LT1109

6

kept to 600mA 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 60mA is to be generated

from a 4.5V input. Recalling Equation 01,

(06)

Energy required from the inductor is

Picking an inductor value of 33

µH with 0.2Ω DCR results

in a peak switch current of

Since 4.77

µJ > 4µJ the 33µH inductor will work. This trial-

and-error approach can be used to select the optimum

inductor. Keep in mind the switch current maximum rating

of 1.2A. If the calculated peak current exceeds this, the

input voltage must be increased or the load decreased.

Capacitor Selection

The output capacitor should be chosen on the basis of its

equivalent series resistance (ESR). Surface-mount tanta-

lum electrolytics can be used provided the ESR value is

sufficiently low. An ESR of 0.1

Ω will result in a 50mV step

at the output of the converter when the peak inductor

current is 500mA. Physically larger capacitors have lower

ESR.

Diode Selection

Speed, forward drop, and leakage current are the three

main considerations in selecting a catch diode for LT1109

converters. General purpose rectifiers such as the 1N4001

are

unsuitable for use in any switching regulator applica-

tion. Although they are rated at 1A, the switching time of

a 1N4001 is in the 10

µs-50µsrange.Atbest,efficiencywill

be severely compromised when these diodes are used; at

worst, the circuit may not work at all. Most LT1109 circuits

will be well served by a 1N5818 Schottky diode. The

combination of 500mV forward drop at 1A current, fast

turn-ON and turn-OFF time, and 4

µA to 10µA leakage

current fit nicely with LT1109 requirements. At peak

switch currents of 100mA or less, a 1N4148 signal diode

may be used. This diode has leakage current in the 1nA to

5nA range at 25

°C and lower cost than a 1N5818.

Table 1. Inductor Manufacturers

MANUFACTURER

PART NUMBERS

Caddell-Burns

7120 Series

258 East Second Street

Mineola, NY 11501

516-746-2310

Coiltronics Incorporated

Surface Mount

6000 Park of Commerce Blvd.

CTX33-1

Boca Raton, FL 33487

407-241-7876

Coilcraft

DT3316 Series

1102 Silver Lake Road

Cary, IL 60013

708-639-6400

Sumida Electric Co., Ltd.

CD54

637 E. Golf Road, Suite 209

CD105

Arlington Heights, IL 60005

Surface Mount

708-956-0666

Table 2. Capacitor Manufacturers

MANUFACTURER

PART NUMBERS

Sanyo Video Components

OS-CON Series

2001 Sanyo Avenue

San Diego, CA 92173

619-661-6835

Matsuo Electronics

267 Series

2134 Main Street, Suite 200

Huntington Beach, CA 92648

714-969-2491

Kemet Electronics Corporation

T491 Series

Box 5928

Greenville, SC 29606

803-963-6621

Philips Components

49MC Series

2001 W. Blue Heron Blvd.

P.O. Box 10330

Riviera Beach, FL 33404

407-881-3200

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