Advanced Analog Technology, Inc.
October 2007
AAT7301
Inductor Selection
A good rule of thumb when choosing the inductance is
to allow the peak inductor current in the inductor to be
approximately 115% of the maximum load current. Also,
make sure that the peak inductor current is below the
2.5A minimum current limit.
The inductance value can be calculated by the
equation:
V
V
OUT
⋅
(1
−
OUT
)
V
I
L
≈
0.3
⋅
I
OUT
⋅
f
SW
Input Capacitor
The input capacitor in a DC-to-DC converter reduces
current peaks drawn from the input power source and
reduces
switching
noise
in
the
controller.
The
impedance of the input capacitor at the switching
frequency should be less than that of the input source
so high-frequency switching currents do not pass
through the input source. Ceramic capacitors are
preferred,
but
tantalum
or
low-ESR
electrolytic
capacitors may also satisfy. For insuring stable
operation the IC must be bypassed with a 0.22
µF
ceramic capacitor placed close to the V
DD
.
Where V
OUT
is the output voltage, V
I
is the input voltage,
f
SW
is the switching frequency (380kHz), and I
OUT
is the
maximum load current.
When the inductance value is determined, then the
peak inductor current can be calculated by the
equation:
V
V
OUT
⋅
(1
−
OUT
)
V
I
I
L(peak)
=
I
OUT
+
2
⋅
f
SW
⋅
L
setting
Output Capacitor
The output capacitor keeps output ripple small and
ensures control-loop stability. The output capacitor
must also have low impedance at the switching
frequency. Ceramic, polymer, and tantalum or low-ESR
electrolytic capacitors are suitable.
Output ripple with a ceramic output capacitor is
approximately as follows:
V
V
OUT
⋅
(1
−
OUT
)
V
I
8
⋅
L
setting
⋅
C
OUT
⋅
f
SW 2
V
ripple
≈
Where I
OUT
is maximum load current, V
OUT
is the output
voltage, V
I
is the input voltage, f
SW
is the switching
frequency (380kHz), and L
setting
is the inductance value.
Choose an inductor that does not saturate under the
peak inductor current.
Where V
OUT
is the output voltage, V
I
is the input voltage,
f
SW
is the switching frequency (380kHz), C
OUT
is the
output capacitance, and L
setting
is the inductance value.
If the capacitor has significant ESR, the output ripple
component due to capacitor ESR is as follows:
V
ripple
≈
2
⋅
(I
L(peak)
−
I
OUT
)
×
R
ESR
Schottky Diode Selection
Choose a Schottky diode whose maximum reverse
voltage rating is greater than the maximum input
voltage, and whose current rating is greater than the
peak inductor current.
Where I
L(peak)
is the peak inductor current, I
OUT
is
maximum load current, and R
ESR
is the capacitor’s
ESR.
–
Advanced Analog Technology, Inc
.
–
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