LTC3558
27
3558f
Figure 11. Error Amplifi er with Type III Compensation
Most applications demand an improved transient response
to allow a smaller output fi lter capacitor. To achieve a higher
bandwidth, Type III compensation is required. Two zeros
are required to compensate for the double-pole response.
Type III compensation also reduces any V
OUT2
overshoot
seen during a start-up condition. A Type III compensa-
tion circuit is shown in Figure 11 and yields the following
transfer function:
V
V
R C
C
sR C
s R
R C
C
OUT
2
2
1
1 1
2
1
2 2 1
1
3 3
=
+
+
+
+
(
)
•
(
) [
(
)
]
ss
sR C
C
sR C
1
2 1
2
1
3 3
+
⎡⎣
⎤⎦
+
(
||
) (
)
A Type III compensation network attempts to introduce
a phase bump at a higher frequency than the LC double
pole. This allows the system to cross unity gain after the
LC double pole, and achieve a higher bandwidth. While
attempting to cross over after the LC double pole, the
system must still cross over before the boost right-half
plane zero. If unity gain is not reached suffi ciently before
the right-half plane zero, then the –180° of phase lag from
the LC double pole combined with the –90° of phase lag
from the right-half plane zero will result in negating the
phase bump of the compensator.
The compensator zeros should be placed either before
or only slightly after the LC double pole such that their
positive phase contributions offset the –180° that occurs
at the fi lter double pole. If they are placed at too low of a
frequency, they will introduce too much gain to the system
and the crossover frequency will be too high. The two high
frequency poles should be placed such that the system
crosses unity gain during the phase bump introduced
by the zeros and before the boost right-half plane zero
and such that the compensator bandwidth is less than
the bandwidth of the error amp (typically 900kHz). If the
gain of the compensation network is ever greater than
the gain of the error amplifi er, then the error amplifi er no
longer acts as an ideal op amp, and another pole will be
introduced at the same point.
Recommended Type III compensation components for a
3.3V output are:
R1:
324k
Ω
R
FB
: 105k
Ω
C1:
10pF
R2:
15kΩ
C2:
330pF
R3:
121k
Ω
C3:
33pF
C
OUT
: 22μF
L
OUT
: 2.2μH
APPLICATIONS INFORMATION
R1
C3
R
FB
3558 F11
0.8V
FB2
V
C2
C2
R2
V
OUT2
–
+
ERROR
AMP
C1
R3
