RT7264E Datasheet, PDF (13/17 Pages) Richtek Technology Corporation – 4A, 21V, 500kHz Synchronous Step-Down Converter

RT7264E
Inductor Selection
Table 2. Suggested Inductors for Typical
For a given input and output voltage, the inductor value
Application Circuit
and operating frequency determine the ripple current. The
Component Supplier Series Dimensions (mm)
ripple current ΔI_L increases with higher V_IN and decreases
with higher inductance.
ΔI_L
=
⎡
⎢⎣
V_OUT
f ×L
⎤
⎥⎦
×
⎡⎢⎣_1−
V_OUT
V_IN
⎤
⎥⎦
TDK
TDK
TAIYO YUDEN
VLF10045
SLF12565
NR8040
10 x 9.7 x 4.5
12.5 x 12.5 x 6.5
8x8x4
Having a lower ripple current reduces not only the ESR Input and Output Capacitors Selection
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.
The input capacitance, C_IN, 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
For the ripple current selection, the value of ΔI_L = 0.24(I_MAX)
will be a reasonable starting point. The largest ripple current
occurs at the highest V_IN. To guarantee that the ripple
current stays below a specified maximum, the inductor
value should be chosen according to the following
equation :
L
=
⎡
⎢⎣
f
×
V_OUT
ΔI_L(MAX)
⎤
⎥⎦
×
⎢⎣⎡^1−
V_OUT
V_IN(MAX)
⎤
⎥⎦
RMS current is given by :
I_RMS
=
I_OUT(MAX)
V_OUT
V_IN
V_IN
V_OUT
−1
This formula has a maximum at V_IN = 2V_OUT, where I_RMS =
I_OUT / 2. This simple worst case condition is commonly
used for design because even significant deviations do
not offer much relief.
Choose a capacitor rated at a higher temperature than
The inductor's current rating (caused a 40°C temperature
rising from 25°C ambient) should be greater than the
required. Several capacitors may also be paralleled to
meet size or height requirements in the design.
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
For the input capacitor, one 22μF low ESR ceramic
capacitors are recommended. For the recommended
capacitor, please refer to Table 3 for more detail.
possible to the recommended inductor values for each
V_OUT as shown in Table 1.
Table 3. Suggested Capacitors for C_IN and C_OUT
Location Component Supplier
Part No.
Capacitance (μF) Case Size
C_IN
MURATA
GRM32ER71C226M
22
C_IN
TDK
C3225X5R1C226M
22
C_OUT
MURATA
GRM31CR60J476M
47
C_OUT
TDK
C3225X5R0J476M
47
C_OUT
MURATA
GRM32ER71C226M
22
C_OUT
TDK
C3225X5R1C226M
22
1210
1210
1206
1210
1210
1210
The selection of C_OUT is determined by the required ESR
to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key
for C_OUT selection to ensure that the control loop is stable.
Loop stability can be checked by viewing the load transient
response.
The output ripple, ΔV_OUT, is determined by :
ΔV_OUT
≤
ΔI_L
⎡⎢⎣_ESR
+
1
8fC_OUT
⎤
⎥⎦
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.
DS7264E-00 November 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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