Datasheet

ADP1828
Rev. C | Page 23 of 36
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.
GAIN
FREQUENCY
PHASE
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
06865-039
Figure 38. LC Filter Bode Plot
The following equations are used for the calculation of the
compensation components as shown in Figure 39 and Figure 40:
I
Z
Z1
CR
f
π
=
2
1
(24)
)(2
1
FF
TOP
FF
Z2
RRC
f
+π
= (25)
HFI
HFI
Z
P1
CC
CC
R
f
+
π
=
2
1
(26)
FFFF
P2
CR
f
π
=
2
1
(27)
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
f
Z
f
P
C
HF
C
I
FB
R
Z
R
BOT
R
TOP
V
OUT
INTERNAL
VREF
EA
COMP
–1
S
L
O
P
E
–1
S
L
O
PE
PHASE
–180°
–270°
G
(dB)
06865-040
Figure 39. 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 39.
Calculate the compensation resistor, R
Z
, with the following
equation:
2
LC
IN
COESRRAMP
TOP
Z
fV
ffVR
R =
(28)
where:
f
CO
is chosen to be 1/10 of f
SW.
V
RAMP
is 1.0 V.
Next, choose the compensation capacitor to set the compensa-
tion zero, f
Z1
, to the lesser of ¼ of the crossover frequency or ½
of the LC resonant frequency
I
Z
SWCO
Z1
CR
ff
f
π
===
2
1
404
(29)
or
I
Z
LC
Z1
f
π
22 CR
f
==
1
(30)
Solving for C
I
in Equation 29 yields
SW
Z
I
fR
C
π
=
20
(31)
Solving for C
I
in Equation 30 yields
LC
Z
I
fR
C
π
=
1
(32)