Download DN190 - Op Amp, Comparator and Reference IC Provides Micropower Monitoring Capability

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Op Amp, Comparator and Reference IC Provides Micropower
Monitoring Capability – Design Note 190
Jim Williams
Introduction
The LTC®1541 combines a micropower amplifier, comparator and 1.2V reference in an 8-pin package. The part
operates from a single 2.5V to 12.6V supply with typical
supply current of 5µA. Both op amp and comparator
feature a common mode input voltage range that extends
from the negative supply to within 1.3V of the positive
supply. The op amp output stage swings from rail-to-rail.
Figure 1 lists additional features along with a block diagram
of the device. The part’s attributes suggest low power
monitoring applications and two such circuits are presented here.
Pilot Light Flame Detector with Low-Battery Lockout
Figure 2 shows a pilot light flame detector with low-battery
lockout. The amplifier (“A”), running open loop, compares
a small portion of the reference with the thermocouplegenerated voltage. When the thermocouple is hot, the
amplifier’s output swings high, biasing Q1 on. Hysteresis,
provided by the 10M resistor, ensures clean transitions,
while the diodes clamp static generated voltages to the
rails. The 100k–2.2µF RC filters the signal to the amplifier.
The comparator (“C”) monitors the battery voltage via the
2M–1M divider and compares it to the 1.2V reference. A
battery voltage above 3.6V holds C’s output high, biasing
Q2 on and maintaining the small potential at A’s negative
input. When the battery voltage drops too low, C goes low,
signaling a low-battery condition. Simultaneously, Q2 goes
off, causing A’s negative input to move to 1.2V. This biases
A low, shutting off Q1. The low outputs alert downstream
circuitry to shut down gas flow.
, LTC and LT are registered trademarks of Linear Technology Corporation.
LTC1541
+
1 AMPOUT
SUPPLY RANGE: 2.5V TO 12.6V
IQUIESCENT: 5µA
OP AMP VOS: 700µV
COMPARATOR VOS: 1mV
COMPARATOR HYSTERESIS: ± 3mV
COMMON MODE RANGE: 0V TO (VSUPPLY – 1.3V)
INPUT BIAS CURRENT: 1nA MAX, 10pA (25°C) TYP
REFERENCE: 1.2V ± 0.4%
OP AMP
VCC
8
COMPOUT
7
REF
6
COMPIN +
5
–
2 AMPIN –
×1
3 AMPIN +
COMP
4 VSS
–
+
DN190 F01
Figure 1. LTC1541 Block Diagram and Features of the Micropower Op Amp, Comparator and Reference
9/98/190
www.BDTIC.com/Linear
VIN = 3.6V TO 9V
1N457
10M
TYPE R
1k
–
+
100k
3
+
10k
1N457
2.2µF
2
8
A
1/2 LTC1541
+
OUTPUT HIGH = “ON”
OUTPUT LOW = “OFF”
1
–
SCR LOAD
2M
ICC = 7µA AT VIN = 5V
Q1
2N3904
5
+
C
1/2 LTC1541
1M
6
7
LOW BATTERY
–
2M
1.2V INTERNAL
REFERENCE
10k
Q2
VN2222
DN190 F02
Figure 2. Pilot Light Flame Detector with Low-Battery Lockout
Tip-Acceleration Detector for Shipping Containers
Figure 3’s circuit is a tip-acceleration detector for shipping
containers. It detects if a shipping container has been
subjected to excessive tipping or acceleration and retains
the detected output. The sensitivity and frequency response are adjustable. A potentiometer with a small pendulous mass biases the amplifier (“A”), operating at a gain of
12. Normally, A’s output is below C’s trip point and circuit
output is low. Any tip-acceleration event that causes A’s
9.1M
10M
9V
+
1M**
*
A
1/2 LTC1540
–
output to swing beyond 1.2V will trip C high. Positive
feedback around C will latch it in this high state, alerting the
receiving party that the shipped goods have been mishandled. Sensitivity is variable with potentiometer mechanical or electrical biasing or A’s gain. Bandwidth is
settable by selection of the capacitor at A’s input. The
circuit is prepared for use by applying power and pushing
the button in C’s output.
10M
9.1M
+
1N4148
C
1/2 LTC1541
NORMALLY CLOSED —
PUSH TO INITIALIZE
OUTPUT
–
820k
*OPTIONAL ACCELERATION/NOISE FILTER,
TYPICALLY 0.1µF, SEE TEXT
**POTENTIOMETER WITH PENDULOUS
WEIGHT ATTACHED
1.2V
DN190 F03
Figure 3. Tip-Acceleration Detector for Shipping Containers Retains Output if Triggered.
Sensitivity is Adjustable Via Amplifier Feedback Values. Capacitor Sets Acceleraton Response Bandwidth
For literature on our Comparators,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 2593
Linear Technology Corporation
dn190f LT/TP 0998 340K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
 LINEAR TECHNOLOGY CORPORATION 1998
www.BDTIC.com/Linear