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ADT7476ARQH

dBCOOL® Remote Thermal Controller and Voltage Monitor

ON Semiconductor 

ADT7476

Table 6. Twos Complement Temperature Data Format

Temperature

Digital Output (10−Bit) (Note 1)

–128°C

1000 0000 00 (diode fault)

–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 LSB of measurement in the Extended

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

Remote Temperature Measurement

The ADT7476 can measure the temperature of two remote

diode sensors or diode−connected transistors connected to

Pin 17 and Pin 18, or Pin 15 and Pin 16.

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. As a result,

this technique is unsuitable for mass production. The

technique used in the ADT7476 is to measure the change in

VBE when the device is operated at two different currents.

Table 7. Extended Range, Temperature Data Format

Temperature

Digital Output (10−Bit) (Note 1)

–64°C

0000 0000 00 (diode fault)

–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 LSB of measurement in the Extended

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

This is given by:

DVBE

+

KT

q

In(N)

(eq. 1)

where:

k is the Boltzmann’s constant.

q is the charge on the carrier.

T is the absolute temperature in Kelvin.

N is the ratio of the two currents.

Figure 25 shows the input signal conditioning used to

measure the output of a remote temperature sensor. This figure

shows the external sensor as a substrate transistor, which is

provided on some microprocessors for temperature monitoring.

It could also be a discrete transistor such as a 2N3904/2N3906.

CPU

VDD

I

N × I IBIAS

REMOTE

SENSING

TRANSISTOR

THERMDA D+

THERMDC D–

BIAS

DIODE

LOW−PASS FILTER

fC = 65kHz

VOUT+

VOUT–

TO ADC

Figure 25. Signal Conditioning for Remote Diode Temperture Senors

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

and is 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 26 and Figure 27

show how to connect the ADT7476 to an NPN or PNP

transistor for temperature measurement. 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.

2N3904

NPN

ADT7476

D+

D–

Figure 26. Measuring Temperature Using an

NPN Transistor

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