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LT1110

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 en-

ergy transfer element, the inductor must fulfill three re-

quirements. First, the inductance 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 maxi-

mum current ratings of the LT1110 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

LT1110 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 resistance so excessive power is not lost as heat

in the windings. An additional consideration is Electro-

Magnetic Interference (EMI). Toroid and pot core type

inductors are recommended 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.

Inductor Selection — Step-Up Converter

In a step-up, or boost converter (Figure 4), power gener-

ated by the inductor makes up the difference between

input and output. Power required from the inductor is

determined by

OUT

IN MIN

OUT

()( )

–( )

where VD is the diode drop (0.5V for a 1N5818 Schottky).

APPLICATI

I FOR ATIO

Energy required by the inductor per cycle must be equal or

greater than

OSC

()

in order for the converter to regulate the output.

When the switch is closed, current in the inductor builds

according to

()

–(

)

–'













103

where R' is the sum of the switch equivalent resistance

(0.8

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

When the drop across the switch is small compared to VIN,

the simple lossless equation

() =

()

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

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

tinuous mode operation” in switching regulator parlance.

Setting “t” to the switch ON time from the LT1110 speci-

fication table (typically 10

µs) will yield IPEAK for a specific

“L” and VIN. Once IPEAK is known, energy in the inductor

at the end of the switch ON time can be calculated as

ELI

PEAK

()

EL must be greater than PL/fOSC for the converter to deliver

the required power. For best efficiency IPEAK should be

kept 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 120mA is to be generated

from a 4.5V to 8V input. Recalling equation (01),

L =+

()(

) =

0 5

4 5

120

960

.– .

. (

)

Energy required from the inductor is

kHz

OSC

960

13 7

(

)

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