APW8802 datasheet(18/25 Pages) ANPEC

18 / 25 page

Rev. A.3 - Jan., 2013

APW8802

www.anpec.com.tw

Application Information

Where 0.75 is the reference voltage, R

TOP is the resistor

connected from converter’s output to FB, and R

GND is the

resistor connected from FB to GND. Suggested R

GND is in

the range from 1k to 20k

Ω. To prevent stray pickup, locate

resistors R

TOP and RGND close to APW8802.

Output Inductor Selection

The duty cycle (D) of a buck converter is the function of the

input voltage and output voltage. Once an output voltage

is fixed, it can be written as:

OUT

RIPPLE

Output Capacitor Selection

The inductor value (L) determines the inductor ripple

current, I

RIPPLE, and affects the load transient reponse.

Higher inductor value reduces the inductor’s ripple cur-

rent and induces lower output ripple voltage. The ripple

current and ripple voltage can be approximated by:

ESR

RIPPLE

ESR

OUT

RIPPLE

OUT

∆

Output Voltage Setting

The output voltage is adjustable from 0.75V to 5.5V with a

resistor-divider connected with FB, GND, and converter’s

output. Using 1% or better resistors for the resistor-di-

vider is recommended. The output voltage is determined

by:

Where F

SW is the switching frequency of the regulator.

Although the inductor value and frequency are increased

and the ripple current and voltage are reduced, a tradeoff

exists between the inductor’s ripple current and the regu-

lator load transient response time.

A smaller inductor will give the regulator a faster load

transient response at the expense of higher ripple current.

Increasing the switching frequency (F

SW) also reduces

the ripple current and voltage, but it will increase the

switching loss of the MOSFETs and the power dissipa-

tion of the converter. The maximum ripple current occurs

at the maximum 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 which is capable of

carrying the required peak current without going into

saturation. In some types of inductors, especially core

that is made of ferrite, the ripple current will increase

abruptly when it saturates. This results in a larger output

ripple voltage. Besides, the inductor needs to have low

DCR to reduce the loss of efficiency.

Output voltage ripple and the transient voltage devia-

tion are factors which have to be taken into consider-

ation when selecting an output capacitor. Higher capaci-

tor value and lower ESR reduce the output ripple and

the load transient drop. Therefore, selecting high per-

formance low ESR capacitors is recommended for

switching regulator applications. In addition to high

frequency noise related to MOSFET turn-on and turn-

off, the output voltage ripple includes the capacitance

voltage drop

∆V

COUT and ESR voltage drop ∆VESR caused

by the AC peak-to-peak inductor’s current. These two

voltages can be represented by:

These two components constitute a large portion of the

total output voltage ripple. In some applications, multiple

capacitors have to be paralleled to achieve the desired

ESR value. If the output of the converter has to support

another load with high pulsating current, more capaci-

tors are needed in order to reduce the equivalent ESR

and suppress the voltage ripple to a tolerable level. A

small decoupling capacitor (1

µF) in parallel for bypass-

ing the noise is also recommended, and the voltage rat-

ing of the output capacitors are also must be considered.

To support a load transient that is faster than the switch-

ing frequency, more capacitors are needed for reducing

the voltage excursion during load step change. Another

aspect of the capacitor selection is that the total AC cur-

rent going through the capacitors has to be less than the

rated RMS current specified on the capacitors in order to

prevent the capacitor from over-heating.







 +

GND

TOP

OUT

0.75

Input Capacitor Selection

The input capacitor is chosen based on the voltage rating

and the RMS current rating. For reliable operation, select-

ing the capacitor voltage rating to be at least 1.3 times

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