ir3821 International Rectifier Corp., ir3821 Datasheet - Page 14

• Manufacturer Part Number: ir3821
• Description: Wide-input Voltage, Synchronous Buck Regulator
• Manufacturer: International Rectifier Corp.

To cancel one of the LC filter poles, place the
zero before the LC filter resonant frequency pole:
Use equations (15) and (16) to calculate C4.
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 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 a configuration, the transfer function is
given by:
The error amplifier gain is independent of the
transconductance under the following condition:
By replacing Z
transformer function can be expressed as:
H
11/04/08
(
g
s
)
m
=
*
sR
Z
C
For
f
F
F
8
z
z
POLE
>>
(
C
=
=
1
4
F
75
0
1
F
P
=
+
.
in
P
75
%
<<
and
C
π
V
V
=
and Z
F
3
*
e
o
2
*
LC
)
F
2
π
2
R
*
=
s
π
3
*
⎡
⎢
⎣
1
(
1
g
R
*
1
f
1
1
L
according to figure 15, the
+
1
m
F
3
+
+
o
−
s
*
*
sR
*
sR
1
g
−
C
g
C
C
Z
m
3
4
4
C
o
m
in
3
1
⎛
⎜ ⎜
⎝
Z
+
C
*
Z
4
C
C
>>
C
C
IN
4
f
≅
4
-
* )
4
POLE
POLE
POLE
- -
+
*
1
π
[
C
1
C
(17)
*
3
+
3
:
R
⎞
⎟ ⎟
⎠
1
sC
-
⎤
⎥
⎦
3
- -
( *
*
7
F
(
1
(18)
R
s
+
8
sR
+
R
10
10
C
)
7
]
)
H(s) dB
As known, the transconductance amplifier has
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
C
impedance
10
π
π
π
π
=
7
*
*
F
R
*
*
R
V
C
Z
1
R
R
3
1
1
OUT
3
7
*
10
3
⎛
⎜ ⎜
⎝
*
( *
C
R
1
R
1
*
C
C
C
7
8
9
C
R
4
*
4
4
7
8
V
+
*
V
+
osc
V
C
in
C
Fb
REF
F
R
3
3
(current
*
Z
10
⎞
⎟ ⎟
⎠
2
IR3821MPbF
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
PD-60331
R
F
3
C
8
P
Comp
3
4
Frequency
output,
Z
Ve
f
14