IR3897 IRF [International Rectifier], IR3897 Datasheet - Page 30

• Manufacturer Part Number: IR3897
• Description: 4A HIGHLY INTEGRATED SUPLRBUCK
• Manufacturer: IRF [International Rectifier]

Again, the transfer function is given by:
By replacing Z
function can be expressed as:
The compensation network has three poles and two
zeros and they are expressed as follows:
H s
|H(s)| dB
( )
F
F
F
P
P
P
1
3
2

 
Gain (dB)


Z
Figure 27: Type III Compensation network
0
2 *

2 *

IN


sR C
C4
R4
30
in
5
R
R C
1
and Z
(
and its asymptotic gain plot
3
4
F
V



*
V
V
Z
2
1
out
C
C C
(1
1
OUT
e

AUGUST 08, 2012 |DATA SHEET | Rev 3.3
R5
2
R6
4
2

f
C

,
*


according to Fig. 27, the transfer
3
sR C
C
) 1
F
H
3



3
3
Z
V



(
2
Fb
REF

3
s

) 1
)
sR


2 *


3
R3





sC
C
C
E
F
R C
Z
1
Z
P
2
2
3
/
IN
4
2
f
*

*
Single‐Input Voltage, Synchronous Buck Regulator

A
C
R
C
C2
C3
3
2
4
3





F



R
P
(1
5
3
Comp


(28)
(30)


(29)
Frequency
sR C
Z
Ve
4
(27)
f
4
- 30 -
)
4A Highly Integrated SupIRBuck
Cross over frequency is expressed as:
Based on the frequency of the zero generated by the output
capacitor and its ESR, relative to crossover frequency, the
compensation type can be different. Table 3 shows the
compensation types for relative locations of the crossover
frequency.
T
The higher the crossover frequency is, the potentially faster
the load transient response will be. However, the crossover
frequency should be low enough to allow attenuation of
switching noise. Typically, the control loop bandwidth or
crossover frequency (F
The DC gain should be large enough to provide high
DC‐regulation accuracy. The phase margin should be greater
than 45
For this design we have:
It must be noted here that the value of the capacitance used
in the compensator design must be the small signal value.
For instance, the small signal capacitance of the 22uF
capacitor used in this design is 10uF at 1.2 V DC bias and
600 kHz frequency. It is this value that must be used for all
ABLE
Compensator
F
F
F
V
V
V
section)
V
L
C
Z
Z
o
o
o
in
o
osc
ref
1
=1.5uH
2
Type III
3:
=1.2V
=4x22uF, ESR≈3mΩ each
Type II

=12V
Type
=0.5V

=1.8V (This is a function of Vin, pls. see feed forward

o
D
R C
for overall stability.
2 *
2 *
IFFERENT TYPES OF COMPENSATORS
3


*
R C
C
4
1
3
*
4
*
* (
V
V
1
osc
F
in
F
R
3
LC
o

o
4
< F
*
) is selected such that:
F


LC
2 *
F

< F
ESR
ESR
1/5
R

5
< F
vs F
O
)
< F
L C
~

1
O
o
< F
1/10
ESR
O
2 *
*
TM

S
/2
o
C

1
*
4
F
*
s
IR3897
R
Typical Output
SP Cap, Ceramic
5
Electrolytic
Capacitor

PD‐97663
(33)
(31)
(32)