LTC3530

12

3530fb

where f = switching frequency in MHz. Therefore frequency

selection is a compromise between the optimal efficiency

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 filter exhibits a double pole response, as

given by:

=

1

Hz

(in buck mode)

=

Hz

(in boost mode)

The output filter zero is given by:

=

1

Hz

output capacitor.

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

zero (RHP), given by:

2

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. The

unity-gain frequency of the error amplifier with the Type

I compensation is given by:

1

Hz (referring to Figure 4).

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 5, the location of the

poles and zeros are given by:

1

2• • 32,000 •R1• CP1

Hz

(which is extremely close to DC)

1

Hz

1

Hz

1

Hz

where resistance is in ohms and capacitance is in

farads.

1.215V

R1

R2

3530 F03

FB

12

11

–

+

ERROR

AMP

1.215V

R1

R2

3530 F04

FB

12

11

–

+

ERROR

AMP

Figure 4. Error Amplifier with Type I Compensation

Figure 5. Error Amplifier with Type III Compensation

APPLICATIONS INFORMATION