DN321 - Dual Micropower Comparator with Integrated 400mV Reference Simplifies Monitor and Control Functions

Dual Micropower Comparator with Integrated 400mV Reference
Simplifies Monitor and Control Functions – Design Note 321
Jon Munson
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 operation (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 applications. 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 configurations, 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 configuration. 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 monitor using the LT6700, thanks to the accurate internal
reference (±2% over temperature). Figure 2 shows an
OUT A 1
LT6700-X
GND 2
IN A 3
4 IN B
DN321 F01
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
Figure 1. Pin Functions of the LT6700 Family
09/03/321_conv
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 operation of the circuit allows it to derive operating power
directly from the monitored voltage using simple Zener
diode techniques.
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.
VBATT
1.4V (MIN)
3V (NOM)
5
R3
1M
R4
1M
LT6700-3
4
+
ALKALINE
AA CELLS
+
+
R2
63.4k
VBATT > 1.6V
–
VR = 400mV
REFERENCE
–
3
R5
1M
COMP B
6
6 OUT B
5 VS
REF
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 remaining. 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.
VS
COMP A
1
VBATT > 2V
+
C1
0.1μF
R1
261k
2
DN321 F02
MONITOR CONSUMES ~10A
HYSTERESIS IS APPROXIMATELY
2% OF TRIP VOLTAGE
Figure 2. Micropower “Gas Gauge” Battery Monitor
+
33k
1.74M
VL
LED OFF
VH
VOUT
VIN
5
VS
1
3
+INA OUTA
7.87k
VIN
–INB OUTB
GND
2
HYSTERESIS ZONES
APPROXIMATELY
2% OF TRIP VOLTAGE
3V/5V
LT6700-1
4
LED ON
22V
CMPZ5251B
27k
33k
1
6
6
0.1μF
VOUT
LOW = (39V < VIN < 70V)
MOC-207
10k
2
5.1V
CMPZ5231B
–
5
DN321 F03
Figure 3. 48V Power Bus Status Monitor
During bus operation within the normal voltage range,
neither comparator output is active, so the LED is on
and the alarm output is low (alarm clear). If the bus
voltage deviates sufficiently, then one of the comparators shunts away the LED drive resulting in assertion of
the alarm. Notice that any failure mode that causes an
open connection to the photo-transistor or prevents the
LED from operating (i.e., other open-circuit conditions),
produces a fail-safe alarm indication at the destination
logic input. The ability of the LT6700 to operate down to
very low voltage assures correct alarm indication even
during deep sags in bus potential (the 22V Zener further
eliminates the possibility of false-fault indication during
a bus power-down transition by disabling the LED).
Micropower Thermostat/Temperature Alarm
Though the 400mV reference is not directly available to
the circuit designer, an inverting comparator section can
be used to scale an external voltage in proportion to the
reference by implementing a simple “bang-bang” servo.
This technique is shown in Figure 4, where the multiplier
is set to two by an equal-resistor feedback path. The
inverting comparator steers current to hold the voltage
on a capacitor such that the feedback “bangs” between
the input hysteresis points. The LT6700 hysteresis is
nominally 6.5mV so this circuit has about 13mVP-P
ripple on the servo capacitor.
The other comparator section actually performs the
alarm decision which is simply based on the imbalance
of a resistor half-bridge. In this circuit, the thermistor
resistance is balanced against a known resistor, so the
temperature threshold is established easily by selecting
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0.1μF
1.4V TO 18V
(IS ≈ 10A)
t7REF
RSET
499k
5
VS
3
1
+INA OUTA
LT6700-1
4
3.3μF
RTH
T
499k
–INB OUTB
GND
220k
220k*
T < TSET
6 10k
3.3μF
2
DN321 F04
RTH = 1M (e.g., YSI 44015, 1.00MΩ AT 25°C)
RSET = RTH AT TSET
*RESISTANCE MAY REQUIRE OPTIMIZATION FOR OPERATION
OVER INTENDED RTH AND VSUPPLY RANGES
HYSTERESIS ZONE ≈ 0.4°C
Figure 4. Micropower Thermostat/Temperature Alarm
RSET from the thermistor table for the temperature of
interest. Since the resistance varies about –4.4%/°C
for the thermistor shown, the temperature hysteresis
of the output signal is about 0.4°C, suitable for most
environmental control applications. The capacitor in
parallel with the thermistor filters away the ripple of the
reference multiplier circuit. The micropower consumption of this circuit permits over two years of continuous
operation from a common 3V coin-cell (i.e., CR2032).
Conclusion
The LT6700 provides compact, micropower solutions
for threshold-based status and control functions. The
extra-wide supply range and Over-The-Top features
offer performance ideal for portable, battery-powered
products as well as industrial applications.
For applications help,
call (408) 432-1900
dn321f_conv LT/TP 0903 351.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2003
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