V

REF

V

IN

V

OUT

Compensation

Network

+

_

Modulator

Filter

Current

Feedback

Network

+

_

V

C

(

)

(

)

ON

SW

TPS5429 x

K t

IN

OUT

6

f

FM

V

V

19.7

e

95

10

L

´

-

=

æ

ö

æ

ö

-

ç

÷

´

+

´

´ ç

÷

ç

÷

ç

÷

è

ø

è

ø

(

)

4

IN

C

6

IN

LOAD

V

FM

2

10

f

V

FM

95

10

1

2

R

-

-

´

´ ´

=

æ

ö

æ

ö

´

´

´

ç

÷

ç

÷

+

ç

÷

ç

÷

´

ç

÷

ç

÷

è

ø

è

ø

(

)

(

)

C

EA

CO

LOAD

OUT

f

K

20 log

1 2

f

2 R

C

æ

ö

ç

÷

= -

´

ç

÷

+ ´ p ´

´

´

´

è

ø

TPS54290, TPS54291, TPS54292

SLUS973 – OCTOBER 2009

www.ti.com

Figure 16. Small Signal Equivalent Block Diagram

To determine the components necessary for compensating the feedback loop, the controller frequency response

characteristics must be understood and the desired crossover frequency selected. The best results are obtained

if 10% of the switching frequency is used as this closed loop crossover frequency. In some cases, up to 20% of

the switching frequency is also possible.

With the output filter components selected, the next step is to calculate the DC gain of the modulator. For

TPS5429x:

(5)

where

•

K = 5.6 ×10

•

K = 1.5 × 10

•

K = 3.6 × 10

The overall DC gain of the converter control-to-output transfer function is approximated Equation 6.

(6)

The next step is to find the desired gain of the error amplifier at the desired crossover frequency. Assuming a

single-pole roll-off, us Equation 6 to evaluate the following expression at the desired crossover frequency.

(7)

where

•

14

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