LT1108CS8 Linear Technology, LT1108CS8 Datasheet - Page 11

• Manufacturer Part Number: LT1108CS8
• Description: IC DC/DC CONV ADJUSTBL OUT 8SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost), Invertingr

Specifications of LT1108CS8

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adjustable
• Current - Output: 400mA
• Frequency - Switching: 19kHz
• Voltage - Input: 2 ~ 30 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Power - Output: 500mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

A
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 80 resistor R1 and out through
the I
between I
switch current flows to develop a V
turns on and injects current into the oscillator, turning off
the switch. Delay through this circuitry is approximately
2 s. The current trip point becomes less accurate for
switch-ON times less than 5 s. Resistor values program-
ming switch-ON time for 2 s or less will cause spurious
response in the switch circuitry although the device will
still maintain output regulation.
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.
PPLICATI
LIM
pin. The value of the external resistor connected
LIM
V
OSCILLATOR
Figure 8. LT1108 Current Limit Circuitry
IN
and V
Q3
O
IN
U
sets the current limit. When sufficient
S
(EXTERNAL)
R
DRIVER
I FOR ATIO
LIM
U
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.
BE
I
W
across R1 + R
LIM
Q1
R1
80
(INTERNAL)
SW1
Q2
LT1108 • F08
SW2
U
LIM
, Q3
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
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
Table 3. Transistor Manufacturers
MANUFACTURER
Zetex Inc.
87 Modular Avenue
Commack, NY 11725
516-543-7100
V
BAT
Figure 9. Setting Low Battery Detector Trip Point
R1
R2
SET
1.245V
REF
+
–
LT1108
GND
V
IN
V
R1 =
R2 = 33k
R3 = 1.6M
LB
= BATTERY TRIP POINT
V
LB
35.1 A
R3
– 1.25V
AO
5V
PART NUMBERS
ZTX 749 (NPN)
ZTX 849 (NPN)
ZTX 949 (PNP)
PART NUMBERS
OCTA-PAC
Series
CD54
CDR74
CDR105
PART NUMBERS
OS-CON Series
PL Series
TPS Series
47k
LT1108
TO
PROCESSOR
11
LT1108 • F09
TM