ADT7490ARQZ-R7 datasheet(19/75 Pages) ONSEMI

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ADT7490

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Table 11. Twos Complement Temperature Data Format

Temperature

Digital Output (10−Bit) (Note 1)

–128°C

1000 0000 00 (diode fault)

–63°C

1100 0001 00

–50°C

1100 1110 00

–25°C

1110 0111 00

–10°C

1111 0110 00

0°C

0000 0000 00

10.25°C

0000 1010 01

25.5°C

0001 1001 10

50.75°C

0011 0010 11

75°C

0100 1011 00

100°C

0110 0100 00

125°C

0111 1101 00

127°C

0111 1111 00

1. Bold numbers denote 2 LSBs of measurement in the Extended

Resolution 2 register (Register 0x77) with 0.25°C resolution.

Table 12. Offset 64 Data Format

Temperature

Digital Output (10−Bit) (Note 1)

–64°C

0000 0000 00 (diode fault)

–63°C

0000 0001 00

–1°C

0011 1111 00

0°C

0100 0000 00

1°C

0100 0001 00

10°C

0100 1010 00

25°C

0101 1001 00

50°C

0111 0010 00

75°C

1000 1001 00

100°C

1010 0100 00

125°C

1011 1101 00

191°C

1111 1111 00

1. Bold numbers denote 2 LSBs of measurement in the Extended

Resolution 2 register (Register 0x77) with 0.25°C resolution.

Thermal Diode Temperature Measurement Method

A simple method of measuring temperature is to exploit

the negative temperature coefficient of a diode, measuring

the base−emitter voltage (VBE) of a transistor operated at

constant current. Unfortunately, this technique requires

calibration to null out the effect of the absolute value of VBE,

which varies from device to device.

The technique used in the ADT7490 is to measure the

change in VBE when the device is operated at three different

currents. Previous devices have used only two operating

currents, but the use of a third current allows automatic

cancellation of resistances in series with the external

temperature sensor.

Figure 28 shows the input signal conditioning used to

measure the output of an external temperature sensor. This

figure shows the external sensor as a substrate transistor, but

it could equally be a discrete transistor, such as a

2N3904/2N3906.

If a discrete transistor is used, the collector is not grounded

and should be linked to the base. If a PNP transistor is used,

the base is connected to the D– input and the emitter to the

D+ input. If an NPN transistor is used, the emitter is

connected to the D– input and the base to the D+ input.

Figure 25 and Figure 26 show how to connect the ADT7490

to an NPN or PNP transistor for temperature measurement.

Figure 25. Measuring Temperature Using an

NPN Transistor

2N3904

NPN

ADT7490

D+

D–

Figure 26. Measuring Temperature Using a

PNP Transistor

2N3906

PNP

ADT7490

D+

D–

To prevent ground noise from interfering with the

measurement, the more negative terminal of the sensor is not

referenced to ground, but is biased above ground by an

internal diode at the D− input. C1 can optionally be added as

a noise filter (recommended maximum value of 1000 pF).

However, a better option in noisy environments is to add a

filter, as described in the section.

Remote Temperature Measurement

The ADT7490 can measure the temperature of two remote

diode sensors or diode−connected transistors connected to

Pin 10 and Pin 11, or Pin 12 and Pin 13.

The forward voltage of a diode or diode−connected

transistor operated at a constant current exhibits a negative

temperature coefficient of about −2 mV/

°C. Unfortunately,

the absolute value of VBE varies from device to device, and

individual calibration is required to null this out. Therefore,

the technique is unsuitable for mass production. The

technique used in the ADT7490 is to measure the change in

VBE when the device is operated at three different currents.

This is given by:

(eq. 2)

DVBE +

In(N)

where:

k is the Boltzmann constant.

q is the charge on the carrier.

T is the absolute temperature in Kelvin.

N is the ratio of the two currents.

To measure

DVBE, the operating current through the

sensor is switched among three related currents. N1 x I and

N2 x I are different multiples of the current I, as shown in

Figure 27. The currents through the temperature diode are

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