(

)

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

V

V

D

×

>

× D

L

Lpeak

O

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

in

V

V

V

i

L

V

fsw

-

D =

×

×

in

out

out

O

in

V

V

V

L

I

LIR

V

fsw

-

=

×

×

×

TPS65281, TPS65281-1

SLVSBH7B – JULY 2012 – REVISED DECEMBER 2012

www.ti.com

Actual core loss of the inductor is independent of core size for a fixed inductor value, but it is dependent on the

inductance value selected. As inductance increases, core losses decrease. Unfortunately, increased inductance

requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core loss

and are preferred for high switching frequencies, so design goals can concentrate on copper loss and preventing

saturation. Ferrite core material saturates hard, which means that inductance collapses abruptly when the peak

design current is exceeded. It results in an abrupt increase in inductor ripple current and consequent output

voltage ripple. Do not allow the core to saturate. It is important that the RMS current and saturation current

ratings are not exceeding the inductor specification. The RMS and peak inductor current can be calculated from

Equation 9 and Equation 10.

(7)

(8)

(9)

(10)

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

a minimum capacitance of 17 µF. A standard 22 µF ceramic capacitor is used in the design.

(11)

maximum output voltage ripple is 50 mV (1% of regulated 5 V). From Equation 8, the output current ripple is 1 A.

From Equation 12, the minimum output capacitance meeting the output voltage ripple requirement is 4.6 µF with

3-m

Ω esr resistance.

(12)

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

Ω of ESR

will 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

9.6 V. Tthe input capacitors must support a ripple current of 1 A RMS.

(13)

22

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