APW1172
Application Description (Cont.)
Inductor (Cont.)
output capacitor induce the output ripple voltage. In
general, the ripple current is usually fixed at 20%~40%
of maximum output current,that is 0.6A~1.2A with
maximum output current equal 3A. The value of
inductor can approximate by (1)
dimension of inductor to save the board space. In
other way, devote the performance by higher ripple
current. If select a greater inductor, the ripple current
will be smaller and a better performance is got. This
tradeoff is an useful method to decide a better
performance or a smaller inductor size.
Output Capacitor
The APW1172 requires a proper output capacitor to
maintain stability and improve transient response over
temperature and current. The output capacitor
selection is dependent upon ESR (equivalent series
resistance) and capacitance of the output capacitor
over the operating temperature.
Consider the output ripple voltage that absorbed in
the application.Output ripple voltage consist of two
parts.It show as (4)
L
=
V
IN
−
V
CE
−
V
O
T
on
∆
I
(1)
Where V
IN
is the input voltage, V
CE
is the voltage
across the pass element when it conduct, V
O
is the
output voltage,
∆I
is the ripple current flowing through
the inductor and Ton is the on period that determined
by V
O
and V
IN
. The exact Ton can obtained by (2) and(3)
D
=
V
O
+
V
D
V
IN
−
V
CE
+
V
D
(2)
Where VD is the forward voltage of the wheeling diode.
T
on
=
DT
S
(3)
V
ripple
=
V
1
+
V
2
(4)
Where T
S
is the period of whole cycle. It equal 1/F
S
where F
S
is the switching frequency of APW1172. For
example, V
IN
= 12V, V
O
= 3.3V, V
D
= 0.7V, I
O
= 3A,
ripple current is I
O
(20%~40%) = 0.6A ~ 1.2A, V
CE
=1.2V, F
S
= 250KHz
D
=
3.3V
+
0.7V
=
34.78%
12V
−
1.2V
+
0.7V
In previously,use the parameter
∆I
to decide the value of
the inductor. As the same manner,use the parameter
∆I
to approximate the value of output capacitor.
The first part of output ripple voltage,V
1
,is related to
the ESR of output capacitor.It show as (5)
by (2)
V
1
=
ESR
× ∆
I
(5)
T
on
=
DT
S
=
34.78%
×
4
µ
s
=
1.3912
µ
s
by (3)
For the worst case ripple current equal 0.6A ~ 1.2A
The second part of output ripple voltage,V
2
,can
calculated by (6)
12V
−
1.2V
−
3.3V
L
1
=
1.3912
µ
s
=
17.39
µ
H
0.6 A
for ripple current is 0.6A… …
V
2
=
∆
I
T
S
8C
(6)
by (1)
L
2
=
12V
−
1.2V
−
3.3V
1.3912
µ
s
=
8.695
µ
H
1. 2 A
for ripple current is 1.2A… …
by (1)
Use the worst case to approximate the minimum value
of inductor. In worst ripple current condition, smaller
These two parameters determine the value of output
ripple voltage and the efficiency. More output ripple
voltage cause the efficiency decreased.The output
ripple voltage means the energy loss in the ESR and
the energy loss in the transition path while the energy
stored and removed in the output capacitor.In other
aspect,the ESR and the value of output capacitor
Copyright
©
ANPEC Electronics Corp.
Rev. A.4 - Aug., 2005
13
www.anpec.com.tw
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
