RT7263A
Inductor Selection
Table 2. Suggested Inductors for Typical
Application Circuit
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ΔIL increases with higher VIN and decreases
with higher inductance.
Component Supplier
Series
Dimensions (mm)
10 x 9.7 x 4.5
TDK
TDK
VLF10045
SLF12565 12.5 x 12.5 x 6.5
NR8040 8 x 8 x 4
V
f ×L
VOUT
⎡
OUT ⎤ ⎡
× 1−
⎥ ⎢
⎤
ΔIL =
⎢
⎣
⎥
⎦
V
IN
⎦ ⎣
TAIYO YUDEN
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. Highest efficiency operation is achieved by reducing
ripple current at low frequency, but it requires a large
inductor to attain this goal.
Input and Output Capacitors Selection
The input capacitance, CIN, is needed to filter the
trapezoidal current at the source of the high side MOSFET.
To prevent large ripple current, a low ESR input capacitor
sized for the maximum RMS current should be used. The
RMS current is given by :
For the ripple current selection, the value of ΔIL = 0.24(IMAX
)
will be a reasonable starting point. The largest ripple current
occurs at the highest VIN. To guarantee that the ripple
current stays below a specified maximum, the inductor
value should be chosen according to the following
equation :
V
V
V
IN
V
OUT
OUT
I
= I
−1
RMS
OUT(MAX)
IN
This formula has a maximum at VIN = 2VOUT, where IRMS
=
IOUT / 2. This simple worst case condition is commonly
used for design because even significant deviations do
not offer much relief.
⎡
⎤ ⎡
⎤
V
f × ΔI
V
OUT
V
IN(MAX)
OUT
L =
× 1−
⎢
⎥ ⎢
⎥
L(MAX)
⎣
⎦ ⎣
⎦
Choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design.
The inductor's current rating (caused a 40°C temperature
rising from 25°C ambient) should be greater than the
maximum load current and its saturation current should
be greater than the short circuit peak current limit. Please
see Table 2 for the inductor selection reference and it is
highly recommended to keep inductor value as close as
possible to the recommended inductor values for each
For the input capacitor, one 22μF low ESR ceramic
capacitors are recommended. For the recommended
capacitor, please refer to table 3 for more detail.
VOUT as shown in Table 1.
Table 3. Suggested Capacitors for CIN and COUT
Location
CIN
Component Supplier
Part No.
Capacitance (μF)
Case Size
1210
MURATA
TDK
GRM32ER71C226M
C3225X5R1C226M
GRM31CR60J476M
C3225X5R0J476M
GRM32ER71C226M
C3225X5R1C226M
22
22
47
47
22
22
CIN
1210
COUT
COUT
COUT
COUT
MURATA
TDK
1206
1210
MURATA
TDK
1210
1210
The selection of COUT is determined by the required ESR
to minimize voltage ripple.
The output ripple, ΔVOUT, is determined by :
1
⎡
⎤
ΔVOUT ≤ ΔIL ESR +
⎢
⎣
⎥
⎦
8fCOUT
Moreover, the amount of bulk capacitance is also a key
for COUT selection to ensure that the control loop is stable.
Loop stability can be checked by viewing the load transient
response.
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
Copyright 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS7263A-01 September 2012
www.richtek.com
13
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