)

2

/(

1

max

_

min

_

CO

O

O

F

R

C

´

´

´

=

p

ú

û

ù

ê

ë

é

+

´

´

-

=

E S R

O

S W

L P P

O P P

R

C

F

D

I

V

4

)

5

.

0

(

(

)

OPPMAX

max

LPP

SW

O

D

0.5

V

ESR

=

I

4

F

C

-

-

´

´

(

)

12

OUT

IN(MAX)

OUT

COUT(RMS)

IN(MAX)

OUT

SW

C

V

×

V

V

1

I

=

×

V

× L

× F

× N

æ

ö

-

ç

÷

ç

÷

è

ø

TPS54233-Q1

www.ti.com

SLUSAZ7A – MARCH 2012 – REVISED MARCH 2012

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 300 kHz frequency of this design, internal circuit limitations of the TPS54233-

Q1 limit the practical maximum crossover frequency to about 25 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:

(11)

Where RO is the output load impedance (VO/IO) and fCO is the desired crossover frequency. For a desired

maximum crossover of 25 kHz the minimum value for the output capacitor is around 3.8

μF. This may not satisfy

the output ripple voltage requirement. The output ripple voltage consists of two components; the voltage change

due to the charge and discharge of the output filter capacitance and the voltage change due to the ripple current

times the ESR of the output filter capacitor. The output ripple voltage can be estimated by:

(12)

Where NC is the number of output capacitors in parallel.

The maximum ESR of the output capacitor is determined by the amount of allowable output ripple as specified in

the initial design parameters; so the maximum specified ESR as listed in the capacitor data sheet is given by

Equation 13:

(13)

Where

ΔVp-p is the desired peak-to-peak output ripple.

To meet the 100 mV p-p ripple requirement, a single 470

μF aluminum electrolytic output capacitor is chosen for

C9. This is a Panasonic, EEVFK1A471P rated at 10 V with a maximum ESR of 160 m

Ω and a ripple current

rating of 600 mA.

The maximum RMS output ripple current can be calculated using Equation 14

(14)

The calculated total RMS ripple current is 216 mA and the maximum total ESR required is 43 m

Ω. These output

capacitors exceed the requirements by a wide margin and will result in a reliable, high-performance design. The

selected output capacitor must be rated for a voltage greater than the desired output voltage plus = the ripple

voltage. Any derating amount must also be included.

Other capacitor types work well with the TPS54233-Q1, depending on the needs of the application.

COMPENSATION COMPONENTS

The external compensation used with the TPS54233-Q1 allows for a wide range of output filter configurations. A

large range of capacitor values and types of dielectric are supported. The design example uses ceramic X5R

dielectric output capacitors, but other types are supported.

A Type II compensation scheme is recommended for the TPS54233-Q1. The compensation components are

chosen to set the desired closed loop cross over frequency and phase margin for output filter components. The

type II compensation has the following characteristics; a dc gain component, a low frequency pole, and a mid

frequency zero / pole pair.

Copyright © 2012, Texas Instruments Incorporated

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