( ) ( )

(

)

( ) ( )

2

2

OH

OL

OUT

2

2

f

i

L

I

I

C

V

V

´

-

=

-

(6)

INPUT CAPACITOR SELECTION

( )

(

)

I

V

OUT

OUT

V

RIPPLE C

C

V

f

IN

IN

SW

IN m in

´

=

´

´

(7)

(

)

1

V

ESR

I

I

C

OUT

RIPPLE

RIPPLE ESR _ C

2

IN

IN

æ

ö

=

´

+ ´

ç

÷

è

ø

(8)

CHOOSING MOSFETS

(

I

2

RIPPLE

I

D

I

RMS1

OUT

12

æ

ö

ç

÷

=

´

+

ç

÷

ç

÷

è

ø

(9)

(

)

( )

2

COND1

RMS1

DS ON 1

P

I

R

=

´

(10)

TPS51113, TPS51163

As a result, to meet the load transient response demand, the minimum output capacitance should be

where

• I

• I

• V

• V

By considering the demand of both output ripple voltage and load transient response, the minimum output

capacitance can be determined.

For a certain rated load current, input and output voltage, the input ripple voltage caused by the input

capacitance value and ESR are shown in Equation 7 and Equation 8, respectively.

Based on the required input voltage ripple, suitable capacitors can be chosen by using the above equations.

Choosing suitable MOSFETs is extremely important to achieve high power conversion efficiency for the

converter. For a buck converter, suitable MOSFETs should not only meet the requirement of voltage and current

rating, but also ensure low power loss.

High-Side MOSFET

value of the current passing through the top MOSFET depends on the average load current, ripple current and

duty cycle the converter is operating.

The conduction loss can, thus, be calculated as follows.

12

Submit Documentation Feedback

Copyright © 2009, Texas Instruments Incorporated

Product Folder Link(s) :TPS51113 TPS51163