11

LTC3401

3401fa

APPLICATIO S I FOR ATIO

components C5 and R6. The amount of feed forward

signal is attenuated with resistor R6 and is given by the

following relationship:

R

VR

V

VI

R

FF

IN

OUT

OUT

6

51 5

5

≈ 







••

• .

•

–

∆

where

The output filter zero is given by:

f

RC

Hz

FILTERZERO

ESR

OUT

=

1

2•

•

•

π

A troublesome feature of the boost regulator topology is

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

f

VR

LV

Hz

RHPZ

IN

O

O

=

2

2

2

•

•• •

π

At heavy loads this gain increase with phase lag can occur

at a relatively low frequency. The loop gain is typically

rolled off before the RHP zero frequency.

The typical error amp compensation is shown in Figure 4.

The equations for the loop dynamics are as follows:

f

C

Hz

which is extremelyclose to DC

f

RC

Hz

f

RC

Hz

POLE

C

ZERO

ZC

POLE

ZC

1

6

1

1

1

2

2

1

220 10

1

2

1

2

≈

=

≈

••

•

•

••

•

••

•

π

π

π

Closing the Feedback Loop

The LTC3401 uses current mode control with internal

adaptive slope compensation. Current mode control elimi-

nates the 2nd order filter due to the inductor and output

capacitor exhibited in voltage mode controllers, and sim-

plifies it to a single-pole filter response. The product of the

modulator control to output DC gain plus the error amp

open-loop gain equals the DC gain of the system.

G

V

I

CONTROL

IN

OUT

=

2•

The output filter pole is given by:

f

I

VC

Hz

FILTERPOLE

OUT

OUT

OUT

=

π ••

Figure 3

3

10

2

6

1

3404 F03

LTC3401

SHDN

MODE/SYNC

PGOOD

SW

FB

GND

4

7

8

9

5

R5

C3

LOAD FEED

FORWARD

SIGNAL

R6

C5

3.3nF

Refer to Application Note AN-76 for more closed loop

examples.

Figure 4

–

+

1.25V

FB

ERROR

AMP

8

3401 F04

R2

R1

9