Datasheet

LTC3556
28
3556f
Closing the Feedback Loop
The LTC3556 incorporates voltage mode PWM control. The
control to output gain varies with operation region (buck,
boost, buck-boost), but is usually no greater than 20. The
output fi lter exhibits a double-pole response given by:
f
LC
Hz
FILTER POLE
OUT
_
•• •
=
1
2 π
where C
OUT
is the output fi lter capacitor.
The output fi lter zero is given by:
f
RC
Hz
FILTER ZERO
ESR OUT
_
•• •
=
1
2 π
where R
ESR
is the capacitor equivalent series resistance.
A troublesome feature in boost mode is the right-half plane
zero (RHP), and is given by:
f
V
ILV
Hz
RHPZ
IN
OUT OUT
=
2
2• • • •π
The loop gain is typically rolled off before the RHP zero
frequency.
A simple Type I compensation network (as shown in
Figure 5) can be incorporated to stabilize the loop but
at the cost of reduced bandwidth and slower transient
response. To ensure proper phase margin, the loop must
cross unity-gain a decade before the LC double pole.
The unity-gain frequency of the error amplifi er with the
Type I compensation is given by:
f
RCP
Hz
UG
=
1
211•• •π
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
OUT3
overshoot
at start-up.
The compensation network depicted in Figure 6 yields the
transfer function:
V
VRCC
sR C s R R C
C
OUT
3
3
1
11 2
1221 133
=
+
+++
()
•
()[()]
sssRCC sRC1212133+
⎡
⎣
⎤
⎦
+(|| )( )
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
APPLICATIONS INFORMATION
7
–
+
ERROR
AMP
0.8V
R1
R2
3556 F05
FB3
8
V
C3
C
P1
V
OUT3
Figure 5. Error Amplifi er with Type I Compensation
7
–
+
ERROR
AMP
0.8V
R1
R3
C3
R
FB
3556 F06
FB3
8
V
C3
C2
C1
R2
V
OUT3
Figure 6. Error Amplifi er with Type III Compensation