TC620/TC621

DS21439C-page 6

© 2006 Microchip Technology Inc.

3.3

Using the TC621

The TC621 operation is similar to that of the TC620, but

requires an external NTC thermistor. Use the resis-

tance versus temperature curve of the thermistor to

determine the values of the programming resistors.

Note that the pin numbers for the HIGH SET and LOW

SET programming resistors for the TC621 are reversed

versus that of the TC620 (i.e., the resistor value on

HIGH SET [Pin 2] should always be lower than the one

connected to LOW SET [Pin 3]). Also note that the

outputs of the TC621 are LOW TRUE when used with

an NTC thermistor.

3.4

TC621 Thermistor Selection

The TC621 uses an external thermistor to monitor the

controlling temperature. A thermistor with a resistance

value

of

approximately

100

k

Ω at 25°C is

recommended.

A temperature set point is selected by picking a resis-

tor whose value is equal to the resistance of the ther-

mistor at the desired temperature. For example, using

the data shown in Figure 3-4, a 30 k

Ω resistor between

temperature trip point at +51°C and a 49 k

Ω resistor on

LOW TEMP (Pin 3) sets the low temperature trip point

to +41°C.

FIGURE 3-4:

Typical NTC Thermistor

3.5

TC620/TC621 Outputs

Both devices have complimentary output stages. They

are rated at a source or sink current of 1 mA maximum.

010

20

30

40

50

100

70

TEMPERATURE (°C)

0

50

60

150

200

250

300

350