BD93942F Datasheet, PDF (18/30 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type)

BD93942F
Datasheet
Selection of inductor L
V_IN
I_L
L
ΔIL
V_OUT
R_CS
C_OUT
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.
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
where, L: Reactance value [H],
V_OUT : DC/DC output voltage [V],
V_IN: Input voltage [V],
I_OUT: Output load current (total output current) [A],
I_IN: Input current [A], and
F_SW: Oscillation frequency [Hz]
.
Figure 19. Inductor current waveform and diagram
Note: 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.
Note: 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)
where R_ESR  Equivalent series resistance of C_OUT.
Note: Select capacitor ratings with an adequate margin for output voltage.
R_ESR
Note: To use an electrolytic capacitor, an adequate margin should be
provided for permissible current. Particularly to apply PWM light modulation
R_CS
C_OUT
to LED, note that a current higher than the set LED current transiently flows.
Figure 20. Output capacitor diagram,
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 “C_OUT breakdown voltage  Rectifier diode”. However, to achieve high-speed switching, select
transistors with small gate capacity (injected charge amount).
Note: Rated current larger than over current protection setting current is recommended.
Note: Selecting transistors with low on resistance can obtain high efficiency.
Selection of rectifier diodes
Select Schottky barrier diodes having current capability higher than the rated current of the inductor L and inverse
breakdown voltage higher that C_OUT breakdown voltage, particularly having low forward voltage VF.
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TSZ02201-0F5F0C100050-1-2
12.Aug.2015 Rev.002