APW7063 datasheet(14/22 Pages) ANPEC

14 / 22 page

Rev. A.10 - Aug., 2009

APW7063

www.anpec.com.tw

Application Information

Component Selection Guidelines

Output Capacitor Selection

The selection of C

OUT is determined by the required effec-

tive series resistance (ESR) and voltage rating rather than

the actual capacitance requirement. Therefore, selecting

high performance low ESR capacitors is intended for

switching regulator applications. In some applications,

multiple capacitors have to be paralled to achieve the

desired ESR value. If tantalum capacitors are used, make

sure they are surge tested by the manufactures. If in doubt,

consult the capacitors manufacturer.

Input Capacitor Selection

The input capacitor is chosen based on the voltage rating

and the RMS current rating. For reliable operation, select

the capacitor voltage rating to be at least 1.3 times higher

than the maximum input voltage. The maximum RMS

current rating requirement is approximately I

OUT/2 where

OUT is the load current. During power up, the input capaci-

tors have to handle large amount of surge current. If tanta-

lum capacitors are used, make sure they are surge tested

by the manufactures. If in doubt, consult the capacitors

manufacturer.

For high frequency decoupling, a ceramic capacitor be-

tween 0.1

µF to 1µF can connect between V

CC and ground

pin.

Inductor Selection

The inductance of the inductor is determined by the out-

put voltage requirement. The larger the inductance, the

lower the inductor’s current ripple. This will translate into

lower output ripple voltage. The ripple current and ripple

voltage can be approximated by:

IN - VOUT

Fs x L

OUT

RIPPLE =

where Fs is the switching frequency of the regulator.

∆V

OUT = IRIPPLE x ESR

A tradeoff exists between the inductor’s ripple current and

the regulator load transient response time. A smaller in-

ductor will give the regulator a faster load transient re-

sponse at the expense of higher ripple current and vice

versa. The maximum ripple current occurs at the maxi-

mum input voltage. A good starting point is to choose the

ripple current to be approximately 30% of the maximum

output current.

Once the inductance value has been chosen, selecting

an inductor is capable of carrying the required peak cur-

rent without going into saturation. In some types of

inductors, especially core that is make of ferrite, the ripple

current will increase abruptly when it saturates. This will

result in a larger output ripple voltage.

Compensation

The output LC filter of a step down converter introduces a

double pole, which contributes with –40dB/decade gain

slope and 180 degrees phase shift in the control loop. A

compensation network between COMP pin and ground

should be added. The simplest loop compensation net-

work is shown in Figure 5.

The output LC filter consists of the output inductor and

output capacitors. The transfer function of the LC filter is

given by:

GAINLC

ESR

OUT

The poles and zero of this transfer function are:

FLC

OUT

FESR

OUT

ESR

The FLC is the double poles of the LC filter, and FESR is

the zero introduced by the ESR of the output capacitor.

COUT

ESR

Output

PHASE

Figure 2. The Output LC Filter

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