Datasheet
ADP1821
Rev. C | Page 16 of 24
Two common compensation schemes are used, which are
sometimes referred to as Type II or Type III compensation,
depending on whether the compensation design includes two
or three poles. (Dominant pole compensations, or single pole
compensation, is referred to as Type I compensation, but it is
not very useful for dealing successfully with switching regulators.)
If the zero produced by the ESR of the output capacitor provides
sufficient phase boost at crossover, Type II compensation is
adequate. If the phase boost produced by the ESR of the output
capacitor is not sufficient, another zero is added to the compen-
sation network, and thus Type III is used.
In
Figure 17, the location of the ESR zero corner frequency gives
significantly different net phase at the crossover frequency.
GAIN
FREQUENCY
PHASE
LC FILTER BODE PLOT
PHASE CONTRIBUTION AT CROSSOVER
OF VARIOUS ESR ZERO CORNERS
f
SW
f
CO
f
ESR3
f
ESR2
f
ESR1
0dB
f
LC
–40dB/dec
–20dB/dec
0°
–90°
–180°
Φ
1
Φ
2
Φ
3
05310-016
Figure 17. LC Filter Bode Plot
Use the following guidelines for selecting between Type II and
Type III compensators.
If
2
CO
ESRZ
f
f ≤
, use Type II compensation.
If
2
CO
ESRZ
f
f >
, use Type III compensation.
The following equations were used for the calculation of the
compensation components as shown in
Figure 18 and Figure 19:
IZ
Z
CR
f
π
2
1
1
=
(27)
)(2
1
2
FFTOPFF
Z
RRC
f
+
=
π
(28)
HFI
HFI
Z
P
CC
CC
R
f
+
=
π
2
1
1
(29)
FFFF
P
CR
f
π
2
1
2
=
(30)
where:
f
Z1
is the zero produced in the Type II compensation.
f
Z2
is the zero produced in the Type III compensation.
f
P1
is the pole produced in the Type II compensation.
f
P2
in the pole produced in the Type III compensation.
Type II Compensator
G
(dB)
PHASE
–180°
–270°
f
Z
f
P
0V
VRAMP
C
HF
C
I
R
Z
R
TOP
R
BOT
FROM
V
OUT
VREF
EA
COMP
TO PWM
–
1
S
L
O
P
E
–
1
S
L
O
P
E
0
5310-017
Figure 18. Type II Compensation
If the output capacitor ESR zero frequency is sufficiently low (≤ ½
of the crossover frequency), use the ESR to stabilize the regulator.
In this case, use the circuit shown in
Figure 18. Calculate the
compensation resistor, Rz, with the following equation:
2
LCIN
COESRRAMPTOP
Z
fV
ffVR
R =
(31)
where:
f
CO
is chosen to be 1/10 of f
SW.
V
RAMP
is 1.25 V.
Next, choose the compensation capacitor to set the compensation
zero,
f
Z1
, to the lesser of ¼ of the crossover frequency or ½ of the
LC resonant frequency
IZ
SWCO
Z
CR
ff
f
π
2
1
404
1
===
(32)
or
IZ
LC
Z
CR
f
f
π
2
1
2
1
==
(33)
Solving for C
I
in Equation 32 yields
SWZ
I
fR
C
π
20
=
(34)