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IR3856WM Datasheet(PDF) 21 Page - International Rectifier

Part # IR3856WM
Description  HIGHLY EFFICIENT INTEGRATED 6A, SYNCHRONOUS BUCK REGULATOR
PDF  34 Pages
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Manufacturer  IRF [International Rectifier]
Direct Link  http://www.irf.com
Logo IRF - International Rectifier

IR3856WM Datasheet(HTML) 21 Page - International Rectifier

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Rev 9.0
21
PD-97506
IR3856WMPbF
The transfer function (V
e/Vo) is given by:
The (s) indicates that the transfer function varies
as a function of frequency. This configuration
introduces a gain and zero, expressed by:
First select the desired zero-crossover frequency
(F
o):
Use the following equation to calculate R3:
VOUT
VREF
R9
R8
C
POLE
C4
R3
Ve
FZ
F
POLE
E/A
Zf
Frequency
Gain(dB)
H(s) dB
Fb
Comp
Z IN
Fig. 14. Type II compensation network
and its asymptotic gain plot
(18)
.....
)
(
4
8
4
3
1
C
sR
C
sR
Z
Z
s
H
V
V
IN
f
o
e
+
−
=
−
=
=
()
(20)
........
..........
..........
*
*
(19)
.........
..........
..........
.........
4
3
8
3
2
1
C
R
F
R
R
s
H
z
π
=
=
()
s
ESR
o
F
F
F
*
1/10
~
1/5
F
and o ≤
>
(21)
.......
..........
..........
*
*
*
*
2
8
3
LC
in
ESR
o
osc
F
V
R
F
F
V
R
=
Where:
V
in = Maximum Input Voltage
V
osc = Oscillator Ramp Voltage
F
o = Crossover Frequency
F
ESR = Zero Frequency of the Output Capacitor
F
LC = Resonant Frequency of the Output Filter
R
8 = Feedback Resistor
To cancel one of the LC filter poles, place the
zero before the LC filter resonant frequency pole:
Use equations (20), (21) and (22) 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 the switching
frequency which results in the capacitor C
POLE:
For a general solution for unconditional stability
for any type of output capacitors, and a wide
range of ESR values, we should implement local
feedback with a type III compensation network.
The typically used compensation network for
voltage-mode controller is shown in figure 15.
Again, the transfer function is given by:
By replacing Z
in and Zf according to figure 15,
the transfer function can be expressed as:
(22)
.......
..........
..........
..........
*
*
.
%
o
o
z
LC
z
C
L
F
F
F
π
2
1
75
0
75
=
=
...(23)
..........
..........
..........
*
*
*
POLE
POLE
P
C
C
C
C
R
F
+
=
4
4
3
2
1
π
(24)
..
..........
..........
s
s
POLE
*F
*R
C
*F
*R
C
3
4
3
1
1
1
π
π
≅
−
=
IN
f
o
e
Z
Z
s
H
V
V
−
=
=
)
(
()
[
]
(25)
....
)
(
*
)
(
)
(
)
(
7
10
3
4
3
4
3
3
4
8
10
8
7
4
3
1
1
1
1
C
sR
C
C
C
C
sR
C
C
sR
R
R
sC
C
sR
s
H
+
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
+
+
+
+
+
−
=



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