SP6120EY datasheet(14/22 Pages) SIPEX

14 / 22 page

Date: 1/21/05

SP6120 Low Voltage, AnyFETTM, Synchronous, Buck Controller

Inductor Selection

There are many factors to consider in selecting

the inductor including cost, efficiency, size and

EMI. In a typical SP6120 circuit, 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 inductor values pro-

vide the smallest size, but cause large ripple

currents, poor efficiency and more output ca-

pacitance to smooth out the larger ripple cur-

rent. The inductor must also be able to handle

the peak current at the switching frequency

without saturating, and the copper resistance in

the winding should be kept as low as possible to

minimize resistive power loss. A good compro-

mise 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

follows:

(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 material 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, on the

other hand, are more expensive and have an

abrupt saturation characteristic with the induc-

tance dropping sharply when the peak design

current is exceeded. Nevertheless, they are pre-

ferred at high switching frequencies because

they present very low core loss and the design

only needs to prevent saturation. In general,

ferrite or molypermalloy materials are better

choice for all but the most cost sensitive appli-

cations.

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.

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)

Output Capacitor Selection

The required ESR (Equivalent Series Resis-

tance) and capacitance drive the selection of the

type and quantity of the output capacitors. The

ESR must be small enough that both the resis-

tive voltage deviation due to a step change in the

load current and the output ripple voltage do not

exceed the tolerance limits expected on the

output voltage. During an output load transient,

the output capacitor must supply all the addi-

tional current demanded by the load until the

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