IRU3137CS IRF [International Rectifier], IRU3137CS Datasheet - Page 11

• Manufacturer Part Number: IRU3137CS
• Description: 8-PIN SYNCHRONOUS PWM CONTROLLER
• Manufacturer: IRF [International Rectifier]

For a general solution for unconditionally stability for
ceramic capacitor with very low ESR and any type of
output capacitors, in a wide range of ESR values we
should implement local feedback with a compensation
network. The typically used compensation network for
voltage-mode controller is shown in Figure 10.
In such configuration, the transfer function is given by:
The error amplifier gain is independent of the transcon-
ductance under the following condition:
By replacing Z
former function can be expressed as:
H(s) =
As known, transconductance amplifier has high imped-
ance (current source) output, therefore, consider should
be taken when loading the E/A output. It may exceed its
source/sink output current capability, so that the ampli-
fier will not be able to swing its output voltage over the
necessary range.
The compensation network has three poles and two ze-
ros and they are expressed as follows:
Rev. 1.0
06/22/04
H(s) dB
g
V
Figure 10 - Compensation network with local
m
V
OUT
Z
Z
sR
e
IN
f
feedback and its asymptotic gain plot.
Gain(dB)
>> 1
6
=
(C
R
C
8
1
12
1 + g
10
1 - g
+C
F
IN
V
Z
and Z
1
11
OUT
and
m
m
)
R
Z
R
Z
6
IN
5
f
[
F
f
1+sR
Vp=V
Z
according to Figure 7, the trans-
(1+sR
2
g
Fb
m
Z
REF
IN
7
(
7
C
>>1
C
R
C
11
12
7
F
12
E/A
) [1+sC
+C
P
C
2
11
11
C
)]
12
F
10
C
P
---(20)
(1+sR
Comp
(R
11
3
Frequency
6
+R
8
8
Z
C
)]
Ve
f
www.irf.com
10
)
Cross Over Frequency:
The stability requirement will be satisfied by placing the
poles and zeros of the compensation network according
to following design rules. The consideration has been
taken to satisfy condition (20) regarding transconduc-
tance error amplifier.
These design rules will give a crossover frequency ap-
proximately one-tenth of the switching frequency. The
higher the band width, the potentially faster the load tran-
sient speed. The gain margin will be large enough to
provide high DC-regulation accuracy (typically -5dB to -
12dB). The phase margin should be greater than 45 for
overall stability.
Based on the frequency of the zero generated by ESR
versus crossover frequency, the compensation type can
be different. The table below shows the compensation
type and location of crossover frequency.
Type II (PI)
Type III (PID)
Method A
Type III (PID)
Method B
Detail information is dicussed in application Note AN-
1043 which can be downloaded from the IR Web-Site.
Compensator
Table - The compensation type and location of zero
F
F
F
F
F
F
Where:
V
V
Lo = Output Inductor
Co = Total Output Capacitors
Type
P1
P2
P3
Z1
Z2
O
IN
OSC
= R
= Maximum Input Voltage
= 0
=
=
=
=
= Oscillator Ramp Voltage
2
2
2
2
7
C
R
R
C
R
10
1
1
Crossover Frequency
7
8
10
7
crossover frequency.
F
F
F
Location of Zero
PO
1
PO
PO
C
C
V
(
V
(R
11
10
1
OSC
< F
< F
< F
C
IN
C
6
12
12
+ R
O
ZO
O
+C
(F
< f
< F
C
< F
2
O
8
11
11
)
)
S
ZO
/2 < F
)
O
≅
< f
< f
Lo Co
≅
1
2
S
S
2
/2
/2
ZO
C
IRU3137
1
10
R
1
Electrolytic,
7
Capacitor
Tantalum,
Tantalum
Ceramic
Ceramic
Typical
R
Output
C
---(21)
6
12
11