Chip information – Rainbow Electronics MAX837 User Manual
Page 5
both parts. For the MAX836, select the ratio of resistors
R1 and R2 so that IN sees 1.204V when the monitor
voltage falls to or rises above the desired trip point
(V
TRIP
). R3 adds hysteresis and is typically an order of
magnitude larger than R1 or R2. The current through R1
and R2 should be at least 500nA to ensure that the
12nA maximum input current does not shift the trip
point significantly. Capacitor C1 adds additional noise
rejection.
Monitoring Voltages Other than V
CC
The MAX836/MAX837 can monitor voltages other than
V
CC
(Figure 3). Calculate V
TRIP
as shown in the section
Programming the Trip Voltage. The monitored voltage
(V
MON
) is independent of V
CC
. V
IN
must be 1V less
than V
CC
.
Heater Temperature Control
Figure 4 shows a basic heater temperature-control cir-
cuit. Upon power-up, OUT is high and the N-channel
MOSFET turns on. Current flows through the heating
element (R4), warming the surrounding area. R2 is a
negative-temperature-coefficient thermistor and as tem-
perature increases, its resistance decreases. As the
thermistor heats up and its resistance decreases, the
MAX837’s voltage at IN decreases until it reaches the
1.204V threshold voltage. At this point, OUT goes low,
turning off the heating element. The thermistor cools
and the voltage at IN rises until it overcomes the
MAX837’s hysteresis (6mV). OUT returns high when this
point is reached, turning on the heating element again.
This cycle repeats as long as power is applied.
Determine the thermistor’s resistance (R2) at the
desired temperature. Then, using R2’s resistance and
half the resistance of R3, calculate R1’s value with the
following formula:
___________________Chip Information
TRANSISTOR COUNT: 54
R1 = (R2 + R3)
V
1.204
- 1
CC
MAX836/MAX837
4-Pin Micropower Voltage Monitors
_______________________________________________________________________________________
5
GND
NOTE: C1 ADDS ADDITIONAL NOISE IMMUNITY
OUT
OUT
IN
V
CC
V
CC
C1
R1
R3
R2
0.1
µ
F
MAX837
Figure 2. Adding Hysteresis to the Comparator
GND
OUT
IN
V
CC
V
MON
V
CC
R2
R1
0.1
µ
F
MAX837
Figure 3. Monitoring Voltages Other than V
CC
IN
THERMISTOR
WITH
NEGATIVE
COEFFICIENT
HEATING
ELEMENT
R4
OUT
GND
V
CC
V
CC
V
CC
R2
T
R3
R1
0.1
µ
F
R1
= (
R2 + R3)
(
- 1
Ω
)
1.204
MAX837
Figure 4. Heater Temperature Control