2-17

TELCOM SEMICONDUCTOR, INC.

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DETAILED DESCRIPTION

The TC620/621 consists of a positive temperature coef-

ficient (PTC) temperature sensor, and a dual threshold

detector. Temperature setpoint programming is easily ac-

complished with external programming resistors from the

and LOW LIMIT outputs remain low as long as measured

temperature is below setpoint values. As measured tem-

perature increases, the LOW LIMIT output is driven high

when temperature equals the LOW SET setpoint (

±3°C

max). If temperature continues to climb, the HIGH LIMIT

output is driven high when temperature equals the HIGH

SET setpoint (Figure 1). The CONTROL (hysteresis) output

is latched in its active state at the temperature specified by

the HIGH SET resistor. CONTROL is maintained active until

temperature falls to the value specified by the LOW SET

resistor.

Programming The TC620

The resistor values to achieve the desired trip-point

temperatures on HIGH SET and LOW SET are calculated

using EQUATION 1 below:

Where: Rtrip = Programming resistor in Ohms

T

= The desired trip point temperature

in degrees Kelvin

Equation 1.

For example, a 50

°C setting on either the HIGH SET or

LOW SET input is calculated using Equation 1 as follows:

Care must be taken to ensure the LOW SET program-

ming resistor is a smaller value than the HIGH SET program-

ming resistor. Failure to do this will result in erroneous

operation of the CONTROL output.

Care must also be taken to ensure the LOW SET

temperature setting is at least 5

°C lower than the HIGH SET

temperature setting. That is:

LOW SET

≤ HIGH SET – 5°C

The nomograph of Figure 2 can help the user obtain an

estimate of the external resistor values required for the

desired LOW SET and HIGH SET trip points.

Built-in Hysteresis

To prevent output "chattering" when measured tem-

perature is at (or near) the programmed trip point values, the

LOW SET and HIGH SET inputs each have built-in hyster-

esis of – 2

°C below the programmed settings (Figure 3).

Figure 3: Built-in Hysteresis on Low Limit and High Limit Outputs

As shown, the outputs remain in their active state

(hysteresis) until temperature falls an additional 2

°C below

the user's setting.

TEMPERATURE (

°C)

-55

-35

-15

5

25

45

65

85

105

125

50

100

150

200

250

SET POINT

(SET POINT – 2

°C)

HIGH LIMIT

or LOW LIMIT

Output

Figure 1: TC620/621 Input vs. Output Logic

LOW SET POINT

HIGH SET POINT

LOW LIMIT OUTPUT

HIGH LIMIT OUTPUT

CONTROL OUTPUT (COOL OPTION)

CONTROL OUTPUT (HEAT OPTION)

TEMPERATURE

Figure 2. TC620 Sense Resistors vs. Trip Temperature

TC620

TC621

5V, DUAL TRIP POINT

TEMPERATURE SENSORS