ir3841 International Rectifier Corp., ir3841 Datasheet - Page 21

• Manufacturer Part Number: ir3841
• Description: Integrated 8a Synchronous Buck Regulator
• Manufacturer: International Rectifier Corp.

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. The table below shows the
compensation
crossover frequency.
F
o
F
F
F
F
F
02/09/09
The compensation network has three poles and
two zeros and they are expressed as follows:
Cross over frequency is expressed as:
H(s) dB
=
P
P
P
Z
Z
1
1
Fig.15. Type III Compensation network and
its asymptotic gain plot
3
2
2
R
=
=
=
=
=
3
*
0
2
2
2
2
Z
π
C
..........
π
π
π
IN
7
Gain(dB)
*
*
*
*
*
R
R
C
R
R
1
C
V
1
V
10
3
7
10
3
osc
7
..........
⎛
⎜
⎜
⎝
in
*
( *
F
1
C
*
C
C
1
V
types
Z
C
R
*
4
1
4
4
OUT
7
8
2
+
*
..........
π
R
R
..........
C
+
C
..........
8
9
*
3
R
3
L
F
1
10
⎞
⎟
⎟
⎠
o
Z
V
and
2
..........
≅
)
*
Fb
REF
..........
..........
C
≅
2
o
π
2
..........
π
*
..........
location
R
R
*
..........
..........
1
E/A
C
3
F
3
1
P
*
7
..........
2
C
*
..........
C
3
R
..........
..........
3
..........
8
..........
..........
F
of
C
P
.....(29)
3
Comp
.......(27
4
......(26)
.....
..
Frequency
(30)
the
(31)
(28)
Z
Ve
f
)
The
potentially faster the load transient response.
However, the crossover frequency should be low
enough to allow attenuation of switching noise.
Typically, the control loop bandwidth or crossover
frequency is selected such that
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:
V
V
V
V
L
C
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 12uF at 1.8 V DC bias and
600 kHz frequency. It is this value that must be
used
compensation. The small signal value may be
obtained from the manufacturer’s datasheets,
design tools or SPICE models. Alternatively, they
may also be inferred from measuring the power
stage transfer function of the converter and
measuring the double pole frequency F
using equation (16) to compute the small signal
C
These result to:
F
F
F
Select crossover frequency: F
Since F
place the pole and zeros.
o
LC
ESR
s
in
o
osc
ref
o
o
=1uH
/2=300 kHz
=1.8V
=6x22uF, ESR=3mOhm each
.
=12V
=0.7V
=18.76 kHz
=1.8V
Compensator
Compensator
Type
Type
=4.4 MHz
higher
for
Type III
Type III
Type II
Type II
LC
<F
all
o
<F
the
F
o
s
computations
/2<F
≤
F
F
(
1/5
LC
LC
crossover
F
F
IR3841MPbF
ESR
<F
<F
F
F
LC
LC
~
ESR
ESR
<F
<F
ESR
ESR
1/10
, TypeIII is selected to
o
for overall stability.
vs F
vs F
o
o
<F
<F
<F
<F
)
*
o
o
o
ESR
ESR
F
<F
<F
=100 kHz
o
o
s
related
s
s
frequency,
/2
/2
Electrolytic
Electrolytic
Tantalum
Tantalum
Output
Output
Capacitor
Capacitor
Tantalum
Tantalum
Ceramic
Ceramic
to
LC
21
and
the
the