IRF7401 datasheet(21/24 Pages) AD

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REV. C

ADN8830

–21–

Using an RTD for Temperature Sensing

The ADN8830 can be used with a resistive temperature device

(RTD) as the temperature feedback sensor. The resistance of an

RTD is linear with respect to temperature, offering an advan-

tage over thermistors that have an exponential relationship to

temperature. A constant current applied through an RTD will

yield a voltage proportional to temperature. However, this volt-

age could be on the order of only 0.5 mV/

°C, thus requiring the

use of additional amplification to achieve a usable signal level.

The ADT70 from Analog Devices can be used to bias and amplify

the voltage across an RTD, which can then be fed directly to the

THERMIN pin on the ADN8830 to provide temperature

feedback for the TEC controller. The ADT70 uses a 0.9 mA

current source to drive the RTD and an instrumentation ampli-

fier with adjustable gain to boost the RTD voltage. Application

notes and typical schematics for this device can be found in the

ADT70 Data Sheet.

Most RTDs have a positive temperature coefficient, also called

tempco, as opposed to thermistors, which have a negative tempco.

For the OUT A output to drive the TEC– input as shown in

Figure 1, the signal from an RTD must be conditioned to create

a negative tempco. This can be easily done using an inverting

amplifier. Alternately, OUT A can be connected to drive TEC+

with OUT B driving TEC– with a positive tempco at THERMIN.

This is highlighted in the Output Driver Amplifiers section.

For the ADN8830, proper operation care should be taken

to ensure the voltage at THERMIN remains within 0.4 V

and 2.0 V. Using a 1 k

Ω RTD with the ADT70 will yield a

THERMIN voltage of 0.9 V at 25

°C. Using the application

circuit shown in Figure 22 will provide a nominal output

voltage of 1.0 V at 25

°C and a total gain of 66.7 mV/Ω.

Using an RTD with a temperature coefficient of 0.375

Ω/°C

will give a THERMIN voltage swing from 1.5 V at 5

°C to

0.5 V at 45

°C, well within the input range of the ADN8830.

Using a Resistive Load as a Heating Element

The ADN8830 can be used in applications that do not neces-

sarily drive a TEC but require only a high current output into a

load resistance. Such applications generally only require heating

above ambient temperature and simply use the power dissipated

by the load element to accomplish this. Because the power

dissipated by such an element is proportional to the square of

the output voltage, the ADN8830 application circuit must be

modified. Figure 24 shows the preferred method for driving a

heating element load.

OUT B

PVDD

OUT A

NO CONNECTION TO P2 REQUIRED

Figure 24. Using the ADN8830 to Drive a Heating Element

Current is delivered from the PWM amplifier through Q3 when

the voltage at THERMIN is lower than TEMPSET. If the object

temperature is greater than the target temperature, Q3 will turn

off and the current through the load goes to zero, allowing the

object to cool back toward the ambient temperature. As the

target temperature is approached, a steady output current should

be reached. Naturally, a proper compensation network must be

found to ensure stability and adequate temperature settling time.

The P2 output from the ADN8830 should be left unconnected.

Suggested Pad Layout for CP-32 Package

Figure 25 shows the dimensions for the PC board pad layout for

the ADN8830, which is a 5

5, 32-lead lead frame chipscale

package. This package has a metallic heat slug that should be

soldered to a copper pad on the PC board. Although the pack-

age slug is electrically connected to the substrate of the IC, the

copper pad should be left electrically floating. This prevents

potential noise injection into the substrate while maintaining

good thermal conduction to the PC board.

0.10

(0.0039)

3.78

(0.1488)

5.36

(0.2110)

3.68

(0.1449)

0.28

(0.0110)

0.50

(0.0197)

0.69

(0.0272)

CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS

(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR

REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN

THERMAL PAD SHOULD BE SOLDERED TO AN ELECTRICALLY FLOATING

PAD ON THE PC BOARD

PACKAGE

OUTLINE

Figure 25. Suggested PC Board Layout for CP-32

Pad Landing

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