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APW7065 Datasheet(PDF) 14 Page - Anpec Electronics Coropration

Part # APW7065
Description  Synchronous Buck PWM Controller
PDF  21 Pages
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Manufacturer  ANPEC [Anpec Electronics Coropration]
Direct Link  http://www.anpec.com.tw
Logo ANPEC - Anpec Electronics Coropration

APW7065 Datasheet(HTML) 14 Page - Anpec Electronics Coropration

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Copyright
© ANPEC Electronics Corp.
Rev. A.6 - Aug., 2009
APW7065
www.anpec.com.tw
14
PWM Compensation
The output LC filter of a step down converter introduces a
double pole, which contributes with -40dB/decade gain
slope and 180 degrees phase shift in the control loop. A
compensation network among COMP, FB and V
OUT should
be added. The compensation network is shown in Fig-
ure 6. The output LC filter consists of the output inductor
and output capacitors. The transfer function of the LC
filter is given by:
Application Information (Cont.)
1
C
ESR
s
C
L
s
C
ESR
s
1
GAIN
OUT
OUT
2
OUT
LC
+
×
×
+
×
×
×
×
+
=
The poles and zero of this transfer functions are:
OUT
LC
C
L
2
1
F
×
×
π
×
=
OUT
ESR
C
ESR
2
1
F
×
×
π
×
=
The F
LC is the double poles of the LC filter, and FESR is the
zero introduced by the ESR of the output capacitor.
PHASE
L
OUTPUT
C
OUT
ESR
Figure 3. The Output LC Filter
FLC
FESR
-40dB/dec
-20dB/dec
Frequency(Hz)
Figure 4. The LC Filter GAIN and Frequency
The PWM modulator is shown in Figure 5. The input is
the output of the error amplifier and the output is the
PHASE node. The transfer function of the PWM modula-
tor is given by:
OSC
IN
PWM
V
V
GAIN
∆
=
Figure 5. The PWM Modulator
Output of Error
Amplifier
ΔV
OSC
PWM
Comparator
Driver
Driver
PHASE
V
IN
OSC
The compensation network is shown in Figure 6. It pro-
vides a close loop transfer function with the highest zero
crossover frequency and sufficient phase margin. The
transfer function of error amplifier is given by:
(
)
×
+
×
×
×
+
+


×
+
+
×
×
+
×
×
×
+
=
+
+
=
=
C3
R3
1
s
C2
C1
R2
C2
C1
s
s
C3
R3
R1
1
s
C2
R2
1
s
C1
R3
R1
R3
R1
sC3
1
R3
R1//
sC2
1
R2
//
sC1
1
V
V
GAIN
OUT
COMP
AMP
The poles and zeros of the transfer function are:
(
)
C3
R3
2
1
F
C2
C1
C2
C1
R2
2
1
F
C3
R3
R1
2
1
F
C2
R2
2
1
F
P2
P1
Z2
Z1
×
×
π
×
=
+
×
×
×
π
×
=
×
+
×
π
×
=
×
×
π
×
=
V
REF
V
OUT
V
COMP
R1
R3
C3
R2
C2
C1
FB
Figure 6. Compensation Network



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