ASC7531A 查看數據表（PDF） - Unspecified

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ASC7531A

LOW-VOLTAGE 1-WIRE DIGITAL TEMPERATURE SENSOR AND VOLTAGE MONITOR

Unspecified 

aSC7531A

Applications Information

Remote Diodes

The aSC7531 is designed to work with a variety of remote

sensors in the form of a diode-connected transistor or the

substrate thermal diode of a CPU or graphics controller.

Actual diodes are not suited for these measurements.

There is some variation in the performance of these diodes,

described in terms of its departure from the ideal diode

equation. This factor is called diode non-ideality, nf .

The equation relating diode temperature to a change in

thermal diode voltage with two driving currents is:

ΔVBE

=

KT

(nf ) ln( N )

q

where:

nf = diode-connected 2N3904 or CPU substrate non-ideality

factor.

K = Boltzman’s constant, (1.38 x 10-23).

T = diode junction temperature in Kelvins.

q = electron charge (1.6 x 10-19 Coulombs).

N = ratio of the two driving currents (10).

The aSC7531A is designed and trimmed for an expected nf

value of 1.0046, based on the typical value for the 2N3904.

There is also a tolerance on the value provided. Table 6

gives a representative sample of what one may expect in the

range of non-ideality.

For the aSC7531A, when thermal diode has a non-ideality

factor other than 1.0046 the difference in temperature

reading at a particular temperature may be interpreted with

the following equation:

where:

Tactual

=

Treported

⎜⎜⎝⎛

1.0046

nactual

⎟⎟⎠⎞

Treported = reported temperature in temperature register.

Tactual = actual remote diode temperature.

nactual = selected diode’s non-ideality factor, nf .

Temperatures are in Kelvins or °C + 273.15.

This equation assumes that the series resistance of the

remote diode 0.6Ω.

Although the temperature error caused by non-ideality

difference is directly proportional to the difference from

1.0046, a small difference in non-ideality results in a

relatively large difference in temperature reading. For

example, if there were a ±1% tolerance in the non-Ideality of

a diode it would result in a ±2.7 degree difference (at 0°C) in

the result (0.01 x 273.15).

The aSC7531B is designed and trimmed for an expected nf

value of 1.009, based on the typical value for the 65nM

Pentium CPU. There is also a tolerance on the value

provided. The values for CPUs may have different nominal

values and tolerances. Consult the CPU or GPU

manufacturer’s data sheet for the nf factor. Table 6 gives a

representative sample of what one may expect in the range

of non-ideality. The trend with CPUs is for a lower value with

a larger spread.

When thermal diode has a non-ideality factor other than

1.009 the difference in temperature reading at a particular

temperature may be interpreted with the following equation:

where:

Tactual

= Treported

⎛

⎝⎜⎜

1.009

n

actual

⎞

⎠⎟⎟

Treported = reported temperature in temperature register.

Tactual = actual remote diode temperature.

nactual = selected diode’s non-ideality factor, nf .

Temperatures are in Kelvins or °C + 273.15.

This equation assumes that the series resistance of the

remote diode 4.52Ω.

Although the temperature error caused by non-ideality

difference is directly proportional to the difference from 1.009,

but a small difference in non-ideality results in a relatively

large difference in temperature reading. For example, if there

were a ±1% tolerance in the non-Ideality of a diode it would

result in a ±2.7 degree difference (at 0°C) in the result (0.01 x

273.15).

This difference varies with temperature such that a fixed

offset value may only be used over a very narrow range.

Typical correction method required when measuring a wide

range of temperature values is to scale the temperature

reading in the host firmware.

Part

nf Min nf Nom nf Max Series

Res

Pentium™ III

(CPUID 68h)

1.0057 1.008 1.0125

Pentium 4,

130nM

1.001 1.002 1.003 3.64

Pentium 4, 90nM

1.011

3.33

Pentium 4, 65nM 1.000 1.009 1.050 4.52

Intel Pentium M 1.0015 1.0022 1.0029 3.06

2N3904

1.003 1.0046 1.005 0.6

Table 6 Representative CPU Thermal Diode and

Transistor Non-Ideality Factors

© Andigilog, Inc. 2006

- 14 -

www.andigilog.com

December 2006 - 70A05012

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