LT1109

5

LT1109 S8 A D 8

ATIO

OPER

reduced by using the 8-pin version since the quiescent

current flows from a lower voltage source. The SHUT-

DOWN pin disables the oscillator when taken to a logic “0.”

If left floating or tied high, the converter operates nor-

mally. With SHUTDOWN low, quiescent current remains

at 320

µA.

The 8-pin versions of the LT1109 have separate pins for

the device to be powered from the (lower) input voltage

rather than the (higher) output voltage. Although quies-

cent

current remains constant, quiescent power will be

S

APPLICATI

I FOR ATIO

Inductor Selection

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

netic flux in an inductor core, and then switching this

energy into the load. To operate as an efficient energy

transfer element, the inductor must fulfill three require-

ments. 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 that maxi-

mum current ratings of the LT1109 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

LT1109 designs, small ferrite surface-mount inductors

will function well. Lastly, the inductor must have suffi-

ciently low DC resistance so that excessive power is not

lost as heat in the windings. Look for DCR values in the

inductors’ specification tables; values under 0.5

Ωwillgive

best efficiency. 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.

Specifying a proper inductor for an application requires

first establishing minimum and maximum input voltage,

output voltage, and output current. In a step-up converter,

the inductive events add to the input voltage to produce the

output voltage. Power required from the inductor is deter-

mined by

(01)

P

F

L

OSC

02

()

It

V

L

t

L

IN

()

04

ELI

L

PEAK

=

()

1

2

05

2

It

V

R

e

L

IN

Rt

L













()

'

–

–'

103

Energy required by the inductor per cycle must be equal or

greater than

in order for the converter to regulate the output.

When the switch is closed, current in the inductor builds

according to

where R' is the sum of the switch equivalent resistance

(0.8 typical at 25

°C) and the inductor DC resistance. When

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 LT1109 speci-

fication table (typically 4.2

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