LT1109
PPLICATI
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.
O U
W
U
A
S I FOR ATIO
are unsuitable for use in any switching regulator applica-
tion. Although they are rated at 1A, the switching time of
a1N4001isinthe10µs-50µsrange.Atbest,efficiencywill
be severely compromised when these diodes are used; at
worst,thecircuitmaynotworkatall.MostLT1109circuits
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 4µA 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.
As an example, suppose 12V at 60mA is to be generated
from a 4.5V input. Recalling Equation 01,
PL = (12V + 0.5V – 4.5V) (60mA) = 480mW.
Energy required from the inductor is
(06)
P
480mW
120kHz
L
=
= 4.0µJ.
07
( )
Table 1. Inductor Manufacturers
F
OSC
MANUFACTURER
PART NUMBERS
Caddell-Burns
7120 Series
Picking an inductor value of 33µH with 0.2Ω DCR results
258 East Second Street
Mineola, NY 11501
516-746-2310
in a peak switch current of
–1.0 • 4.2µs
4.5V
Coiltronics Incorporated
6000 Park of Commerce Blvd.
Boca Raton, FL 33487
407-241-7876
Surface Mount
CTX33-1
I
=
1– e
= 538mA.
08
( )
33µH
PEAK
1.0
Ω
Substituting IPEAK into Equation 03 results in
Coilcraft
DT3316 Series
1102 Silver Lake Road
Cary, IL 60013
708-639-6400
2
1
2
E = 33µH 0.538A = 4.77µJ.
)(
09
( )
(
)
L
Sumida Electric Co., Ltd.
637 E. Golf Road, Suite 209
Arlington Heights, IL 60005
708-956-0666
CD54
Since4.77µJ>4µJthe33µHinductorwillwork. Thistrial-
and-error approach can be used to select the optimum
inductor.Keepinmindtheswitchcurrentmaximumrating
of 1.2A. If the calculated peak current exceeds this, the
input voltage must be increased or the load decreased.
CD105
Surface Mount
Table 2. Capacitor Manufacturers
MANUFACTURER
PART NUMBERS
Sanyo Video Components
2001 Sanyo Avenue
San Diego, CA 92173
619-661-6835
OS-CON Series
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 0.1Ω 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.
Matsuo Electronics
267 Series
2134 Main Street, Suite 200
Huntington Beach, CA 92648
714-969-2491
Kemet Electronics Corporation
Box 5928
Greenville, SC 29606
803-963-6621
T491 Series
49MC Series
Diode Selection
Philips Components
2001 W. Blue Heron Blvd.
P.O. Box 10330
Riviera Beach, FL 33404
407-881-3200
Speed, forward drop, and leakage current are the three
main considerations in selecting a catch diode for LT1109
converters.Generalpurposerectifierssuchasthe1N4001
6
Linear [ Linear ]
