ADT14

–14–

REV. 0

Intelligent Temperature Control System—Interfacing the

8051

C with the ADT14

Connecting the 8051, or any modern microcontroller, with the

ADT14 yields a powerful temperature control tool. For ex-

ample, by using digitally controlled resistors (AD8402/3), a user

may automatically set the temperature trip points via computer

or touch pad. The ADT14 outputs are fed back to the controller

allowing it to sense whether or not a setpoint has been exceeded.

The higher temperature trip-point data can then be used to turn

on a fan to either a high or low setting. In addition, the lower

temperature trip-point data may be used to turn on a heater to

either a high or low setting.

The AD8403 is shown in a single ladder resistor configuration

with an additional fix resistor to set the voltages of all four

setpoints. The microcontroller program that runs the AD8403

can also be set to ensure that no out-of-bound conditions occur.

Finally, the AD8403 also has shutdown pin which, when acti-

vated, will lower power consumption to 5

µA max (0.01 µA

typical), while still retaining memory of the potentiometer set-

tings.

Power-Down in Battery Powered Applications

Battery powered applications require low power consumption

components. The ADT14 is just such a device. The low supply

current (< 600

µA) of the ADT14 makes it possible to power

the device from any suitable logic output pin. Figure 29 is a

schematic of an 'HC04 CMOS inverter driving the ADT14. When

the SLEEP pin is high, the ADT14 is powered down. Figure 5

shows a typical start-up response of an ADT14. Most tempera-

ture measurements vary slowly over time, which allows designs

P3.2

P3.1

P3.0

P3.3

P2.0

P2.1

P2.2

P2.3

P2.4

P1.0/INT2

P0.0 TO

P0.7

USER

INPUTS

CS

CLK

SDI

SHDN

GND

10 F

+5V

SET1

SET2

SET3

SET4

HYS

TRIP1

TRIP2

TRIP3

TRIP4

GND

+5V

0.1 F

ADT14

AD8403

80CL51 C

TO 2ND

AD8403

RS

DIS

THR

TRIG

OUT

CONT

TLC555

GND

+5V

0.1 F

0.1 F

NC

NC = NO CONNECT

Figure 28. Typical Microcontroller Setup Configuration

that consume very little power over very long periods of time by

using a duty-cycling technique. The basic idea of the duty-

cycling technique is to simply measure the temperature at se-

lected time intervals and leave the device powered down the rest

of the time. Instead of a CMOS inverter, any other logic pin

with sufficient drive capability (such as the output of a micro-

processor) can be used to power on and off the ADT14.

ADT14

V+

'HC04

SLEEP

Figure 29. Using a CMOS Inverter to Power Down the

ADT14

Combining Trip Point Outputs to Yield 25%, 50%, 75%, and

100% Power Outputs

It is often desirable to power a device at varying levels based

upon ambient temperature. For example, suppose a liquid is

below a desired temperature, the lowest trip point temperature.

In this case, full power should be delivered to the heating ele-

ment, causing the temperature to rise as rapidly as possible.

When the lowest trip-point is finally crossed, a reduction in

power should occur, slowing the rate of increase in the heating

process. This process continues until all trip points are crossed

and the current to the heating element is reduced to zero.