LTC3530 LINER [Linear Technology], LTC3530 Datasheet - Page 12

• Manufacturer Part Number: LTC3530
• Description: Wide Input Voltage Synchronous Buck-Boost DC/DC Converter
• Manufacturer: LINER [Linear Technology]

LTC3530
APPLICATIONS INFORMATION
where f = switching frequency in MHz. Therefore frequency
selection is a compromise between the optimal effi ciency
and the smallest solution size.
Closing the Feedback Loop
The LTC3530 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 fi lter exhibits a double pole response, as
given by:
where L is in henries and C
The output fi lter zero is given by:
where R
output capacitor.
A troublesome feature in boost mode is the right-half plane
zero (RHP), given by:
12
(in buck mode)
(in boost mode)
f
f
f
f
FILTER
FILTER
FILTER
RHPZ
Figure 4. Error Amplifi er with Type I Compensation
ESR
=
—
—
—
POLE
POLE
2 • •I
ZERO
is the equivalent series resistance of the
=
=
=
OUT
ERROR
2 • • L • C
2 • V
2 • • R
AMP
V
IN
• L • V
+
–
OUT
2
1
ESR
• • L • C
OUT
1
OUT
FB
12
V
11
V
C
IN
1.215V
• C
OUT
is in farads.
Hz
OUT
C
P1
Hz
Hz
OUT
V
OUT
Hz
R1
R2
3530 F03
The loop gain is typically rolled off before the RHP zero
frequency.
A simple Type I compensation network can be incorporated
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 amplifi er with the Type
I compensation is given by:
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, providing
two zeros to compensate for the double-pole response of
the output fi lter. Referring to Figure 5, the location of the
poles and zeros are given by:
where resistance is in ohms and capacitance is in
farads.
(which is extremely close to DC)
f
f
f
f
f
UG
POLE1
POLE2
ZERO1
ZERO2
Figure 5. Error Amplifi er with Type III Compensation
=
2 • • R1• C
=
=
=
2 • • 32,000 • R1• CP1
2 • • R
2 • • R
2 • • R1• C
ERROR
AMP
1
+
–
1
1
1
Z
Z
P1
• C
• C
1
FB
12
V
11
Hz (referring to Figure 4).
C
Z1
P1
1.215V
P2
Hz
Hz
Hz
R
Z
C
P2
C
Hz
P1
3530 F04
V
OUT
R1
R2
C
Z1
3530fb