Datasheet

ADP1828
Rev. C | Page 24 of 36
Use the larger value of C
I
from Equation 31 or Equation 32.
Because of the finite output current drive of the error amplifier,
C
I
needs to be less than 10 nF. If it is larger than 10 nF, choose a
larger R
TOP
and recalculate R
Z
and C
I
until C
I
is less than 10 nF.
Next, choose the high frequency pole, f
P1
, to be ½ of f
SW
.
SW
P1
ff
2
1
= (33)
Because C
HF
<< C
I
, Equation 26 is simplified to
HF
Z
P1
CR
f
π
=
2
1
(34)
Combine Equation 33 and Equation 34, and solve for C
HF
,
Z
SW
HF
Rf
C
π
=
1
(35)
Type III Compensator
G
(dB)
PHASE
–270°
–90°
f
Z
f
P
C
HF
C
I
R
Z
C
FF
R
TOP
R
BOT
V
OUT
INTERNAL
VREF
FB
EA
COMP
R
FF
–
1
S
L
O
P
E
–
1
S
L
O
P
E
+
1
S
L
O
P
E
06865-041
Figure 40. Type III Compensation
If the output capacitor ESR zero frequency is greater than ½ of
the crossover frequency, use the Type III compensator as shown
in Figure 40. Set the poles and zeros as follows:
SW
P2P1
fff
2
1
== (36)
I
Z
SWCO
Z2Z
CR
ff
ff
π
====
2
1
404
1
(37)
or
I
Z
LC
Z2Z
CR
f
ff
π
===
2
1
2
1
(38)
Use the lower zero frequency from Equation 37 or Equation 38.
Calculate the compensator resistor, R
Z
(39)
Next, calculate C
I
,
2
LC
IN
CO
Z1
RAMP
TOP
Z
fV
ffVR
R =
Z1Z
I
fR
C
π
=
2
1
(40)
Because of the finite output current drive of the error amplifier,
C
I
needs to be less than 10 nF. If it is larger than 10 nF, choose a
C
I
until C
I
is less than 10 nF.
Because C
HF
<< C
I
, combining Equation 26 and Equation 36
yields
larger R
TOP
and recalculate R
Z
and
Z
SW
HF
C =
1
Rfπ
(41)
Next, calculate the feedforward capacitor C
FF
. Assuming R
FF
<<
lified to R
TOP
, then Equation 25 is simp
TOP
FF
Z2
RC
f
π
=
2
1
(42)
Solving C
FF
in Equation 42 yields
Z2TOP
FF
fR
C
π
=
2
1
(43)
where f
Z2
is obtained from Equation 37 or Equation 38.
The feedforward resistor, R
FF
, can be calculated by combining
Equation 27 and Equation 36
SW
FF
FF
fC
R
π
=
1
(44)
Check that the calculated component values are reasonable. For
instance, capacitors smaller than about 10 pF should be avoided.
In addition, the ADP1828 error amplifier has a finite output
current drive, so R
Z
values less than 3 k and C
I
values greater
than 10 nF should be avoided. If necessary, recalculate the compen-
sation network with a different starting value of R
TOP
. If R
Z
is too
small or C
I
is too big, start with a larger value of R
TOP
. This com-
pensation technique should yield a good working solution.
olytic capacitors have high ESR, and
uate. However, if several aluminum
electrolytic capacitors are connected in parallel, and produce a
ve ESR, then Type III compensation is needed. In
addition, ceramic capacitors have very low ESR (only a few
milliohms) making Type III compensation a better choice.
Type III compensation offers better performance than Type II
in terms of more low frequency gain and more phase margin
and less high frequency gain at the crossover frequency.
In general, aluminum electr
Type II compensation is adeq
low effecti