APW8713 datasheet(17/24 Pages) ANPEC

17 / 24 page

Rev. A.3 - Sep., 2013

APW8713

www.anpec.com.tw

Application Information

Output Inductor Selection

The output voltage is adjustable from 0.8V to 12V 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 0.8 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 APW 8713.

)

0.8

GND

TOP

OUT

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:

Output Capacitor Selection

Output voltage ripple and the transient voltage deviation

are factors that have to be taken into consideration when

selecting an output capacitor. Higher capacitor value and

lower ESR reduce the output ripple and the load transient

drop. Therefore, selecting high performance low ESR

capacitors is recommended for switching regulator

applications. In addition to high frequency noise related

to MOSFET turn-on and turnoff, the output voltage ripple

includes the capacitance voltage drop

∆V

COUT and ESR

voltage drop

∆V

ESR caus ed by the AC peak-to -peak

inductor’s current. These two voltages can be represented

by:

OUT

The inductor value (L) determines the inductor ripple

current, I

RIPPLE, and affects the load transient response.

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:

OUT

RIPPLE

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 in-

ductor that is capable of carrying the required peak cur-

rent 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 re-

sults in a larger output ripple voltage. Besides, the induc-

tor needs to have low DCR to reduce the loss of efficiency.

OUT

RIPPLE

OUT

∆

ESR

RIPPLE

ESR

∆

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 capacitors

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 bypassing the

noise is also recommended, and the voltage rating 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.

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