APW7061
Application Information (Cont.)
Inductor Selection (Cont.)
The poles and zero of this transfer function are:
into lower output ripple voltage. The ripple current and
ripple voltage can be approximated by:
1
FLC
=
2´ p´ L´ COUT
VIN - VOUT
Fs x L
VOUT
VIN
IRIPPLE
=
x
1
FESR
=
2´ p ´ ESR´ COUT
DVOUT = IRIPPLE x ESR
The FLC is the double poles of the LC filter, and FESR is
thezero introducedby the ESRof the output capacitor.
where Fs is the switching frequency of the regulator.
There is a tradeoff exists between the inductor’s ripple
current and the regulator load transient response time
A smaller inductor will give the regulator a faster load
transient response at the expense of higher ripple
current and vice versa. The maximum ripple current
occurs at the maximum input voltage. A good starting
point is to choose the ripple current tobe approximately
30% of the maximum output current.
L
Output
PHASE
COUT
ESR
Figure 1. The Output LC Filter
Once the inductance value has been chosen, select
an inductor that is capable of carrying the required
peak current without going into saturation. In some
type of inductors, especially core that is make of
ferrite, the ripple current will increase abruptly when it
saturates. This will result in a larger output ripple
voltage.
F
LC
-40dB/dec
FESR
Gain
-20dB/dec
Compensation
The output LC filter introduces a double pole, which
contributes with –40dB/decade gain slope and 180
degrees phase shift inthe control loop.A compensation
network between COMP pin and ground should be
added. The simplest loop compensation network is
shown in Figure. 4.
Frequency
Figure 2. The Output LC Filter Gain & Frequency
The PWM modulator is shown in Figure. 3. The input
is the output of the error amplifierand the output is the
PHASE node. The transfer function of the PWM
modulator is given by:
The output LC filter consists of the output inductor
and output capacitors. The transfer function of the LC
filter is given by:
VIN
1+s´ ESR´ COUT
s2 ´ L´ COUT+ s´ ESR+1
GAINLC
=
GAINPWM =
DVOSC
Copyright ã ANPECElectronicsCorp.
11
www.anpec.com.tw
Rev. A.7 - Nov., 2005
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
