LT1108_15 Datasheet, PDF (11/12 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1108
APPLICATI S I FOR ATIO
Figure 8 details current limit circuitry. Sense transistor Q1,
whose base and emitter are paralleled with power switch
Q2, is ratioed such that approximately 0.5% of Q2’s
collector current flows in Q1’s collector. This current
passed through internal 80Ω resistor R1 and out through
the I_LIM pin. The value of the external resistor connected
between I_LIM and V_IN sets the current limit. When sufficient
switch current flows to develop a V_BE across R1 + R_LIM, Q3
turns on and injects current into the oscillator, turning off
the switch. Delay through this circuitry is approximately
2µs. The current trip point becomes less accurate for
switch-ON times less than 5µs. Resistor values program-
ming switch-ON time for 2µs or less will cause spurious
response in the switch circuitry although the device will
still maintain output regulation.
R_LIM
(EXTERNAL)
V_IN
Q3
OSCILLATOR
DRIVER
I_LIM
R1
80Ω
(INTERNAL)
SW1
Q1
Q2
SW2
LT1108 • F08
Figure 8. LT1108 Current Limit Circuitry
USING THE GAIN BLOCK
The gain block (GB) on the LT1108 can be used as an error
amplifier, low battery detector or linear post regulator. The
gain block itself is a very simple PNP input op amp with an
open collector NPN output. The negative input of the gain
block is tied internally to the 1.245V reference. The positive
input comes out on the SET pin.
Arrangement of the gain block as a low battery detector
is straightforward. Figure 9 shows hookup. R1 and R2
need only be low enough in value so that the bias current
of the SET input does not cause large errors. 33k for R2
is adequate. R3 can be added to introduce a small amount
of hysteresis. This will cause the gain block to “snap”
when the trip point is reached. Values in the 1M to 10M
range are optimal. The addition however, of R3 will
change the trip point.
R1
V_BAT
V _IN
LT1108
1.245V
REF
–
SET
+
5V
47k
AO
TO
PROCESSOR
R2
GND
R3
R1 = ^{VLB – 1.25V}
35.1µA
V_LB = BATTERY TRIP POINT
R2 = 33k
R3 = 1.6M
LT1108 • F09
Figure 9. Setting Low Battery Detector Trip Point
Table 1. Inductor Manufacturers
MANUFACTURER
Coiltronics International
984 S.W. 13th Court
Pompano Beach, FL 33069
305-781-8900
Sumida Electric Co. USA
708-956-0666
PART NUMBERS
OCTA-PAC^TM
Series
CD54
CDR74
CDR105
Table 2. Capacitor Manufacturers
MANUFACTURER
Sanyo Video Components
1201 Sanyo Avenue
San Diego, CA 92073
619-661-6322
Nichicon America Corporation
927 East State Parkway
Schaumberg, IL 60173
708-843-7500
AVX Corporation
Myrtle Beach, SC
803-946-0690
PART NUMBERS
OS-CON Series
PL Series
TPS Series
Table 3. Transistor Manufacturers
MANUFACTURER
Zetex Inc.
87 Modular Avenue
Commack, NY 11725
516-543-7100
PART NUMBERS
ZTX 749 (NPN)
ZTX 849 (NPN)
ZTX 949 (PNP)
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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