User guide

AP3595
Document number: DS36749 Rev. 1 - 2
15 of 23
www.diodes.com
January 2014
© Diodes Incorporated
AP3595
A Product Line of
Diodes Incorporated
Application Information (Cont.)
The compensation network is shown in Figure 9. It provides a close loop transfer function with the highest zero cross over frequency and sufficient
phase margin. The transfer function of error amplifier is given by:
)
33
1
()
212
21
(
}
3)31(
1
{)
22
1
(
131
31
)
3
1
3//(1
)
2
1
2//(
1
1
CR
s
CCR
CC
ss
CRR
s
CR
s
CRR
RR
sC
RR
sC
R
sC
V
V
Gain
OUT
COMP
AMP
















The pole and zero frequencies of the transfer function are:
222
1
1
CR
f
Z



3)31(2
1
2
CRR
f
Z



)
21
21
(22
1
1
CC
CC
R
f
P





332
1
2
CR
f
P



R3
V
OUT
+
-
V
REF
V
COMP
C3
C1
R2
C2
R1
FB
Figure 9.Compensation Network
The closed loop gain of the converter can be written as:
AMPPW MLC
GainGainGain 
Figure 10 shows the asymptotic plot of the closed loop converter gain, and the following guidelines will help to design the compensation network.
Using the below guidelines will give a compensation similar to the curve plotted. A stable closed loop has a-20dB/decade slope and a phase
margin greater than 45 degree.
1. Choose a value for R1, usually between 1k and 5k.
2. Select the desired zero crossover frequency.
SWO
ff  )10/1~5/1(
Use the following equation to calculate R2:
12 R
f
f
V
V
R
LC
O
IN
OSC



3. Place the first zero f
Z1
before the output LC filter double pole frequency f
LC
.
LCZ
ff  75.0
1