Datasheet

TC620/TC621
DS21439D-page 6  2001-2012 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 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 therm-
istor at the desired temperature. For example, using
the data shown in Figure 3-4, a 30 k resistor between
HIGH TEMP (Pin 2) and V
DD
(Pin 8) sets the high
temperature trip point at +51°C and a 49 k 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.
01020304050
100
70
TEMPERATURE (°C)
0
50
60
150
200
250
300
350
THERMISTOR RESISTANCE (kΩ)