REV. B

TMP12

–9–

Preserving Accuracy Over Wide Temperature Range

Operation

The TMP12 is unique in offering both a wide-range temperature

sensor and the associated detection circuitry needed to imple-

ment a complete thermostatic control function in one monolithic

device. The voltage reference, setpoint comparators, and output

buffer amplifiers have been carefully compensated to maintain

accuracy over the specified temperature ranges in this applica-

tion. Since the TMP12 is both sensor and control circuit, in

many applications, the external components used to program

and interface the device are subjected to the same temperature

extremes. Thus, it is necessary to place components in close

thermal proximity minimizing large temperate differentials, and

to account for thermal drift errors where appropriate, such as

resistor matching temperature coefficients, amplifier error drift,

and the like. Circuit design with the TMP12 requires a slightly

different perspective regarding the thermal behavior of electronic

components.

PC Board Layout Considerations

The TMP12 also requires a different perspective on PC board

layout. In many applications, wide traces and generous ground

planes are used to extract heat from components. The TMP12

is slightly different in that ideal path for heat is via the cooling

system air flow. Thus, heat paths through the PC traces should

be minimized. This constraint implies that minimum pad sizes

and trace widths should be specified to reduce heat conduc-

tion. At the same time, analog performance should not be

compromised. In particular, the bottom of the setpoint resistor

ladder should be located as close to GND as possible, as

discussed in the Understanding Error Sources section of this

data sheet.

Thermal Response Time

The time required for a temperature sensor to settle to a specified

accuracy is a function of the thermal mass of the sensor and

the thermal conductivity between the sensor and the object being

sensed. Thermal mass is often considered equivalent to capaci-

tance. Thermal conductivity is commonly specified using the

symbol Q, and is the inverse of thermal resistance. It is commonly

specified in units of degrees per watt of power transferred across

the thermal joint. TPCs 2 and 4 illustrate the typical RC time

constant response to a step change in ambient temperature. Thus,

the time required for the TMP12 to settle to the desired accuracy is

dependent on the package selected, the thermal contact established

in the particular application, and the equivalent thermal conductiv-

ity of the heat source. For most applications, the settling time is

probably best determined empirically.

Switching Loads with the Open-Collector Outputs

In many temperature sensing and control applications, some

type of switching is required. Whether it be to turn on a heater

when the temperature goes below a minimum value or to turn

off a motor that is overheating, the open-collector outputs can

be used. For the majority of applications, the switches used need

to handle large currents on the order of 1 A and above. Because the

TMP12 is accurately measuring temperature, the open-collector

outputs should handle less than 20 mA of current to minimize

self-heating. Clearly, the trip point outputs should not drive the

equipment directly. Instead, an external switching device is

required to handle the large currents. Some examples of these

are relays, power MOSFETs, thyristors, IGBTs, and Darlington

transistors.

This section shows a variety of circuits where the TMP12 con-

trols a switch. The main consideration in these circuits, such as

the relay in Figure 10, is the current required to activate the switch.

NC

12V

NC = NO CONNECT

R1

R2

R3

IN4001

OR EQUIV

140

MOTOR

SHUTDOWN

2604-12-311

COTO

12V

8

7

6

5

1

2

3

4

TEMPERATURE

SENSOR AND

VOLTAGE

REFERENCE

WINDOW

COMPARATOR

HYSTERESIS

GENERATOR

100

TMP12

VPTAT

Figure 10. Reed Relay Drive

It is important to check the particular relay you choose to ensure

that the current needed to activate the coil does not exceed the

TMP12’s recommended output current of 20 mA. This is easily

determined by dividing the relay coil voltage by the specified

coil resistance. Keep in mind that the inductance of the relay

will create large voltage spikes that can damage the TMP12

output unless protected by a commutation diode across the coil,

as shown. The relay shown has a contact rating of 10 W maxi-

mum. If a relay capable of handling more power is desired, the

larger contacts will probably require a commensurably larger

coil, with lower coil resistance and thus higher trigger current.

As the contact power handling capability increases, so does the

OP193

VPTAT

200k

TMP12

SET

HIGH

SET

LOW

GND

V+

OVER

UNDER

HEATER

1

8

2

7

3

6

4

5

5V

NC

5V

NC = NO CONNECT

NC

5V

300k

130k

1 F

–1.5V

10k

0.1 F

Figure 9. An Analog Measurement Circuit for VPTAT