(

)

(

)

æ

ö

-

´

æ

ö

-

´

´

´

W

ç

÷

ç

÷

ç

÷

´ ´

è

ø

è

ø

W =

m

IN

OUT

OUT

OCP

DS(on)

SW

IN

TRIP

TRIP

V

V

V

1

I

32 R

m

2 L f

V

R

(k )

I

( A)

(

)

( )

(

)

(

)

(

)

-

´

=

+

´

´

´

IN

OUT

OUT

max

TRIP

IND peak

SW

IN

DS on

max

V

V

V

V

1

I

32 R

L f

V

(

)

(

)

(

)

(

)

(

)

(

)

(

)

-

´

-

´

=

´

=

´

´

´

IN

OUT

OUT

IN

OUT

OUT

max

max

SW

IN

OUT

SW

IN(max)

IND ripple

max

max

V

V

V

V

V

V

1

3

L

I

f

V

I

f

V

TPS53353

SLUSAK2B – AUGUST 2011 – REVISED FEBRUARY 2015

www.ti.com

8.2.2.2 Detailed Design Procedure

8.2.2.2.1

External Component Selection

Refer to the External Component Selection Using All Ceramic Output Capacitors section for guidelines for this

design with all ceramic output capacitors.

The external components selection is a simple process when using organic semiconductors or special polymer

output capacitors.

1. SELECT OPERATION MODE AND SOFT-START TIME

Select operation mode and soft-start time using Table 3.

2. SELECT SWITCHING FREQUENCY

Select the switching frequency from 250 kHz to 1 MHz using Table 1.

3. CHOOSE THE INDUCTOR

The inductance value should be determined to give the ripple current of approximately 1/4 to 1/2 of maximum

output current. Larger ripple current increases output ripple voltage and improves signal-to-noise ratio and

helps ensure stable operation, but increases inductor core loss. Using 1/3 ripple current to maximum output

current ratio, the inductance can be determined by Equation 8.

(13)

The inductor requires a low DCR to achieve good efficiency. It also requires enough room above peak

inductor current before saturation. The peak inductor current can be estimated in Equation 9.

(14)

4. EXTERNAL COMPONENT SELECTION WITH ALL CERAMIC OUTPUT CAPACITORS

Refer to the External Component Selection Using All Ceramic Output Capacitors section to select external

components because ceramic output capacitors are used in this design.

5. CHOOSE THE OVERCURRENT SETTING RESISTOR

where

•

•

(15)

1.7 m

Ω for overcurrent level of approximately 10 A.

8.2.2.2.2

External Component Selection Using All Ceramic Output Capacitors

When a ceramic output capacitor is used, the stability criteria in Equation 6 cannot be satisfied. The ripple

injection approach as shown in Figure 41 is implemented to increase the ripple on the VFB pin and make the

system stable. In addition to the selections made using Steps 1 through Step 6 in the section, the ripple injection

components must be selected. The C2 value can be fixed at 1 nF. The value of C1 can be selected from 10 nF

to 200 nF.

24

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