LTC3533

13

3533f

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

LC

Hz

in buck

e

f

FILTER POLE

OUT

F

_

••

•

(

mod )

=

1

2

π

IILTER POLE

IN

OUT

OUT

V

VL C

Hz

in boost

_

•• •

•

(

=

2

π

m

mod )

e

The output filter zero is given by:

f

RC

Hz

FILTER ZERO

ESR

OUT

_

••

•

=

1

2

π

output capacitor.

A troublesome feature in boost mode is the right-half plane

zero (RHP), given by:

f

V

IL V

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 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

RC

Hz

UG

P

••

•

=

1

21

1

π

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

eR C

Hz

which is a

POLE

P

1

3

1

1

210

1

••

•

•

(

=

π

very llow frequency)

••

•

f

RC

Hz

f

ZERO

ZP

ZERO

1

1

2

1

2

=

π

==

=

1

21

1

2

1

2

2

••

•

••

•

π

π

RC

Hz

f

RC

Hz

Z

POLE

ZP

where resistance is in Ohms and capacitance is in Farads.

1.22V

R1

R2

3533 F05

FB

12

11

–

+

ERROR

AMP

1.22V

R1

R2

3533 F06

FB

12

11

–

+

ERROR

AMP

Figure 5. Error Amplifier with Type I Compensation

Figure 6. Error Amplifier with Type III Compensation