LT1173CS8PBF LINEAR TECH, LT1173CS8PBF Datasheet - Page 11

• Manufacturer Part Number: LT1173CS8PBF
• Manufacturer: LINEAR TECH

A
Using the Gain Block
The gain block (GB) on the LT1173 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 posi-
tive input comes out on the SET pin.
Arrangement of the gain block as a low battery detector is
straightforward. Figure 12 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. 100k 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-10M range are
optimal. The addition of R3 will change the trip point,
however.
Table 1. Component Selection for Common Converters
G = Gowanda
C = Coiltronics
* Add 68 from I
** Add 100 from I
PPLICATI
VOLTAGE
2.0-3.1
2.0-3.1
2.0-3.1
2.0-3.1
6.5-9.5
INPUT
12-20
20-30
12
–5
–5
5
5
5
5
5
LIM
VOLTAGE
OUTPUT
LIM
to V
O
12
12
12
12
15
30
–5
–5
12
5
5
5
5
5
5
to V
IN
U
IN
S
I FOR ATIO
CURRENT (MIN)
U
OUTPUT
300mA
300mA
250mA
150mA
90mA
10mA
50mA
10mA
90mA
30mA
50mA
25mA
50mA
75mA
75mA
W
CIRCUIT
FIGURE
6
6
7
8
5
5
5
5
5
5
5
5
6
7
8
U
INDUCTOR
VALUE
220 H
150 H
120 H
150 H
120 H
100 H
220 H
470 H
100 H
470 H
100 H
100 H
47 H
47 H
47 H
V
BAT
Figure 12. Setting Low Battery Detector Trip Point
G GA10-472K, C CTX50-1
G GA10-223K, C CTX
G GA10-472K, C CTX50-1
G GA10-153K
G GA10-123K
G GA10-153K
G GA10-123K C CTX100-4
G GA10-103K, C CTX100-4
G GA10-472K, C CTX50-1
G GA20-223K
G GA20-473K
G GA10-103K, C CTX100-4
G GA40-473K
G GA10-103K, C CTX100-4
G GA10-103K, C CTX100-4
R1
R2
PART NUMBER
INDUCTOR
SET
1.245V
REF
–
+
GND
LT1173
V
IN
V
R1 =
R2 = 100k
R3 = 4.7M
LB
= BATTERY TRIP POINT
V
LB
11.7 A
R3
CAPACITOR
– 1.245V
10 F, 50V
VALUE
AO
100 F
100 F
100 F
220 F
470 F
100 F
220 F
220 F
22 F
47 F
22 F
47 F
47 F
47 F
+5V
100k
LT1173
TO
PROCESSOR
LT1173 • TA16
NOTES
11
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