APW1172
Application Description (Cont.)
Inductor(Cont.)
dimension of inductor to save the board space. In
other way, devote the performance by higher ripple
output capacitor induce the output ripple voltage. In
general, the ripple current isusuallyfixedat 20%~40% current. If select a greater inductor, the ripple current
will be smaller and a better performance is got. This
maximum output current equal 3A. The value of tradeoff is an useful method to decide a better
of maximum output current,that is 0.6A~1.2A with
performance or a smaller inductor size.
inductor can approximate by (1)
VIN - VCE - VO
Output Capacitor
L =
Ton
(1)
DI
The APW1172 requires a proper output capacitor to
maintain stability andimprove transient responseover
temperature and current. The output capacitor
selection is dependent upon ESR (equivalent series
resistance) and capacitance of the output capacitor
over the operating temperature.
Where VIN is the input voltage, VCE is the voltage
across the pass element when it conduct, VO is the
output voltage, DI is the ripple current flowing through
the inductor and Ton is the on period that determined
byVO and V . The exact Toncan obtained by (2) and(3)
INVO +VD
Consider the output ripple voltage that absorbed in
the application.Output ripple voltage consist of two
parts.It show as (4)
D =
(2)
VIN - VCE +VD
WhereVDistheforward voltageof thewheeling diode.
(3)
Ton = DTS
(4)
Vripple =V1 +V2
Where TS is the period of whole cycle. It equal 1/FS
where FS is the switching frequency of APW1172. For
example, VIN = 12V, VO = 3.3V, VD = 0.7V, IO = 3A,
ripple current is IO (20%~40%) = 0.6A ~ 1.2A, VCE
=1.2V, FS = 250KHz
Inpreviously,usethe parameter DItodecidethevalueof
the inductor.Asthesame manner,use the parameter DI
to approximate the value of output capacitor.
The first part of output ripple voltage,V1,is related to
the ESR of output capacitor.It show as (5)
3.3V +0.7V
D =
= 34.78%
by (2)
by (3)
(5)
V = ESR´ DI
1
12V - 1.2V + 0.7V
The second part of output ripple voltage,V2,can
calculated by (6)
Ton = DTS = 34.78%´ 4ms = 1.3912ms
For the worst case ripple current equal 0.6A ~ 1.2A
DI
(6)
V2 =
TS
12V - 1.2V - 3.3V
8C
L1 =
1.3912ms = 17.39mH
0.6 A
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
by (1)
for ripple current is 0.6A… …
12V - 1.2V - 3.3V
L2 =
1.3912ms = 8.695mH
1.2A
for ripple current is 1.2A… …
by (1)
Use the worst caseto approximatethe minimum value
of inductor. In worst ripple current condition, smaller
Copyright ã ANPEC Electronics Corp.
13
www.anpec.com.tw
Rev. A.4 - Aug., 2005
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
