TC649 datasheet(10/28 Pages) MICROCHIP

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TC649

DS21449C-page 10

 2002 Microchip Technology Inc.

5.1

Temperature Sensor Design

The temperature signal connected to VIN must output a

voltage in the range of 1.25V to 2.65V (typical) for 0%

to 100% of the temperature range of interest. The

circuit in Figure 5-2 illustrates a convenient way to

provide this signal.

FIGURE 5-2:

Temperature Sensing

Circuit.

Figure 5-2 shows a simple temperature dependent

voltage divider circuit. RT1 is a conventional NTC ther-

mistor, while R1 and R2 are standard resistors. The

supply voltage, VDD, is divided between R2 and the

parallel combination of RT1 and R1. For convenience,

the parallel combination of RT1 and R1 will be referred

to as RTEMP. The resistance of the thermistor at various

temperatures is obtained from the manufacturer’s

specifications. Thermistors are often referred to in

terms of their resistance at 25°C.

Generally, the thermistor shown in Figure 5-2 is a non-

linear device with a negative temperature coefficient

(also called an NTC thermistor). In Figure 5-2, R1 is

used to linearize the thermistor temperature response

and R2 is used to produce a positive temperature

coefficient at the VIN node. As an added benefit, this

configuration produces an output voltage delta of 1.4V,

which is well within the range of the VC(SPAN)

specification of the TC649. A 100 k

Ω NTC thermistor is

selected for this application in order to keep IDIV at a

minimum.

For the voltage range at VIN to be equal to 1.25V to

2.65V, the temperature range of this configuration is

0°C to 50°C. If a different temperature range is required

from this circuit, R1 should be chosen to equal the

resistance value of the thermistor at the center of this

new temperature range. It is suggested that a maxi-

mum temperature range of 50°C be used with this cir-

cuit due to thermistor linearity limitations. With this

change, R2 is adjusted according to the following

equations:

EQUATION

More information about thermistors may be obtained

from AN679, “Temperature Sensing Technologies”,

and

AN685,

“Thermistors

Single

Supply

Temperature Sensing Circuit”, which can be down-

loaded

from

Microchip’s

website

www.microchip.com.

5.2

Auto-Shutdown Temperature

Design

A voltage divider on VAS sets the temperature at which

the part is automatically shut down if the sensed tem-

perature at VIN drops below the set temperature at VAS

(i.e. VIN < VAS). As with the VIN input, 1.25V to 2.65V

(typ.) corresponds to the temperature range of interest

from T1 to T2, respectively. Assuming that the temper-

ature sensor network designed above is linearly related

to temperature, the shutdown temperature TAS is

related to T2 and T1 by:

EQUATION

For example, if 1.25V and 2.65V at VIN corresponds to

a temperature range of T1 = 0°C to T2 = 125°C, and the

auto-shutdown temperature desired is 25°C, then VAS

voltage is:

EQUATION

The VAS voltage may be set using a simple resistor

divider, as is shown in Figure 5-3.

R2 = 23.2 kΩ

R1 =100 kΩ

RT1

NTC

Thermistor

100 k

Ω @ 25˚C

IDIV

VIN

VDD

VDD x R2

RTEMP (T1) + R2

= V(T1)

RTEMP (T2) + R2

= V(T2)

VDD x R2

Where T1 and T2 define the temperature range of the

circuit. RTEMP is the parallel equivalent of the

thermistor and R1 at those temperatures.

2.65V - 1.25V

T2 - T1

VAS - 1.25

TAS - T1

VAS - 1.25

VAS =

(

(TAS - T1) + 1.25

1.4V )

T2 - T1

VAS =

(25 - 0) + 1.25 = 1.53V

1.4V

(125 - 0)

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