SP7653 datasheet(7/12 Pages) SIPEX

7 / 12 page

Date: 03/25/05

SP7653 Wide Input Voltage Range, 1.3MHz, Buck Regulator

APPLICATIONS INFORMATION

Inductor Selection

There are many factors to consider in selecting

the inductor including core material, inductance

vs. frequency, current handling capability, effi-

ciency, size and EMI. In a typical SP7653 cir-

cuit, the inductor is chosen primarily for value,

saturation current and DC resistance. Increasing

the inductor value will decrease output voltage

ripple, but degrade transient response. Low in-

ductor values provide the smallest size, but

cause large ripple currents, poor efficiency and

more output capacitance to smooth out the larger

ripple current. The inductor must be able to

handle the peak current at the switching fre-

quency without saturating, and the copper resis-

tance in the winding should be kept as low as

possible to minimize resistive power loss. A

good compromise between size, loss and cost is

to set the inductor ripple current to be within

20% to 40% of the maximum output current.

The switching frequency and the inductor oper-

ating point determine the inductor value as fol-

lows:

(max)

(max )

(max)

)

(

OUT

−

where:

fS = switching frequency

Kr = ratio of the AC inductor ripple current to

the maximum output current

The peak to peak inductor ripple current is:

OUT

(max)

)

(

−

Once the required inductor value is selected, the

proper selection of core material is based on

peak inductor current and efficiency require-

ments. The core must be large enough not to

saturate at the peak inductor current

(max)

OUT

PEAK

and provide low core loss at the high switching

frequency. Low cost powdered iron cores have

a gradual saturation characteristic but can intro-

duce considerable AC core loss, especially when

the inductor value is relatively low and the

ripple current is high. Ferrite materials, although

more expensive, have an abrupt saturation char-

acteristic with the inductance dropping sharply

when the peak design current is exceeded. Nev-

ertheless, they are preferred at high switching

frequencies because they present very low core

loss while the designer is only required to

prevent saturation.

In general, ferrite or

molypermalloy materials are a better choice for

all but the most cost sensitive applications.

Optimizing Efficiency

The power dissipated in the inductor is equal to

the sum of the core and copper losses. To mini-

mize copper losses, the winding resistance needs

to be minimized, but this usually comes at the

expense of a larger inductor. Core losses have a

more significant contribution at low output cur-

rent where the copper losses are at a minimum,

and can typically be neglected at higher output

currents where the copper losses dominate. Core

loss information is usually available from the

magnetic vendor. Proper inductor selection can

affect the resulting power supply efficiency by

more than 15%!

The copper loss in the inductor can be calculated

using the following equation:

WINDING

RMS

)

(

)

(

where IL(RMS) is the RMS inductor current that

can be calculated as follows:

IL(RMS) = IOUT(max) 1 +

( IPP )

IOUT(max)

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