MC623 Datasheet, PDF (3/4 Pages) ON Semiconductor – 3V, Dual Trip Point Temperature Sensor

MC623
DETAILED DESCRIPTION
MC623 Operation
The MC623 consists of a positive temperature coefficient
(PTC) temperature sensor and dual threshold detector.
Temperature set point programming is easily accomplished
with external programming resistors from the HIGH SET
and LOW SET inputs to V_CC. The HIGH LIMIT and LOW
LIMIT outputs remain inactive (low) as long as the
measured temperature is below setpoint values. As
temperature increases, the LOW LIMIT is driven high when
temperature exceeds the LOW LIMIT setpoint (±3°C). If
temperature continues to climb, the HIGH LIMIT output is
driven high when temperature exceeds the HIGH LIMIT
setpoint (±3°C). Figure 1 shows the relationship between the
sense resistance values and trip point temperature.
To prevent output ”chattering” when measured
temperature is at (or near) the programmed trip point values,
the LOW SET and HIGH SET inputs each have a built–in
hysteresis of - 2°C max. As a result, the HIGH LIMIT and
LOW LIMIT outputs remain active until the measured
temperature falls a maximum of 2°C below the programmed
HIGH SET and LOW SET thresholds as shown in Figure 3.
The programmed setting threshold of Figure 3 is
user–programmed temperature trip points of either the
LOW SET or HIGH SET inputs. The LOW LIMIT or HIGH
LIMIT output is driven active when temperature equals the
programmed setpoint value (to within 3°C). The output
remains active until the temperature falls an additional 2°C
below the setpoint due to hysteresis.
250
Programmed
Setpoint
200
–2°C Hysteresis
150
100
50
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
Figure 1. MC623 Sense Resistors vs. Trip
Temperature
The CONTROL output is driven high when the HIGH
LIMIT output goes high, and is reset low when the LOW
LIMIT output goes low. This output provides the logic for
simple ON/OFF fan control. Figure 2 shows overall MC623
operation.
High Set Point
Low Set Point
Low Limit Output
Temperature
High Limit Output
Control Output
(Cool Option)
Figure 2. MC623 Temperature vs. Output
High Limit or
Low Limit Output
Figure 3. High Set and Low Set Thresholds
APPLICATIONS
Mounting
If the MC623 is used to measure the temperature of
another device, it is important that the top surface of the
MC623 package be in intimate contact with the measured
device. Good thermal conductivity and no air space is
critical to accurate temperature measurement in applications
of this type.
Trip Point Programming
The resistance values required for the HIGH SET and
LOW SET inputs are calculated using the formula below:
R_TRIP = 0.5997 x T ^2.1312
Where Rtrip = Programming resistor value in Ohms
T = Desired trip temperature in degrees Kelvin.
For example, to program a trip point of 50°C, the
programming resistor is:
W R_TRIP = 0.5997 x (50 + 273.15)^2.1312) = 133,652
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