APW7074 datasheet(13/20 Pages) ANPEC

13 / 20 page

Rev. A.2 - Feb., 2006

APW7074

www.anpec.com.tw

Application Information (Cont.)

where Fs is the switching frequency of the regulator.

Although increase of the inductor value and frequency

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. Increasing the switching frequency (F

S) also

reduces the ripple current and voltage, but it will

increase the switching 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 core that is made of

ferrite, 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

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 parallel to

achieve the desired 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

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 IOUT 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 1uF 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

determined by the R

DS(ON), reverse transfer capacitance

RSS) 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 (1+ TC)(RDS(ON))D + (0.5)( IOUT)(VIN)( tSW)FS

LOWER = IOUT (1+ TC)(RDS(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 temperature dependency of the R

DS(ON)

and can be extracted from the “R

DS(ON) vs Temperature”

curve of the power MOSFET.

Output Inductor Selection (Cont.)

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