6

LT1108

INDUCTOR SELECTION

General

A DC/DC converter operates by storing energy as mag-

netic flux in an inductor core, and then switching this

energy into the load. Since it is flux, not charge, that is

stored, the output voltage can be higher, lower, or oppo-

site in polarity to the input voltage by choosing an appro-

priate switching topology.

To operate as an efficient energy transfer element, the

inductor must fulfill three requirements. First, the induc-

tance must be low enough for the inductor to store adequate

energy under the worst case condition of minimum input

voltage and switch-ON time. The inductance must also be

high enough so maximum current ratings of the LT1108

and inductor are not exceeded at the other worst case

condition of maximum input voltage and ON-time.

Additionally, the inductor core must be able to store the

required flux; i.e., it must not

saturate. At power levels

generally encountered with LT1108 based designs, small

surface mount ferrite core units with saturation current

ratings in the 300mA to 1A range and DCR less than 0.4

Ω

(depending on application) are adequate.

Lastly, the inductor must have sufficiently low DC resis-

tance so excessive power is not lost as heat in the windings.

An additional consideration is Electro-Magnetic Interfer-

ence (EMI). Toroid and pot core type inductors are recom-

mended in applications where EMI must be kept to a

minimum; for example, where there are sensitive analog

circuitry or transducers nearby. Rod core types are a less

expensive choice where EMI is not a problem. Minimum

and maximum input voltage, output voltage and output

current must be established before an inductor can be

selected.

Step-Up Converter

In a step-up, or boost converter (Figure 1), power generated

by the inductor makes up the difference between input and

output. Power required from the inductor is determined by

PV

V

V

I

L

OUT

D

IN MIN

OUT

=+

()( )

–( )

01

S

APPLICATI

I FOR ATIO

Energy required by the inductor per cycle must be equal or

greater than

P f

L

OSC

/(

)

02

in order for the converter to regulate the output.

When the switch is closed, current in the inductor builds

according to

It

V

R

e

L

IN

Rt

L

()

'

–(

)

–'

=













103

where R' is the sum of the switch equivalent resistance

(0.8

Ω typical at 25°C) and the inductor DC resistance.

the simple lossless equation

It

V

L

t

L

IN

()

04

can be used. These equations assume that at t = 0,

inductor current is zero. This situation is called “discontinu-

ous mode operation” in switching regulator parlance.

Setting “t” to the switch-ON time from the LT1108 specifi-

cation table (typically 36

end of the switch-ON time can be calculated as

ELI

L

PEAK

=

1

2

05

2

()

to 1A or less. Higher switch currents will cause excessive

drop across the switch resulting in reduced efficiency. In

general, switch current should be held to as low a value as

possible in order to keep switch, diode and inductor losses

at a minimum.

As an example, suppose 12V at 30mA is to be generated

from a 2V to 3V input. Recalling equation (01),

P

V

V

V

mA

mW

()(

12

0 5

2

30

315

06

.–

(

)