)

2

/(

1

max

_

min

_

CO

O

O

F

R

C

´

´

´

=

p

(

)

´

-

´

´

´

OUT

IN(MAX)

OUT

L(PK)

OUT(MAX)

IN(MAX)

OUT

SW

V

V

V

I

= I

+

1.6

V

L

F

(

)

æ

ö

´

-

ç

÷

´

ç

÷

´

´

´

è

ø

2

OUT

IN(MAX)

OUT

2

L(RMS)

OUT(MAX)

IN(MAX)

OUT

SW

V

V

V

1

I

= I

+

12

V

L

F

0.8

(

)

OUT

IN(MAX)

OUT

LPP

IN(MAX)

OUT

SW

V

× V

V

I

=

V

× L

ƒ

0.8

-

´

´

(

)

OUT(MAX)

IN(MAX)

OUT

MIN

IN(MAX)

IND

OUT

SW

V

V

V

L

=

V

K

I

F

´

-

´

´

´

TPS54232

SLVS876D – NOVEMBER 2008 – REVISED NOVEMBER 2014

www.ti.com

9.2.2.4 Output Filter Components

Two components need to be selected for the output filter, L1 and C3. Since the TPS54232 is an externally

compensated device, a wide range of filter component types and values can be supported.

9.2.2.4.1

Inductor Selection

To calculate the minimum value of the output inductor, use Equation 8.

(8)

KIND is a coefficient that represents the amount of inductor ripple current relative to the maximum output current.

In general, this value is at the discretion of the designer; however, the following guidelines may be used. For

designs using low ESR output capacitors such as ceramics, a value as high as KIND = 0.4 may be used. When

using higher ESR output capacitors, KIND = 0.2 yields better results.

For this design example, use KIND = 0.35 and the minimum inductor value is calculated to be 2.97 μH. For this

design, a large value was chosen: 3.3

μH.

For the output filter inductor, it is important that the RMS current and saturation current ratings not be exceeded.

The peak-to-peak inductor current is calculated using Equation 9.

(9)

The RMS inductor current can be found from Equation 10.

(10)

and the peak inductor current can be determined with Equation 11.

(11)

For this design, the RMS inductor current is 2.01 A and the peak inductor current is 2.39 A. The chosen inductor

is a Coilcraft MSS7341-332NL 3.3

μH. It has a saturation current rating of 3.28 A and an RMS current rating of

3.95 A, meeting these requirements. Smaller or larger inductor values can be used depending on the amount of

ripple current the designer wishes to allow so long as the other design requirements are met. Larger value

inductors will have lower ac current and result in lower output voltage ripple, while smaller inductor values will

increase ac current and output voltage ripple. Inductor values for use with the TPS54232 are in the range of 1

μH

to 47

μH.

9.2.2.4.2

Capacitor Selection

The important design factors for the output capacitor are dc voltage rating, ripple current rating, and equivalent

series resistance (ESR). The dc voltage and ripple current ratings cannot be exceeded. The ESR is important

because along with the inductor current it determines the amount of output ripple voltage. The actual value of the

output capacitor is not critical, but some practical limits do exist. Consider the relationship between the desired

closed loop crossover frequency of the design and LC corner frequency of the output filter. In general, it is

desirable to keep the closed loop crossover frequency at less than 1/5 of the switching frequency. With high-

switching frequencies such as the 1 MHz frequency of this design, internal circuit limitations of the TPS54232

limit the practical maximum crossover frequency to about 70 kHz. In general, the closed loop crossover

frequency should be higher than the corner frequency determined by the load impedance and the output

capacitor. This limits the minimum capacitor value for the output filter to:

(12)

16

Submit Documentation Feedback

Copyright © 2008–2014, Texas Instruments Incorporated

Product Folder Links: TPS54232