(

)

(

)

(

)

(

)

(

)

(

)

(

)

f

f

OUT

OUT

OUT

OUT

IN max

IN max

IND(ripple)

SW

SW

IN max

OUT max

IN max

V

V

V

V

V

V

1

3

L

I

V

I

V

-

´

-

´

=

´

=

´

´

´

(

)

(

)

(

)

f

OUT

OUT

IN max

TRIP

IND(peak )

DS(on)

SW

IN max

V

V

V

V

1

I

8 R

L

V

-

´

=

+

´

´

´

(

)

f

f

OUT

SW

SW

IND(ripple)

V

10 mV

1 D

10 mV

L

L

ESR

0.7 V

I

0.7 V

70

´

´

-

´ ´

é

ù

é

ù

´

ë

û

ë

û

=

=

=

W

é ù

ë û

´

é ù

é ù

ë û

ë û

t

SW

t – Time

0

10

t

V

RIPPLE(FB)

TPS51218

SLUS935B

– MAY 2009 – REVISED FEBRUARY 2012

www.ti.com

EXTERNAL COMPONENTS SELECTION

Selecting external components is simple in D-CAP

™ mode.

1. Choose the inductor.

The inductance value should be determined to give the ripple current of approximately 1/4 to 1/2 of maximum

output current. Larger ripple current increases output ripple voltage and improves S/N ratio and helps stable

operation.

(6)

The inductor also needs to have low DCR to achieve good efficiency, as well as enough room above peak

inductor current before saturation. The peak inductor current can be estimated in Equation 7.

(7)

2. Choose the output capacitor(s).

Organic semiconductor capacitor(s) or specialty polymer capacitor(s) are recommended. For loop stability,

capacitance and ESR should satisfy Equation 2. For jitter performance, Equation 8 is a good starting point to

determine ESR.

where

•

D is the duty ratio

•

•

(8)

Figure 18. Ripple Voltage Down Slope

16

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