Datasheet
LTC3442
15
3442fa
1.22V
R1
R2
3442 F07
FB
12
V
C
C
P1
V
OUT
11
–
+
ERROR
AMP
where f = switching frequency in MHz. Therefore fre-
quency selection is a compromise between the optimal
efficiency and the smallest solution size.
Closing the Feedback Loop
The LTC3442 incorporates voltage mode PWM control.
The control to output gain varies with operation region
(buck, boost, buck/boost), but is usually no greater than
15. The output filter exhibits a double pole response, as
given by:
f
LC
FILTER POLE
OUT
—
•• •
=
1
2 π
Hz
(in buck mode)
f
V
VLC
FILTER POLE
IN
OUT OUT
—
••••
=
2 π
Hz
(in boost mode)
where L is in henries and C
OUT
is in farads.
The output filter zero is given by:
f
RC
FILTER ZERO
ESR OUT
—
•• •
=
1
2 π
Hz
where R
ESR
is the equivalent series resistance of the
output cap.
A troublesome feature in boost mode is the right-half
plane zero (RHP), given by:
f
V
ILV
RHPZ
IN
OUT OUT
=
2
2• • • •π
Hz
The loop gain is typically rolled off before the RHP zero
frequency.
A simple Type I compensation network can be incorpo-
rated to stabilize the loop, but at a cost of reduced
bandwidth and slower transient response. To ensure
proper phase margin using Type I compensation, the loop
must be crossed over a decade before the LC double pole.
The unity-gain frequency of the error amplifier with the
Type I compensation is given by:
Most applications demand an improved transient response
to allow a smaller output filter capacitor. To achieve a
higher bandwidth, Type III compensation is required,
providing two zeros to compensate for the double-pole
response of the output filter. Referring to Figure 8, the
location of the poles and zeros are given by:
f
eRCP
Hz
which is extremely close to DC
f
RC
Hz
f
RC
Hz
f
RC
Hz
POLE
ZERO
ZP
ZERO
Z
POLE
ZP
1
3
1
1
2
1
2
2
1
232 11
1
2
1
21
1
2
≅
=
=
=
•• • •
()
•• •
•• •
•• •
π
π
π
π
where resistance is in ohms and capacitance is in farads.
Figure 7. Error Amplifier with Type I Compensation
APPLICATIO S I FOR ATIO
WUUU
Figure 8. Error Amplifier with Type III Compensation
1.22V
R1
R2
3442 F08
FB
12
V
C
C
P1
C
Z1
R
Z
V
OUT
11
C
P2
–
+
ERROR
AMP
f
RC
Hz
UG
P
=
1
21
1
•• •π
referring to Figure 7.