ADT7475ARQZ(2011) 查看數據表（PDF） - ON Semiconductor

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ADT7475ARQZ
(Rev.:2011)

dbCOOL™ Remote Thermal Monitor and Fan Controller

ON Semiconductor 

ADT7475

Temperature Measurement Method

Local Temperature Measurement

The ADT7475 contains an on−chip band gap temperature

sensor whose output is digitized by the on−chip, 10−bit

ADC. The 8−bit MSB temperature data is stored in the

temperature registers (0x25, 0x26, and 0x27). Because both

positive and negative temperatures can be measured, the

temperature data is stored in Offset 64 format or twos

complement format, as shown in Table 3 and Table 4.

Theoretically, the temperature sensor and ADC can

measure temperatures from −128°C to +127°C (or −64°C to

+191°C in the extended temperature range) with a

resolution of 0.25°C.

However, this exceeds the operating temperature range of

the device, so local temperature measurements outside the

ADT7475 operating temperature range are not possible.

Remote Temperature Measurement

The ADT7475 can measure the temperature of two remote

diode sensors or diode−connected transistors connected to

Pin 10 and Pin 11 or to 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. Because the

absolute value of VBE varies from device to device and

individual calibration is required to null this out, the

technique is unsuitable for mass production.

CPU

VDD

I

N y I IBIAS

REMOTE

SENSING

TRANSISTOR

THERMDA D+

THERMDC D–

BIAS

DIODE

LOW−PASS FILTER

fC = 65kHz

VOUT+

VOUT–

TO ADC

Figure 20. Signal Conditioning for Remote Diode Temperature Sensors

The technique used in the ADT7475 is to measure the

change in VBE when the device is operated at two different

currents.

This is given by:

DVBE + kTńq In(N)

(eq. 1)

where:

k is 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 20 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,

provided for temperature monitoring on some

microprocessors. It could also 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 21 and Figure 22 show how to connect the ADT7475

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.

To measure DVBE, the sensor is switched between

operating currents of I and N x I. The resulting waveform is

passed through a 65 kHz low−pass filter to remove noise and

to a chopper stabilized amplifier that performs the functions

of amplification and rectification of the waveform to

produce a dc voltage proportional to DVBE. This voltage is

measured by the ADC to give a temperature output in

10−bit, twos complement format. To further reduce the

effects of noise, digital filtering is performed by averaging

the results of 16 measurement cycles.

A remote temperature measurement takes nominally

38 ms. The results of remote temperature measurements are

stored in 10−bit, twos complement format, as shown in

Table 3. The extra resolution for the temperature

measurements is held in the Extended Resolution Register 2

(0x77). This gives temperature readings with a resolution of

0.25°C.

Noise Filtering

For temperature sensors operating in noisy environments,

previous practice was to place a capacitor across the D+ pin

and D− pin to help combat the effects of noise. However,

large capacitance’s affect the accuracy of the temperature

measurement, leading to a recommended maximum

capacitor value of 1000 pF.

This capacitor reduces the noise but does not eliminate it.

Sometimes, this sensor noise is a problem in a very noisy

environment. In most cases, a capacitor is not required

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