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LTC3113

3113f

APPLICATIONS INFORMATION

CLOSING THE FEEDBACK LOOP

The LTC3113 incorporates voltage mode PWM control.

The control-to-output gain varies with the operation region

(buck, buck-boost, boost), but is usually no greater than

15. The output ﬁlter exhibits a double pole response, as

given by:

FILTER _ POLE =

π LC

OUT

()

In Buck Region

()

FILTER _ POLE =

π LC

OUT

()

In Boost Region

()

where L is in Henries and COUT is in Farads. The output

ﬁlter zero is given by:

FILTER _ ZERO =

πR

ESRCOUT

()

where RESR is the equivalent series resistance out the

output capacitor in ohms.

A troublesome feature in the boost and buck-boost region

is the right-half plane (RHP) zero, given by:

RHPZ =

πI

OUTLVOUT

()

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 at the 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 4,

the unity-gain frequency of the error ampliﬁer with the

Type I compensation is given by:

UG =

πR2C

()

–

ERROR

AMP

0.6V

VOUT

3113 F04

CP1

Figure 4. Error Ampliﬁer with Type I Compensation

–

ERROR

AMP

0.6V

VOUT

3113 F05

CP2

CZ1

CP1

Figure 5. Error Ampliﬁer with Type III Compensation

Most applications demand an improved transient response

to allow a smaller output 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:

POLE1 =

π10

5R2C

()

ZERO1 =

πR

Z CP1

()

ZERO2 =

πR2C

()

POLE2 =

πR

Z CP2

()

POLE3 =

πR

P CZ1

()

where resistance is in Ohms and capacitance is in

Farads.

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