5

LT1109A

The LT1109A is a micropower step-up DC/DC converter.

It uses Burst Mode

operation yet still deliver more than 2W of output power

from a 5V supply. Circuit operation can be best under-

stood by referring to the LT1109A block diagram. With

SHUTDOWN high, comparator A1 compares the feedback

(FB) pin voltage with the 1.25V reference signal. When FB

drops below 1.25V, A1 switches on the 120kHz oscillator.

The driver amplifier boosts the signal level to drive the

output NPN power switch. When the FB voltage is sufficient

to trip A1, the oscillator is turned off. A low signal on the

shutdown pin gates off the oscillator, overriding A1. With

SHUTDOWN low, quiescent current remains at 360

µA.

Burst Mode

OPERATIO

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: inductance value, saturation current and DC resis-

tance. A fourth requirement is physical size. The inductors

recommended with the LT1109A circuits are small, sur-

face-mountable and are designed for switch-mode appli-

cations. Avoid using RF chokes or air core units since they

have very low peak current ratings. The LT1109A works

best in situations where the input voltage does not vary

much since the device has no internal switch current limit

function. For situations where the input voltage varies,

such as battery inputs, the LT1107 or LT1111 is suggested

instead.

Capacitor Selection

The output capacitor should be chosen on the basis of its

equivalent series resistance (ESR) and capacitance value.

Low ESR tantalum surface-mountable capacitors such as

those made by AVX are well-suited for DC/DC converter

applications. Inexpensive aluminum electrolytics may have

excessive ESR, resulting in high output ripple. These

should be avoided.

Diode Selection

Speed, forward drop, and leakage current are the three

main considerations in selecting a diode for LT1109A

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 to 50µs range. At best, efficiency

will be severely compromised if this diode is used; at

worst, the circuit may not work at all. The 1N5818 is an

ideal choice for LT1109A circuits. Surface-mountable

versions, such as the MBRS130T3, are available as well.

Table 1. Inductor Manufacturers

MANUFACTURER

PART NUMBERS

Coiltronics International

Surface Mount

984 S.W. 13th Court

OCTA-PAC

Pompano Beach, FL 33069

305-781-8900

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

AVX

TPS Series

Myrtle Beach, SC 29578

803-946-0690

Philips Components

49MC Series

2001 W. Blue Heron Blvd.

P.O. Box 10330

Riviera Beach, FL 33404

407-881-3200

Sanyo Video Components

OS-CON Series

1201 Sanyo Avenue

San Diego, CA 92073

619-661-6322

OCTA-PAC