APPLICATION INFORMATION

EXTERNAL PARTS SELECTION

CHOOSING THE INDUCTOR

(

)

V

V

V

IN

OUT

OUT

L

V

I

f

IN

RIPPLE

SW

-

´

=

´

´

(2)

CALCULATING OUTPUT CAPACITANCE

RIPPLE

1

V

I

ESR

8

C

f

OUT

SW

æ

ö

ç

÷

D =

´

+

ç

÷

´

´

è

ø

(3)

( ) ( )

2

2

1

E

L

I

I

L

OH

OL

2

æ

ö

=

´ ´

-

ç

÷

è

ø

(4)

( ) ( )

(

)

2

2

C

OUT

1

E

C

Vf

Vi

2

=

´

´

-

(5)

TPS51113, TPS51163

The value of the output filtering inductor determines the magnitude of the current ripple, which also affects the

output voltage ripple for a certain output capacitance value. Increasing the inductance value reduces the ripple

current, and thus, results in reduced conduction loss and output ripple voltage. On the other hand, low

inductance value is needed due to the demand of low profile and fast transient response. Therefore, it is

important to obtain a compromise between the low ripple current and low inductance value.

In practical application, to ensure high power conversion efficiency at light load condition, the peak-to-peak

current ripple is usually designed to be between 1/4 to 1/2 of the rated load current. Since the magnitude of the

current ripple is determined by inductance value, switching frequency, input voltage and output voltage, the

required inductance value for a certain required ripple

∆I is shown in Equation 2,

where

• V

• V

• I

• f

When the inductance value is determined, the output capacitance value can also be derived according to the

output ripple voltage and output load transient response requirement. The output ripple voltage is a function of

both the output capacitance and capacitor ESR. Considering the worst case and assume the capacitance value

Thus, output capacitors with suitable ESR and capacitance value should be chosen to meet the ripple voltage

(

ΔV) requirement.

Minimum capacitance value is also calculated according to the demand of the load transient response. When the

load current changes, the energy that the inductor needs to release or absorb is derived in Equation 4.

At the same time, the energy that is delivered to or provided by the output capacitor can also be derived as

shown in Equation 5.

Copyright © 2009, Texas Instruments Incorporated

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