BD9470AEFV Datasheet, PDF (30/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
●Selection of Inductor
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
lower.
increasingly
ΔIL = ^{(VOUT − VIN ) × VIN} 　 [ A]　　　　・・・・・　　(1)
L × V_OUT × f _SW
Efficiency as shown by Equation (2), peak input current is given as Equation
ΔIL
(3).
η = ^{VOUT × IOUT} 　　　　　・・・・・　　(2)
V_IN × I _IN
V_IN
IL_MAX
=
I _IN
+^ΔIL
2
=
V_OUT × I_OUT
V_IN ×η
　　+ 　　Δ^IL
2
　・・・・・　　(3)
I_L
L
Here,
L：Reactance value [H] VOUT：DC/DC output voltage[V]
V_OUT
VIN：input voltage[V] IOUT：output current(LED total current)[A]
IIN：input current[A]
FSW：oscillation frequency[Hz]
If a current in excess of the rated current of the inductor applies to the
coil, the inductor will cause magnetic saturation, resulting in lower
efficiency.
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 induct
or, select an inductor with low resistance component (DCR or AC
R).
R_CS
C_OUT
Figure33.
DCDC application circuit and coil current
●Selection of switching MOSFET transistors
There will be no problem for switching MOSFET transistors having absolute maximum rating higher than rated current of
the inductor L and VF higher than “COUT breakdown voltage + Rectifier diode”. However, to achieve high-speed switching,
select transistors with small gate capacity (injected charge amount).
・Rated current larger than current protection setting current is recommended
・Selecting transistors with low On resistance can obtain high efficiency.
●Selection of rectifier diodes
Select current capability higher than the rated current of the inductor L and inverse breakdown voltage higher that COUT
break-down voltage, particularly having low forward voltage VF.
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TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003