Technical Note
7/13
www.rohm.com
2011.05 - Rev.B
© 2011 ROHM Co., Ltd. All rights reserved.
BD95830MUV
●External Component Selection
1. Inductor (L) selection
※Passing a current larger than the inductor’s rated current will cause magnetic saturation in the inductor and decrease
system efficiency. When selecting an inductor, be sure to allow enough margin to assure that peak current does not
exceed the inductor’s rated current value.
※To minimize possible inductor damage and maximize efficiency, choose a inductor with a low (DCR, ACR) resistance.
2. Output Capacitor (C
OUT) Selection
Also, give due consideration to the conditions in formula (7) below for output capacitance, bearing in mind that output rise
time must be established within the soft start time frame. As output capacitor capacitance, bypass capacitor will be
connected to output load side (C
EXT, figure above). Please set the over current detection value with regards to these
capacitance.
Note: an improper output capacitor may cause startup malfunctions.
3. Input Capacitor (C
IN) Selection
A low-ESR capacitor is recommended to reduce ESR loss and maximize efficiency.
The inductance value has a major influence on output ripple current.
s formula (3) below indicates, the greater the inductance or
switching frequency, the lower the ripple current.
ΔI
L=
(V
IN -VOUT)×VOUT
L×VIN×f
A
・・・
3
The proper output ripple current setting is about 30% of maximum
output current.
ΔI
L=0.3×IOUTmax. [A]・・・(4)
L=
(VIN -VOUT)×VOUT
ΔIL×VIN×f
H
・・・
5
(ΔI
L: output ripple current, f: switching frequency)
In order to prevent transient spikes in voltage, the input capacitor selected must have
a low enough ESR resistance to fully support a large ripple current on the output.
The formula for ripple current IRMS is given in equation (8) below:
I
RMS=IOUT×
VOUT
VIN -VOUT
VIN
[A]・・・
8
√
Where V
IN =2×VOUT, IRMS=
I
OUT
2
Output Capacitor (COUT) has a considerable influence on output
voltage regulation due to a rapid load change and smoothing outpu
ripple voltage. Determine the capacitor by considering the value o
capacity, the equivalent series resistance, and equivalent series
inductance. Also, make sure the capacitor’s voltage rating is high
enough for the set output voltage (including ripple).
Output ripple voltage is determined as in formula (6) below.
ΔVOUT=ΔIL/(8×COUT×f)+ESR×ΔIL +ESL×ΔIL / TON・・・(6)
(ΔI
L: Output ripple current; ESR: Equivalent series resistance,
ESL: Equivalent series inductance)
C
OUT≦
1.3ms×(Limit-I
OUT)
V
OUT
・・・
7
Limit: Over current detection
I
OUT: Output current
ΔIL
VIN
IL
L
Co
V
OUT
Output Ripple Current
HG
SW
LG
Input Capacitor
VIN
L
C
OUT
VOUT
CIN
HG
SW
LG
VIN
L
C
OUT
VOUT
ESR
Output Capacitor
ESL
HG
SW
LG
Load
CEXT
