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Dual Micropower Comparator with Integrated 400mV Reference
Simplifi es Monitor and Control Functions – Design Note 321
Jon Munson
09/03/321_conv
L, LT, LTC, LTM, Linear Technology, the Linear logo and Over-The-Top are
registered trademarks and ThinSOT is a trademark of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
Introduction
The LT
®
6700 dual comparator incorporates features to
reduce part count in space-critical designs, including
a trimmed on-chip 400mV bandgap derived reference
and internal hysteresis mechanisms. The LT6700 also
features low voltage micropower single supply opera-
tion (1.4V to 18V, 7μA typical) and Over-The-Top
®
I/O
capability to maximize versatility and provide solutions
especially useful in portable battery-powered applica-
tions. The outputs are open collector to permit logical
wire-AND functionality, and can drive relatively heavy
loads (up to 40mA) such as relays or LED indicators.
The LT6700 supports a wide range of design confi gu-
rations, but still offers a minimum pin count package
(ThinSOT™, 6-lead). This is made possible by offering
the LT6700 in three different versions, each with a
different input confi guration. The LT6700-1 provides
the designer with one inverting and one noninverting
input, especially useful in window detection functions;
the LT6700-2 provides two inverting inputs; and the
LT6700-3 offers two noninverting inputs. The internal
reference is connected to one of the inputs of each
comparator section, as shown in Figure 1, and the
remaining two connections are brought out for signal
sensing by the user.
“Gas Gauge” Battery Monitor
It is easy to create a simple and accurate battery moni-
tor using the LT6700, thanks to the accurate internal
reference (±2% over temperature). Figure 2 shows an
implementation of a 2-threshold “alkaline-cell” battery
monitor. For the resistor values shown, the Pin 1 output
goes low when the pack voltage falls below 2V (1V per
cell) which corresponds to about 30% capacity remain-
ing. The Pin 6 output goes low as well at 1.6V (0.8V per
cell) as the battery pack reaches its rated end-of-life
voltage. The number of threshold points may easily be
increased by extending the resistor-divider chain and
using additional comparators.
Simple Window-Function Status Monitor
The LT6700-1 lends itself nicely to window comparison
applications, where the output wire-AND feature can
be exploited. Figure 3 shows a 48V power bus monitor
that provides an optoisolated alarm indication when
voltage limits are exceeded. The micropower opera-
tion of the circuit allows it to derive operating power
directly from the monitored voltage using simple Zener
diode techniques.
Figure 1. Pin Functions of the LT6700 Family Figure 2. Micropower “Gas Gauge” Battery Monitor
4
5
6
3
2
1
REF
LT6700-X
THREE AVAILABLE INPUT POLARITY OPTIONS:
LT6700-1 INVERTING (B) AND NONINVERTING (A)
LT6700-2 BOTH (A AND B) INVERTING
LT6700-3 BOTH (A AND B) NONINVERTING
OUT A
GND
IN A
OUT B
V
S
IN B
DN321 F01
–
+
–
+
COMP B
R4
1M
COMP A
ALKALINE
AA CELLS
V
R
= 400mV
REFERENCE
5
6
4
3
1
2
C1
0.1μF
R3
1M
LT6700-3
R2
63.4k
R1
261k
R5
1M
V
BATT
> 1.6V
V
BATT
> 2V
MONITOR CONSUMES ~10A
HYSTERESIS IS APPROXIMATELY
2% OF TRIP VOLTAGE
DN321 F02
V
BATT
1.4V (MIN)
3V (NOM)
+
+
V
S
