Equation (3).
  ^{VOUT  I OUT} 　　　　　・・・・・　　(2)
V_IN  I _IN
IL_MAX
 I _IN
 ^ΔIL
2
 ^{VOUT  I OUT}
V_IN 
　　 　　Δ^IL 　・・・・・　(　3)
2
Here,
L：Inductor value[H]
V_OUT：DC/DC output voltage[V]
V_IN：input voltage[V]
I_OUT：output total current[A]
I_IN：input current[A]
F_SW：Oscillation frequency[Hz]
Basically, make setting of ∆IL to approximately 30% to 50% of the output load
current.
※ If a current in excess of the rated current of the inductor applies to the coil, the inductor will cause magnetic
saturation, resulting in efficiency degradation.
Select an inductor with an adequate margin so that peak current will not exceed the rated current of the inductor.
※ To reduce power dissipation from and increase efficiency of inductor, select an inductor with low resistance
component (DCR or ACR).
○Selection of output capacitor C_OUT
V_IN
I_L
L
V_OUT
Select a capacitor on the output side taking into account the stability region
of output voltage and equivalent series resistance necessary to smooth
ripple voltage. Note that higher output ripple voltage may result in a drop in
LED pin voltage, making it impossible to supply set LED current.
The output ripple voltage ∆V_OUT is given by Equation (4).
ΔV_OUT
 ILMAX
 R_ESR

1
C_OUT

I _OUT


1
f _SW
　[V ]　・・・・・　　(4)
Here, R_ESR  Equivalent series resistance of C_OUT.
R_ESR
R_CS
C_OUT
※ Select capacitor ratings with an adequate margin for output voltage.
※ To use an electrolytic capacitor, an adequate margin should be provided for permissible current. Particularly to
apply PWM light modulation to LED, note that a current higher than the set LED current transiently flows.
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TSZ22111 • 15 • 001
17/27
TSZ02201-0F1F0C100310-1-2
25.Sep.2015 Rev.003