APW7065 View Datasheet(PDF) - Anpec Electronics

Part Name

Description

Manufacturer

APW7065

Synchronous Buck PWM Controller

Anpec Electronics 

APW7065

Application Information (Cont.)

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:

IRIPPLE

= VIN − VOUT × VOUT

FS × L

VIN

∆VOUT = IRIPPLE × ESR

where F is the switching frequency of the regulator.

S

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 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 ca-

pable of carrying the required peak current without

going into saturation. In some types of inductors, es-

pecially core that is made of ferrite, the ripple current

will increase abruptly when it saturates. This will re-

sult in a larger output ripple voltage.

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 IOUT/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 RDS(ON), reverse transfer capacitance

(C ) and maximum output current requirement. There

RSS

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:

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

Output Capacitor Selection

P = I (1+ TC)(R )(1-D)

LOWER OUT

DS(ON)

Higher capacitor value and lower ESR reduce the Where IOUT is the load current

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

TC is the temperature dependency of R

DS(ON)

FS is the switching frequency

tSW 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, tSW, is a function of the

reverse transfer capacitance C . The (1+TC) term is

RSS

Copyright © ANPEC Electronics Corp.

12

www.anpec.com.tw

Rev. A.1 - Feb., 2006

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