APW7074 datasheet(14/21 Pages) ANPEC

14 / 21 page

Rev. A.4 - Aug., 2008

APW7074

www.anpec.com.tw

Application Information

Output Voltage Selection

The output voltage can be programmed with a resistive

divider. The use of 1% or better resistors for the resistive

divider is recommended. The FB pin is the inverter input

of the error amplifier, and the reference voltage is 0.8V.

The output voltage is determined by:









GND

OUT

0.8

Where R

OUT is the resistor connected from VOUT to FB and

GND is the resistor connected from FB to GND.

Output Inductor Selection

The inductor value determines the inductor ripple cur-

rent and affects the load transient response. Higher in-

ductor value reduces the inductor’s ripple current and

induces lower output ripple voltage. The ripple current

and the ripple voltage can be approximated by:

OUT

RIPPLE

−

ESR

RIPPLE

OUT

∆

where F

S is the switching frequency of the regulator.

Although the increase of the inductor value and frequency

reduces the ripple current and voltage, there is a tradeoff

between the inductor’s ripple current and the regula-

tor 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

S) also reduces the

ripple current and voltage, but it will increase the switch-

ing loss of the MOSFET and the power dissipation 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, select an inductor that is capable of

carrying the required peak current without going into

saturation. In some types of inductors, especially those

with ferrite core, the ripple current will increase abruptly

when it saturates. This will result in a larger output ripple

voltage.

Output Capacitor Selection

Higher capacitor value and lower ESR reduce the out-

put ripple and the load transient drop. Therefore, select-

ing high performance low ESR capacitors is intended

for switching regulator applications. In some applications,

multiple capacitors have to be parallel to achieve the de-

sired ESR value. A small decoupling capacitor in parallel

for bypassing the noise is also recommended, and the

voltage rating of the output capacitors also must be

considered. 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 rat-

ing 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 I

OUT is the load current. During power up, the

input capacitors have to handle large amount of surge

current. If tantalum 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 1

µF can connect between

the drain of upper MOSFET and the source of lower

MOSFET.

MOSFET Selection

The selection of the N-channel power MOSFETs are de-

termined by the R

DS(ON), reverse transfer capacitance (CRSS)

and maximum output current requirement. There are two

components of loss in the MOSFETs: conduction

loss and transition loss. For the upper and lower

MOSFET, the losses are approximately given by the fol-

lowing equations:

UPPER = IOUT

(1+ TC)(R

DS(ON))D + (0.5)( IOUT)(VIN)( tSW)FS

LOWER = IOUT

(1+ TC)(R

DS(ON))(1-D)

Where I

OUT is the load current

TC is the temperature dependency of R

DS(ON)

S is the switching frequency

SW is the switching interval

D is the duty cycle

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