IR3629 International Rectifier Corp., IR3629 Datasheet - Page 15

• Manufacturer Part Number: IR3629
• Description: High Frequency Synchronous Pwm Buck Controller With Power Good Output
• Manufacturer: International Rectifier Corp.

H
To cancel one of the LC filter poles, place the
zero before the LC filter resonant frequency pole:
Using equations (15) and (16) to calculate C9.
One more capacitor is sometimes added in
parallel with C4 and R3. This introduces one
more pole which is mainly used to suppress the
switching noise.
The additional pole is given by:
The pole sets to one half of the switching
frequency which results in the capacitor C
For a general solution for unconditional stability
for any type of output capacitors, in a wide range
of ESR values we should implement local
feedback with a compensation network (type III).
The typically used compensation network for
voltage-mode controller is shown in figure 15.
In such configuration, the transfer function is
given by:
The error amplifier gain is independent of the
transconductance under the following condition:
By replacing Z
transfer function can be expressed as:
(
11/29/2007
s
g
)
=
m
sR
*
C
Z
POLE
8
f
(
F
F
C
>>
z
z
1
4
=
=
=
+
75
0
1
F
π
C
.
in
P
75
%
*
3
V
V
and
=
and Z
)
R
F
e
o
*
*
2
LC
3
2
π
⎡
⎢
⎣
=
*
1
(
π
1
1
*
F
1
+
1
s
R
f
+
g
L
according to figure 15, the
sR
1
−
+
3
−
m
sR
o
*
C
*
g
*
1
1
3
g
C
C
C
3
4
⎛
⎜ ⎜
⎝
Z
C
m
m
4
4
C
o
C
Z
≅
4
Z
in
+
*
4
4
* )
IN
C
C
π
f
>>
+
*
-
POLE
POLE
[
C
*
1
C
- -
R
3
1
+
3
(16)
1
3
⎞
⎟ ⎟
⎠
sC
⎤
⎥
⎦
*
( *
F
7
-
s
(
1
R
- -
+
8
POLE
sR
(17)
+
R
10
10
C
:
)
7
]
)
H(s) dB
As known, the transconductance amplifier has a
high
therefore, consideration should be taken when
loading the error amplifier output. It may exceed
its source/sink output current capability, so that
the amplifier will not be able to swing its output
voltage over the necessary range.
The compensation network has three poles and
two zeros and they are expressed as follows:
Cross over frequency is expressed as:
F
F
F
F
F
Fig.15: Compensation network with local
Z
P
P
P
1 z
z
2
1
2
3
IN
Gain(dB)
=
=
=
=
=
F
R
feedback and its asymptotic gain plot
0
2
2
2
2
o
IR3629/IR3629A MPbF
C
impedance
10
π
π
π
π
=
7
*
F
*
*
*
R
R
V
C
Z
R
R
1
3
1
1
OUT
3
7
*
10
3
C
⎛
⎜ ⎜
⎝
*
( *
R
1
R
1
*
C
C
C
7
8
9
C
R
*
4
4
4
F
7
8
V
+
V
*
Z
osc
V
+
C
in
C
2
Fb
REF
R
3
3
*
(current
10
⎞
⎟ ⎟
⎠
2
)
π
≅
≅
*
2
R
1
L
2
π
3
F
o
E/A
π
P
*
*
2
C
*
R
1
C
o
1
C
3
source)
7
*
3
*
C
R
F
3
C
8
P
Comp
3
4
Frequency
output,
Z
Ve
f
15