TC623

DS21441B-page 4

© 2002 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

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)

-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