iru3038 International Rectifier Corp., iru3038 Datasheet - Page 9

• Manufacturer Part Number: iru3038
• Description: Synchronous Pwm Controller For Termination Power Supply Applications
• Manufacturer: International Rectifier Corp.

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 7.
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. 2.0
09/12/02
H(s) dB
g
V
V
m
Figure 7 - Compensation network with local
OUT
Z
e
sR
Z
IN
feedback and its asymptotic gain plot.
f
Gain(dB)
>> 1
6
=
(C
R
C
8
1
1 +
12
10
1 -
+C
F
IN
V
Z
g
g
1
and Z
OUT
11
and
m
m
)
R
R
3
Z
Z
6
5
f
IN
[
F
f
1+sR
Vp=V
Z
according to Figure 7, the trans-
(1+sR
2
Fb
g
m
Z
REF
7
IN
(
7
C
C
R
>>1
C
11
12
7
F
)3[1+sC
12
E/A
+C
P
C
2
11
11
C
)]
12
F
3(1+sR
C
10
P
Comp
3
(R
11
---(14)
Frequency
6
+R
Z
8
8
)]
C
V e
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 (14) regarding transconduc-
tance error amplifier.
1) Select the crossover frequency:
2) Select R
3) Place first zero before LC’s resonant frequency pole.
4) Place third pole at the half of the switching frequency.
5) Place R
F
F
F
F
F
F
Where:
V
V
Lo = Output Inductor
Co = Total Output Capacitors
Fo < F
F
C
F
C
C
If not, change R
C
P1
P2
P3
Z1
Z2
O
Z1
P3
IN
OSC
11
12
12
10
= R
=
= 0
=
=
=
=
= Maximum Input Voltage
=
=
> 50pF
[
= Oscillator Ramp Voltage
75% F
2p3R
2p3R
2p3C
2p 3 R
2p3R
2p 3 F
f
2
7
ESR
S
2p 3 Lo 3 F
3C
7
7
, so that R
in equation (15) and calculate C
and Fo [ (1/10 ~ 1/6)3 f
10
1
1
LC
8
7
3
10
7
1
3C
3C
1
3
Z1
7
3(R
1
3 F
(
V
7
3 R
V
R
10
1
11
selection.
OSC
C
IN
C
7
6
12
12
P3
7
+ R
3
3C
7
O
+C
>>
3 Co
2p3Lo3Co
8
11
)
11
g
)
2
m
1
2p3C
3
2p3R
V
V
OSC
IRU3038
1
IN
S
10
1
3R
7
3C
---(15)
6
10
12
:
9