LT1173

6

S

APPLICATI

I FOR ATIO

LT1173 • TA06

+

–

Ω

SET

V

+12V

+

µ

1 F*

µ

1000 F

100

V1

V2

LTC1050

Ω

1M

LT1173

CIRCUIT

*NON-POLARIZED

Figure 1. Test Circuit Measures No Load Quiescent Current of

LT1073 Converter

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. 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

appropriate switching topology. To operate as an efficient

energy transfer element, the inductor must fulfill three

requirements. 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

maximum current ratings of the LT1173 and inductor are

not exceeded at the other worst case condition of maxi-

mum 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

LT1173 based designs, small axial leaded units with

saturation current ratings in the 300mA to 1A range

(depending on application) are adequate. Lastly, the in-

ductor must have sufficiently low DC resistance so that

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 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.

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

(02)

Energy required by the inductor per cycle must be equal or

greater than

P

F

L

OSC

03

()

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













()

'

–

–'

104

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

()

05

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 LT1173 speci-

fication table (typically 23

the end of the switch ON time can be calculated as

ELi

L

PEAK

=

()

1

2

06

2

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.