5
LT1109A
The LT1109A is a micropower step-up DC/DC converter.
It uses Burst Mode
TM
operation to achieve micropower
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 360µA.
Burst Mode
TM
is a trademark of Linear Technology Corporation
OPERATIO
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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 10µs 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
TM
Series
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
TM
is a trademark of Coiltronics International
