ADT14

–8–

REV. 0

Safety Considerations

In heating and cooling system design, designers should antici-

pate potential system fault conditions which may result in sig-

nificant safety hazards which are outside the control of, and

cannot be corrected by, the ADT14 based circuit. Governmen-

tal and industrial regulations regarding safety requirements and

standards for such designs should be observed where applicable.

Self-Heating Effects

In some applications the user should consider the effects of self-

heating due to the power dissipated by the open-collector out-

puts, which are capable of sinking 5 mA each continuously.

Under full load, the ADT14 open-collector output device is

dissipating,

which, in the small outline package, accounts for a temperature

increase due to self-heating of

∆T = P

81

°C

W

= 0.97°C

This will directly affect the accuracy of the ADT14 and will, for

example, cause the device to switch the heating output off 0.97

degrees early. Alternatively, bonding the same package to a

moderate heatsink limits the self-heating effect to approximately

∆T = P

27

°C

W

= 0.32°C

and VPTAT outputs are also capable of delivering sufficient

current to contribute heating effects and should not be ignored.

Buffering the Voltage Reference

setpoint programming voltages for the ADT14. The onboard

µA output drive into as

much as a 50 pF load. Exceeding this load will affect the accu-

racy of the reference voltage, will increase thermal errors due to

internal heat generation, and may induce oscillations. External

optimal reference accuracy if a large load current is required.

Amplifiers that offer low drift, low power consumption, and low

cost appropriate to this application include the OP295 and

members of the OP90, OP97, OP177 families, and others shown

in the following applications circuits.

voltage reference for data acquisition and transducer excitation

applications as well.

Preserving Accuracy Over Wide Temperature Range Operation

The ADT14 is unique in offering both a wide-range tempera-

ture sensor and the associated detection circuitry needed to

implement 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

application. Since the ADT14 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 to minimize temperature 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 ADT14 requires a slightly

different perspective regarding the thermal behavior of elec-

tronic components.

Thermal Response Time

The time required for a temperature sensor to settle to a speci-

fied 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

capacitance. Thermal resistance is commonly specified in units

of degrees per watt of power transferred across the thermal joint.

Figure 3 illustrates the typical response to a step change in am-

bient temperature for PDIP and SOIC packages. Thus, the time

required for the ADT14 to settle to the desired accuracy is

dependent on the package selected, the thermal contact estab-

lished in the particular application, and the equivalent thermal

conductivity of the heat source. For most applications, the set-

tling 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’s 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 amp and above. Because

the ADT14 is accurately measuring temperature, the open-

collector outputs should handle less than 5 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 power MOSFETs, thyristors, IGBTs, and Darlingtons.

Figures 18a–18d show a variety of circuits where the ADT14

controls a switch. The main consideration in these circuits is the

current required to activate the switch.

Power FETs are popular for handling a variety of high current

DC loads. Figure 18b shows the ADT14 driving a P-channel

MOSFET transistor for a simple heater circuit. When the out-

put transistor turns on, the gate of the MOSFET is pulled down

to approximately 0.6 V, turning it on. For most MOSFETs a

sufficient to turn on the device.

Isolated Gate Bipolar Transistors (IGBT) combine many of the

benefits of power MOSFETs with bipolar transistors, and are

used for a variety of high power applications. Because IGBTs

have a gate similar to MOSFETs, turning the devices on and off

is relatively simple as shown in Figure 18c. The turn-on voltage

for the IGBT shown (IRGB40S) is between 3.0 and 5.5 volts.

This part has a continuous collector current rating of 50 A and a

maximum collector-to-emitter voltage of 600 V, enabling it to

work in very demanding applications.

The last class of high power devices discussed here are thyris-

tors, which include SCRs and triacs. Triacs are a useful alter-

native to relays for switching ac line voltages. The 2N6073A

shown in Figure 18d is rated to handle 4 A (rms). The opto-

isolated MOC3021 triac shown features excellent electrical

isolation from the noisy AC line and complete control over the

high power triac with only a few additional components.