IR3623MPBF_1 IRF [International Rectifier], IR3623MPBF_1 Datasheet - Page 19

• Manufacturer Part Number: IR3623MPBF_1
• Description: HIGH FREQUENCY 2-PHASE, SINGLE OR DUAL OUTPUT SYNCHRONOUS STEP DOWN CONTROLLER WITH OUTPUT TRACKING AND SEQUENCING
• Manufacturer: IRF [International Rectifier]

Feedback Compensation
The IR3623 is a voltage mode controller; the
control loop is a single voltage feedback path
including error amplifier and error comparator. To
achieve fast transient response and accurate
output regulation, a compensation circuit is
necessary. The goal of the compensation
network is to provide a closed loop transfer
function with the highest 0dB crossing frequency
and adequate phase margin (greater than 45
The output LC filter introduces a double pole, –
40dB/decade gain slope above its corner
resonant frequency, and a total phase lag of 180
(see figure 18). The resonant frequency of the LC
filter expressed as follows:
figure 16 shows gain and phase of the LC filter.
Since we already have 180
the output filter, the system risks being unstable.
The IR3623’s error amplifier is a differential-input
transconductance
available for DC gain control or AC phase
compensation.
The E/A can be compensated either in type II or
typeIII compensation. When it is used in typeII
compensation the transconductance properties of
the E/A become evident and can be used to
cancel one of the output filter poles. This will be
accomplished with a series RC circuit from Comp
pin to ground as shown in figure 19.
This method requires that the output capacitor
should have enough ESR to satisfy stability
requirements. In general the output capacitor’s
ESR generates a zero typically at 5kHz to 50kHz
which is essential for an acceptable phase
margin. The ESR zero of the output capacitor
expressed as follows:
www.irf.com
0dB
Gain
Fig. 18: Gain and Phase of LC filter
F
LC
=
F
2
LC
∗
Frequency
π
-40dB/decade
1
L
o
∗
amplifier.
C
o
-180
0
Phase
-
o
- -
phase shift just from
-(14)
The
F
LC
output
Frequency
o
).
is
o
The transfer function (Ve/Vo) is given by:
The (s) indicates that the transfer function varies
as a function of frequency. This configuration
introduces a gain and zero, expressed by:
The gain is determined by the voltage divider and
E/A’s transconductance gain.
First select the desired zero-crossover frequency
(Fo):
Use the following equation to calculate R
Where:
V
V
F
F
F
g
m
o
ESR
LC
in
osc
= Crossover Frequency
= Error Amplifier Transconductance
= Maximum Input Voltage
= Resonant Frequency of the Output Filter
= Oscillator Ramp Voltage
H
= Zero Frequency of the Output Capacitor
(
s
R
Fig. 19: TypeII compensation network
V
)
4
OUT
[
F
=
H(s) dB
H
=
z
F
R
⎛
⎜ ⎜
⎝
F
R
( )
=
V
o
s
g
ESR
6
and its asymptotic gain plot
5
osc
m
>
2
]
V
=
π
*
F
V
=
*
REF
Gain(dB)
Fb
R
ESR
⎛
⎜ ⎜
⎝
F
in
*
2
g
o
5
R
*
1
R
∗
m
*
+
and
F
4
π
5
F
*
LC
*
R
2
ESR
R
C
*
6
*
ESR
5
9
⎞
⎟ ⎟
⎠
F
E/A
R
1
R
( *
+
o
*
5
F
5
1
≤
R
R
*
Z
IR3623MPbF
+
5
g
6
-
(
*
1/5
sC
sR
⎞
⎟ ⎟
⎠
- -
+
m
C
Frequency
*
Comp
R
o
9
-(18)
R
4
~
6
C
4
)
1/10
9
R
C
-
4
- -
9
-
-
Ve
)
-
- -
- -
-(15)
*
- -
F
-(19)
-(17)
s
-(16)
C
4
:
POLE
19