SP6123EB【PDF 9/18 页】EVAL BOARD FOR SP6123

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APPLICATIONS INFORMATION

Inductor Selection

There are many factors to consider in selecting

the inductor including cost, efficiency, size and

EMI. In a typical SP6123 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 provide

the smallest size, but cause large ripple currents,

poor efficiency and more output capacitance to

smooth out the larger ripple current. The induc-

tor 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 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:

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.

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

L =

V OUT ( V IN (max) ? V OUT )

V IN (max) F S K r I OUT ( max)

using the following equation:

P L ( Cu ) = I L 2 ( RMS ) R WINDING

where:

F S = switching frequency

K r = ratio of the peak to peak inductor ripple

where I L(RMS) is the RMS inductor current that

can be calculated as follows:

I L(RMS) = I OUT(max) 1 +

current to the maximum output current

The peak to peak inductor ripple current is:

1

3

Output Capacitor Selection

(

I PP

I OUT(max)

)

2

I PP =

V OUT ( V IN (max) ? V OUT )

V I N (max) F S L

The required ESR (Equivalent Series Resis-

tance) and capacitance drive the selection of the

type and quantity of the output capacitors. The

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

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-

I PEAK = I OUT (max) +

I PP

2

tional current demanded by the load until the

SP6123 adjusts the inductor current to the new

value. Therefore the capacitance must be large

and provide low core loss at the high switching

frequency. Low cost powdered iron cores have

enough so that the output voltage is held up

while the inductor current ramps up or down to

Date: 9/13/04

SP6123 Low Voltage, Synchronous Step Down PWM Controller

9

? Copyright 2004 Sipex Corporation

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