ADT14GP AD [Analog Devices], ADT14GP Datasheet - Page 8

• Manufacturer Part Number: ADT14GP
• Description: Quad Setpoint, Programmable Temperature Monitor and Controller
• Manufacturer: AD [Analog Devices]

ADT14
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
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
which is a much more tolerable error in most systems. The V
and VPTAT outputs are also capable of delivering sufficient
current to contribute heating effects and should not be ignored.
Buffering the Voltage Reference
The reference output V
setpoint programming voltages for the ADT14. The onboard
V
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
buffering of V
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.
With excellent drift and noise characteristics, V
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
REF
output buffer is capable of 500 A output drive into as
T
T
P
P
REF
P
DISS
DISS
DISS
with a low drift voltage follower will ensure
0.6V 0.005 A 4 12 mW
JC
REF
JA
is used to generate the temperature
0.012 W
0.012 W
81 C
27 C
W
W
REF
0.97 C
0.32 C
offers a good
REF
–8–
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
gate-to-source voltage, or V
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.
GS
, on the order of –2 V to –5 V is
REV. 0