Datasheet
Data Sheet ADP2325
Rev. A | Page 23 of 32
COMPENSATION COMPONENTS DESIGN
In peak current mode control, the power stage can be simplified
to a voltage controlled current source supplying current to the
output capacitor and load resistor. It is composed of one domain
pole and a zero contributed by the output capacitor ESR. The
control-to-output transfer function is shown in the following
equations:
××
+
××
+
××==
P
Z
VI
COMP
OUT
VD
f
s
f
s
RA
sV
sV
sG
π
π
2
1
2
1
)(
)(
)(
OUT
ESR
Z
CR
f
×××
=
π
2
1
( )
OUT
ESR
P
CRR
f
×+××
=
π
2
1
where:
A
VI
= 8.33 A/V.
R is the load resistance.
C
OUT
is the output capacitance.
R
ESR
is the equivalent series resistance of the output capacitor.
The ADP2325 uses a transconductance amplifier for the error
amplifier to compensate the system. Figure 51 shows the
simplified peak current mode control small signal circuit.
Figure 51. Simplified Peak Current Mode Control Small Signal Circuit
The compensation components, R
C
and C
C
, contribute a zero,
and the optional C
CP
and R
C
contribute an optional pole.
The closed-loop transfer equation is as follows:
(s)G
s
CC
CCR
s
sCR
CC
g
RR
R
(s)T
VD
CPC
CPCC
CC
CPC
m
TOPBOT
BOT
V
×
×
+
××
+×
××+
×
+
−
×
+
=
1
1
The following design guidelines show how to select the compen-
sation components, R
C
, C
C
, and C
CP
, for ceramic output capacitor
applications.
4. Determine the cross frequency (f
C
). Generally, the f
C
is
between f
SW
/12 and f
SW
/6.
5. R
C
can be calculated by using the following equation:
VI
m
C
OUTOUT
C
Ag
fCV
R
××
××××
=
V6.0
2
π
6. Place the compensation zero at the domain pole (f
P
).
C
C
can be determined by
( )
C
OUT
ESR
C
R
CRR
C
×+
=
7. C
CP
is optional. It can be used to cancel the zero caused by
the ESR of the output capacitor.
C
OUT
ESR
CP
R
CR
C
×
=
The ADP2325 has an internal 10 pF capacitor at the COMPx
pin; therefore, if C
CP
is smaller than 10 pF, no external capacitor
is required.
R
ESR
R
+
–
g
m
R
C
C
CP
C
OUT
C
C
R
TOP
R
BOT
–
+
A
VI
V
OUT
V
COMP
V
OUT
10036-047