TC623
DS21441D-page 4 2001-2012 Microchip Technology Inc.
3.0 DETAILED DESCRIPTION
3.1 TC623 Operation
The TC623 has a positive temperature coefficient
(Silicon) temperature sensor and dual threshold detec-
tor. Temperature set point programming is accom-
plished with external 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 set point values. As
temperature increases, the LOW LIMIT is driven high
when temperature equals the LOW LIMIT set point
(±3°C). If temperature continues to climb, the HIGH
LIMIT output is driven high when temperature equals
the HIGH LIMIT set point (±3°C).
Figure 3-1 shows the relationship between the sense
resistance values and trip point temperature.
FIGURE 3-1: TC623 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 3-2 shows
overall TC623 operation.
FIGURE 3-2: TC623 TEMPERATURE
VS. OUTPUT
To prevent output “chattering” when measured temper-
ature is at (or near) the 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 3-3.
The programmed setting threshold of Figure 3-3 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 set point value (to within 3°C). The output remains
active until the temperature falls an additional 2°C
below the set point due to hysteresis.
FIGURE 3-3: HIGH SET AND LOW SET
THRESHOLDS
TEMPERATURE (°C)
RESISTANCE, R
TRIP
(kΩ)
-55 -35 -15 5 25 45 65 85 105 125
50
100
150
200
250
Low Set Point
High Set Point
Low Limit Output
High Limit Output
Control Output (Cool Option)
Temperature
Programmed Setpoint
-2°C Hysteresis
High Limit or Low Limit Ouput
