2-18

TELCOM SEMICONDUCTOR, INC.

5V, DUAL TRIP POINT

TEMPERATURE SENSORS

TC620

TC621

Using The TC621

The TC621 operation is identical to that of the TC620,

but requires an external NTC thermistor. Use the resistance

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.

TYPICAL NTC THERMISTOR

010

20

30

40

50

100

70

TEMPERATURE (

°C)

0

50

60

150

200

250

300

350

Figure 4. Typical Thermistor Resistance vs. Temperature

TC621 Thermistor Selection

The TC621 uses an external thermistor to monitor the

controlling temperature. A thermistor with a resistance value

of approximately 100k

Ω at 25°C is recommended.

A temperature setpoint is selected by picking a resistor

whose value is equal to the resistance of the thermistor at the

desired temperature. For example, a 30k

Ω resistor between

ture trip point at +51

°C and a 49kΩ resistor on LOW TEMP

(pin 3) sets the low temperature trip point to +41

°C.

APPLICATIONS

Dual Speed Temperature Control

The Dual Speed Temperature Control uses a TC620

and a TC4469 quad driver. Two of the drivers are configured

in a simple oscillator. When the temperature is below the

LOW TEMP set point, the output of the driver is OFF. When

the temperature exceeds the LOW TEMP set point, the

TC4469 gates the oscillator signal to the outputs of the

driver. This square wave signal modulates the remaining

outputs and drives the motor at a low speed. If this speed

cannot keep the temperature below the HIGH TEMP set

point, then the driver turns on continuously which increases

the fan speed to high. The TC620 will monitor the tempera-

ture and only allow the fan to operate when needed, and at

the required speed to maintain the desired temperature. A

higher power option can be designed by adding a resistor

and a power MOSFET.

Temperature Controlled Fan

In this application, a high and a low temperature is

selected by two ‘set’ resistors. The TC620 monitors the

ambient temperature and turns the FET switch on when the

temperature exceeds the HIGH TEMP set point. The fan

remains on until the temperature decreases to the LOW

TEMP set point. This provides the hysteresis. In this appli-

cation, the fan turns on only when required.

The TC621 uses an external thermistor to monitor the

ambient temperature. This adds one part, but allows more

flexibility in location of the sensor.

TC620/621 Outputs

Both devices have complimentary output stages. They

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