AN929 Datasheet, PDF (5/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
Thermistor Logic Output Sensor
V_{DD Non-Inverting Comparator}
R₃
R_S
V_IN
R₁
V_DD
R_T
R₂
Thermistor ^R4
V_DD
V_REF
V_OUT
MCP6541
Silicon Logic Output Sensor
V_DD
Fan
R_SET
OUT NC
T_{SET OUT}
TC622 GND
Assume: V_OH = V_DD, V_OL = 0 and R₃ >> R₁
V_REF = V_DD x (R₄ / (R₂ + R₄))
V_TL ≅ [(R₁ + R₃)V_REF - (R₁ x V_DD)) / R₃]
V_TH ≅ [((R₁ + R₃)V_REF) / R₃]
V_HYS = V_TH - V_TL
≅ [(R₁ / R₃) x V_DD]
V_OUT
V_OH
V_OL
V_TL V_TH
V_IN
R_TRIP = 0.5997 x T^2.132
R_TRIP = Programming Resistor Value (Ω)
T = Set point Temperature (°C)
Set point Temp.
Hysteresis Temp.
Output Voltage
Fan
Off
On
Off
FIGURE 8:
Logic Output Sensors.
Most logic output sensors are available in either a hot
or cold option, as shown in Figure 9. The hot and cold
options are used to allow the flexibility of using the
switch in either hot (temperature increasing) or cold
(temperature decreasing) sensing applications. The
hot and cold options ensure that the hysteresis is in the
appropriate position, either below or above the
temperature set point.
For example, assume that the temperature switch is
being used to turn on a cooling fan. The setpoint is
75°C and the hysteresis is 10°C. A sensor with the hot
option will switch to the active logic level at 75°C. The
sensor will remain in the on condition and the fan will
run until the temperature drops to 65°C, or 10°C below
the temperature setpoint.
In contrast, assume that a cold option switch is used in
an application that requires a heater to turn on to pre-
vent freezing. Assume that the temperature set point is
0°C and the hysteresis is 10°C. The cold option sensor
will switch to the active logic level at 0°C and turn on the
heater. The sensor will remain in the on condition until
the temperature increases to 10°C, or 10°C above the
temperature set point.
Hot Option
T_SET > T_HYS
“ON” if temperature > T_SET
“OFF” if temperature < T_HYS
Voltage
OUT
Temperature
“Cold” T_HYS T_SET “Hot”
Cold Option
T_SET < T_HYS
“ON” if temperature < T_SET
“OFF” if temperature > T_HYS
Voltage
OUT
Temperature
“Cold” T_SET T_HYS “Hot”
FIGURE 9:
Logic Output Sensors - Hot
and Cold Options.
 2004 Microchip Technology Inc.
DS00929A-page 5