I

COUT(RMS) +

1

12

V

OUT

V

IN(MAX) *

V

OUT

V

IN(MAX)

L

OUT

F

SW

N

C

V

(MAX)=

PP

()

ESR

xV

xV

-V

MAX

OUT

IN(MAX)

OUT

N

xV

xL

C

IN(MAX)

ESR

1

2p

C

OUT

f

CO

C

1

3357

L

OUT

f

CO

V

OUT

f

f

LC

2

85 V

OUT

17

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SLVS632I – JANUARY 2006 – REVISED APRIL 2017

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Copyright © 2006–2017, Texas Instruments Incorporated

8.2.1.2.4.2

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 ripple 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. Due to the

design of the internal compensation, it is desirable to keep the closed loop crossover frequency in the range 3

kHz to 30 kHz as this frequency range has adequate phase boost to allow for stable operation. For this design

example, it is assumed that the intended closed loop crossover frequency will be between 2590 Hz and 24 kHz

and also below the ESR zero of the output capacitor. Under these conditions the closed loop crossover

frequency is related to the LC corner frequency by:

(7)

And the desired output capacitor value for the output filter to:

(8)

For a desired crossover of 18 kHz and a 15 μH inductor, the calculated value for the output capacitor is 220 μF.

The capacitor type should be chosen so that the ESR zero is above the loop crossover. The maximum ESR

should be:

(9)

The maximum ESR of the output capacitor also determines the amount of output ripple as specified in the initial

design parameters. The output ripple voltage is the inductor ripple current times the ESR of the output filter.

Check that the maximum specified ESR as listed in the capacitor data sheet results in an acceptable output

ripple voltage:

where

•

•

•

(10)

For this design example, a single 220 μF output capacitor is chosen for C3. The calculated RMS ripple current is

143 mA and the maximum ESR required is 40 m

Ω. A capacitor that meets these requirements is a Sanyo

Poscap 10TPB220M, rated at 10 V with a maximum ESR of 40 m

Ω and a ripple current rating of 3 A. An

additional small 0.1 μF ceramic bypass capacitor may also used, but is not included in this design.

The minimum ESR of the output capacitor should also be considered. For good phase margin, the ESR zero

when the ESR is at a minimum should not be too far above the internal compensation poles at 24 kHz and 54

kHz.

The selected output capacitor must also be rated for a voltage greater than the desired output voltage plus one

half the ripple voltage. Any derating amount must also be included. The maximum RMS ripple current in the

output capacitor is given by Equation 11:

where

•

•

(11)