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AN2728
Selecting components for your applications
9/20
The maximum and minimum duty cycles are:
Equation 3
and
Equation 4
where VF it is the voltage drop across the internal NMOS and VSW the voltage drop across
the internal PMOS. Considering the range DMIN to DMAX it is possible to determine the
maximum IRMS flowing through the input capacitor.
The use of ceramic capacitors with voltage ratings in the range of 1.5 times the maximum
output voltage is recommended.
2.3
Output capacitor
The output capacitor is very important to satisfy the output voltage ripple requirement. Using
a small inductor value is useful to reduce the size of the coil, but increases the current ripple.
So, in order to reduce the output voltage ripple a low ESR capacitor is required. The output
voltage ripple (VOUT_RIPPLE) is:
Equation 5
where
∆ I is the ripple current and FSW is the switching frequency.
The use of ceramic capacitors with voltage ratings in the range of 1.5 times the maximum
output voltage is recommended.
2.4
Inductor
The inductor value is very important because it fixes the ripple current flowing through
output capacitor. The ripple current is usually fixed at 20-40 % of IOUT_MAX, that is
0.14-0.28 A with IOUT_MAX = 0.7 A. The inductor value is approximately obtained by the
following formula:
Equation 6
where TON is the ON time of the internal switch, given by D x T.
SW
INMIN
F
OUT
MAX
V
V
V
V
D
-
+
=
SW
INMIN
F
OUT
MAX
V
V
V
V
D
-
+
=
SW
INMAX
F
OUT
MAX
V
V
V
V
D
-
+
=
SW
INMAX
F
OUT
MAX
V
V
V
V
D
-
+
=
×
×
+
×
∆ I
=
SW
OUT
RIPPLE
OUT
F
C
ESR
8
1
V
_
[]
×
×
+
×
∆ I
=
SW
OUT
RIPPLE
OUT
F
C
ESR
8
1
V
_
[]
ON
OUT
IN
T
V
V
L
×
∆ I
-
=
ON
OUT
IN
T
V
V
L
×
∆ I
-
=



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