LT1108
O U
W
U
PPLICATI
A
S I FOR ATIO
5V
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 ILIM pin. The value of the external resistor connected
betweenILIM andVIN setsthecurrentlimit. Whensufficient
switchcurrentflowstodevelopaVBE across R1 +RLIM, 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.
V
IN
LT1108
47k
R1
R2
1.245V
REF
–
+
AO
V
TO
BAT
PROCESSOR
SET
GND
R3
V
– 1.25V
35.1µA
LB
R1 =
V
= BATTERY TRIP POINT
LB
R2 = 33k
R3 = 1.6M
LT1108 • F09
Figure 9. Setting Low Battery Detector Trip Point
R
I
LIM
LIM
(EXTERNAL)
Table 1. Inductor Manufacturers
MANUFACTURER
V
IN
R1
80Ω
(INTERNAL)
PART NUMBERS
OCTA-PACTM
Series
Coiltronics International
984 S.W. 13th Court
Pompano Beach, FL 33069
305-781-8900
Q3
SW1
Q2
DRIVER
Q1
OSCILLATOR
Sumida Electric Co. USA
708-956-0666
CD54
CDR74
CDR105
SW2
LT1108 • F08
Figure 8. LT1108 Current Limit Circuitry
Table 2. Capacitor Manufacturers
MANUFACTURER
PART NUMBERS
USING THE GAIN BLOCK
Sanyo Video Components
1201 Sanyo Avenue
San Diego, CA 92073
619-661-6322
OS-CON Series
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
blockistiedinternallytothe1.245Vreference.Thepositive
input comes out on the SET pin.
Nichicon America Corporation
927 East State Parkway
Schaumberg, IL 60173
708-843-7500
PL Series
AVX Corporation
Myrtle Beach, SC
803-946-0690
TPS Series
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
isadequate. R3canbeaddedtointroduceasmallamount
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.
Table 3. Transistor Manufacturers
MANUFACTURER
PART NUMBERS
Zetex Inc.
ZTX 749 (NPN)
ZTX 849 (NPN)
ZTX 949 (PNP)
87 Modular Avenue
Commack, NY 11725
516-543-7100
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-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
11
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