LTC3533 Datasheet, PDF (13/16 Pages) Linear Technology – 2A Wide Input Voltage Synchronous Buck-Boost DC/DC Converter

LTC3533
APPLICATIONS INFORMATION
Closing the Feedback Loop
The LTC3533 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 _{FILTER_POLE} = _{2 • π •}
1
Hz
L • C_OUT
(in buck mode)
f _{FILTER _POLE}
=
2•
V_OUT
V_IN
•π•
Hz
L • C_OUT
(in boost mode)
where L is in Henries and C_OUT is in Farads.
The output filter zero is given by:
f
FILTER
_
ZERO
=
2
•
π
•
1
R_ESR
•
C_OUT
Hz
where R_ESR is the equivalent series resistance of the
output capacitor.
A troublesome feature in boost mode is the right-half plane
zero (RHP), given by:
f
RHPZ
=
2
•
π
V_IN2
•I_OUT • L
•
V_OUT
Hz
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. Referring
to Figure 5, the unity-gain frequency of the error amplifier
with the Type I compensation is given by:
f
UG
=
2
•
π
•
1
R1•
C_P1
Hz
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 6, the location of the
poles and zeros are given by:
f
POLE1
=
2
•
π
•
1
10e³
•
R1•
C_P1
Hz
(which is a very low frequency)
f
ZERO1
=
2
•
π
1
• R_Z
•
Hz
C_P1
f
ZERO2
=
2
•
π
•
1
R1•
C_Z1
Hz
f
POLE2
=
2
•
π
1
• R_Z
• C_P2
Hz
where resistance is in Ohms and capacitance is in Farads.
V_OUT
ERROR
AMP
1.22V
FB
12
V_C
C_P1
11
V_OUT
R1
R2
3533 F05
Figure 5. Error Amplifier with Type I Compensation
ERROR
AMP
1.22V
FB
12
R1
C_Z1
V_C
R_Z ^CP1
R2
11
C_P2
3533 F06
Figure 6. Error Amplifier with Type III Compensation
3533f
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