IR3897 IRF [International Rectifier], IR3897 Datasheet - Page 29

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

The ESR zero of the output capacitor is expressed as
follows:
H (s) dB
The transfer function (V
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
F
o
V
V

out
Z
e
F
IN
Figure 26: Type II compensation network

G ain (dB )
F
ESR
H s
ESR
29
F
( )
H s
z
F
and its asymptotic gain plot
and F


 
V
Z
 
2
O U T
2 *

AUGUST 08, 2012 |DATA SHEET | Rev 3.3
π* ESR* C
R 5
R 6

Z
Z
o
R
1
IN
e
R
f
R
1
/V
V
3

5
3
Fb
*
R EF
 
out

C

) is given by:
3
1/5~1/10 *
1
F

o
R 3
P O LE

sR C

sR C
E /A
5
3
3
Single‐Input Voltage, Synchronous Buck Regulator
(20)
(21)
C
3
C 3

(18)
PO LE

F
C om p
s
(19)

F requency
(22)
o
Z
):
V e
f
- 29 -
4A Highly Integrated SupIRBuck
Use the following equation to calculate R3:
Where:
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 C3.
One more capacitor is sometimes added in parallel with C3
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, and a wide range of ESR values, a type III
compensation network can be used, as shown in Fig. 27.
C
V
V
F
F
F
R
o
ESR
LC
5
in
osc
F
POLE
= Crossover Frequency
= Feedback Resistor
= Maximum Input Voltage
= Resonant Frequency of the Output Filter
P
= Amplitude of the oscillator Ramp Voltage
= Zero Frequency of the Output Capacitor

F
F

z
z
2 *


R


3
75 % *
0.75*
* R * F

R
V
3
osc
3
*
2
F

1
*
C
1
C
LC
V
F F
3
3
in
s
L C
o
1

POLE
*
o

*
*
C
*
C
F
C
:
POLE
1
POLE
LC
ESR
2
o
3


*

R

TM
5
* R * F

1
3
IR3897
(23)
s

(24)
PD‐97663
(25)
(26)