OUT max

IN

IN

SW

I

0.25

V

C

f

×

D

=

×

(

)

INmin

OUT

OUT

INRMS

OUT

INmin

INmin

V

V

V

I

I

V

V

-

=

×

×

OUT

L

1

1

Co

V

8 fsw

ESR

i

>

×

D

×

-

D

2

OUT

OUT

OUT

I

L

Co

2 V

V

D

×

>

×

× D

L

O

Lpeak

i

I

I

2

+

D

=

2

OUT

INmax

OUT

2

INmax

LRMS

O

V

(V

V

)

(

)

V

L fsw

i

I

12

×

-

× ×

=

+

IN

OUT

OUT

L

O

IN

V

V

V

i

I

V

fsw

-

D =

×

×

IN

OUT

OUT

O

IN

V

V

V

L

I

LIR

V

fsw

-

=

×

×

×

TPS5403

www.ti.com

SLVSBK5 – SEPTEMBER 2012

(3)

(4)

(5)

(6)

34% of 2-A DC load current.

Output Capacitor Selection

There are two primary considerations for selecting the value of the output capacitor. The output capacitors are

selected to meet load transient and output ripple’s requirements.

Equation 7 gives the minimum output capacitance to meet the transient specification. For this example,

L = 13 µH,

a minimum capacitance of 24 µF. A standard 33-µF ceramic is chosen in the design.

(7)

is the maximum allowable output voltage ripple, and

output voltage ripple is 33 mV (1% of regulated 3.3 V). From Equation 4, the output current ripple is 0.6 A. From

Equation 8, the minimum output capacitance meeting the output voltage ripple requirement is 8.3 µF with 3-m

Ω

ESR resistance.

(8)

After considering both requirements, for this example, one 22-µF, 6.3-V X7R ceramic capacitor with 3-m

Ω ESR

should be used.

Input Capacitor Selection

A minimum 10-µF X7R/X5R ceramic input capacitor is recommended to be added between VIN and GND. These

capacitors should be connected as close as physically possible to the input pins of the converters as they handle

from Equation 9, the input capacitors must support a ripple current of 1.15-A RMS.

(9)

The input capacitance value determines the input ripple voltage of the regulator. The input voltage ripple can be

an input voltage ripple of 167 mV.

(10)

To prevent large voltage transients, a low ESR capacitor sized for the maximum RMS current must be used.

Copyright © 2012, Texas Instruments Incorporated

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