(

)

(

)

(

)

(

)

(

)

(

)

2

2

2

2

2

2

1

1

1

RIPPLE1

OUT1

RIPPLE1

L1 rms

L1 avg

12

12

12

I

I

I

I

I

10

2.5

10.026 A

=

+

´

=

+

´

=

+

´

=

(

)

D

´

D

D

D

´

<

´ D =

´

=

´

2

OUT1

OUT1

OUT1

OUT1

OVER1

OUT1

OUT1

OUT1

OUT1

OUT1

I

L1

I

I

I

L1

V

t

C

C

V

V

C

(

)

(

)

2

OUT1

OUT1

OUT1

OUT1

UNDER1

OUT1

OUT1

IN

OUT1

IN

OUT1

OUT1

I

L1

I

I

I

L1

V

t

C

C

V

V

V

V

C

D

´

D

D

D

´

<

´ D =

´

=

-

-

´

(

)

2

2

OUT1

OUT1(min)

OUT

OVER1

I

L1

5

0.88 H

C

458 F

V

V

1.2

40 mV

D

´

´

m

=

=

=

m

´

´

RIPPLE1

RIPPLE1

RIPPLE1

RIPPLE(cap)

OUT1

SW

MAX

RIPPLE1

RIPPLE1

I

2.5 A

V

24 mV

V

V

8

C

f

8

458 F

500 kHz

ESR

9 m

I

I

2.5 A

æ

ö

æ

ö

-

-

ç

÷

ç

÷

-

´

´

´

m

´

è

ø

è

ø

=

=

=

=

W

(

)

OUT1

OUT1

CHARGE

SS

1.2 V

2

220 F

2

10 F

2

3.3 F

1 F

V

C

I

0.281A

t

2ms

´

´

m

+

´

m

+

´

m

+ m

´

=

=

=

(

)

(

)

(

)

1

1

OUT1(max)

RIPPLE1

CHARGE

L1 peak

2

2

I

I

I

I

10 A

2.5A

0.281A

11.53 A

=

+

´

+

=

+

´

+

=

TPS40322

SLUSAF8

– JUNE 2011

www.ti.com

The RMS current through the inductor is approximated by the equation:

(9)

Output Capacitor Selection (C10 through C16)

The selection of the output capacitor is typically driven by the output transient response requirement.

Equation 10and Equation 11 over-estimate the voltage deviation to account for delays in the loop bandwidth and

can be used to determine the required output capacitance:

(10)

(11)

capacitance.

(12)

With a minimum capacitance, the maximum allowable ESR is determined by the maximum ripple voltage and is

approximated by Equation 13.

(13)

Two 220-

µF, 4-V, aluminum electrolytic capacitors were chosen for load response requirements. Additionally two

0805 10-

µF, X7R, along with two 0603, 3.3-µF X5R, and one 1-µF, X5R ceramic capacitors are selected for low

ESR and high frequency decoupling.

Peak Current Rating of Inductor

With the output capacitance known, it is possible to calculate the charge current during start-up and determine

the minimum saturation current rating for the inductor. The start-up charging current is approximated using

Equation 14.

(14)

(15)

Table 3. Inductor Requirements Summary

PARAMETER

VALUE

UNITS

L1

Inductance

0.88

µH

IL1_RMS

RMS current (thermal rating)

10.026

A

IL1_PEAK

Peak current (saturation rating)

11.53

A

22

Copyright

© 2011, Texas Instruments Incorporated