APW7065 datasheet(13/21 Pages) ANPEC

13 / 21 page

Rev. A.6 - Aug., 2009

APW7065

www.anpec.com.tw

Application Information

Where R

OUT is the resistor connected from VOUT to FB and

GND is the resistor connected from FB to GND.

Output Voltage Selection

The output voltage can be programmed with a resistive

divider. Use 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

Output Inductor Selection

The inductor value determines the inductor ripple current

and affects the load transient response. Higher inductor

value reduces the inductor’s ripple current and induces

lower output ripple voltage. The ripple current and ripple

voltage can be approximated by:

∆V

OUT = IRIPPLE x ESR

where F

S is the switching frequency of the regulator.

Although increase of the inductor value reduces the ripple

current and voltage, a tradeoff will exist between the

inductor’s ripple current and the regulator load transient

response time.

A smaller inductor will give the regulator a faster load

transient response at the expense of higher ripple current.

The maximum ripple current occurs at the maximum in-

put 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 core that is made of ferrite, the

ripple current will increase abruptly when it saturates.

This will result in a larger output ripple voltage.

OUT

RIPPLE

−

Output Capacitor Selection

Higher capacitor value and lower ESR reduce the output

ripple and the load transient drop. Therefore, selecting

high performance low ESR capacitors is intended for

switching regulator applications. In some applications,

multiple capacitors have to be paralleled to achieve the

desired ESR value. A small decoupling capacitor in par-

allel 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 ca-

pacitors manufacturer. For high frequency decoupling, a

ceramic capacitor 1

µF can be connected 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 following:

UPPER = IOUT

2 (1+ TC)(R

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

LOWER = IOUT

2(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

Note that both MOSFETs have conduction loss while the

upper MOSFET include an additional transition loss. The

switching internal, t

SW, is a function of the reverse transfer

capacitance C

RSS. The (1+TC) term is to factor in the tem-

perature dependency of the R

DS(ON) and can be extracted

from the “R

DS(ON) vs Temperature” curve of the power

MOSFET.

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