APW7079
Application Information
Input Capacitor Selection
The input capacitor is chosen based on the voltage rating
and the RMS current rating. For reliable operation, it is
recommended to select the capacitor voltage rating at
least 1.3 times higher than the maximum input voltage.
The maximum RMS current rating of the input capacitor is
calculated as the following equation:
I
RMS
=
where
1
V
IN
⋅
T
ON
L
3
⋅
Since the output ripple is the product of the peak inductor
current and the output capacitor ESR, using low-ESR tan-
talum capacitors for the best performance or connecting
two or more filter capacitors in parallel is recommended.
Inductor Selection
The inductor value determines the inductor ripple current
and affects the load transient response. It is recom-
mended to select the boost inductor in order to keep the
maximum peak inductor current below the current limit
threshold of the power switch. For example, the current
limit threshold of the APW7079’ switch is 600mA. For
s
choosing an inductor which has peak current passed,
firstly, it is necessary to consider the output load (I
OUT
),
input (V
IN
), and output voltage (V
OUT
). Secondly, the de-
sired current ripple in the inductor also needed to be
taken into account. The current was calculated in “Output
Capacitor Selection”. Since the output ripple is the prod-
uct of the peak inductor current and the output capacitor
ESR, the larger inductor value reduces the inductor cur-
rent ripple and output voltage ripple but typically offers a
larger physical size.
The inductor value also slightly affects the maximum out-
put current. The maximum output current can be calcu-
lated as below:
V
IN
V
OUT
V
OUT
−
V
IN
I
LIM
−
T
OFF
2
×
L
⋅ η
T
ON
= main switch max. on-time (4µs typical)
V
IN
= input voltage
L = inductor value in
µH
The capacitors should be placed close to the inductor
and the GND.
Output Capacitor Selection
An output capacitor is required to filter the output and sup-
ply the load transient current. The output ripple is the sum
of the voltages across the ESR and the ideal output
capacitor. The peak-to-peak voltage of the ESR is calcu-
lated as the following equations:
∆
V
ESR
=
I
PEAK
x ESR
I
PEAK
=
V
OUT
⋅
I
OUT
V
IN
⋅
T
ON
+
≤
I
LIM
V
IN
⋅ η
2
⋅
L
Where
I
PEAK
= peak current of inductor in amp
where
I
OUT (MAX )
=
η
=
efficiency (0.85 typical)
The peak-to-peak voltage of the ideal output capacitor is
calculated as the following equation:
∆
V
COUT
=
I
OUT
×
T
ON
C
OUT
T
OFF
= main switch min. off-time (0.9µs typical)
Therefore, to consider the balance of the efficiency and
component size, an inductor value of 22µH to 47µH is
recommended in most applications.
V
IN
I
IN
I
LX
LX
I
SWP
C
IN
N-FET
I
SWN
P-FET
ESR
C
OUT
I
OUT
V
OUT
For the applications using tantalum capacitors, the
∆V
COUT
is much smaller than the V
ESR
and can be ignored.
Therefore, the AC peak-to-peak output voltage (∆V
OUT
) is
shown as below:
∆
V
OUT
=
I
PEAK
x ESR
Copyright
©
ANPEC Electronics Corp.
Rev. A.3 - Oct., 2008
9
www.anpec.com.tw
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
