ADM1023

–6–

REV. A

values are then stored before a comparison with the stored limits is

made. However, if the part is powered up in standby mode (STBY

pin pulled low), no new values are written to the register before

a comparison is made. As a result, both RLOW and LLOW are

tripped in the Status Register thus generating an

ALERT output.

This may be cleared in one of two ways:

1. Change both the local and remote lower limits to –128

°C

and read the status register (which in turn clears the

ALERT

output).

2. Take the part out of standby and read the status register

(which in turn clears the

ALERT output). This will work

only if the measured values are within the limit values.

MEASUREMENT METHOD

A simple method of measuring temperature is to exploit the nega-

tive temperature coefficient of a diode, or the base-emitter voltage

of a transistor, operated at constant current. Thus, the temperature

V

nKT

q

I

I

BE

C

S

=× ln

()

(1)

Unfortunately, this technique requires calibration to null out

to device.

The technique used in the ADM1023 is to measure the change

currents.

This is given by:

∆V

nKT

q

N

BE

=× ln ( )

(2)

where:

K is Boltzmann’s constant

q is charge on the electron (1.6

T is absolute temperature in Kelvins

N is ratio of the two collector currents

n is the ideality factor of the thermal diode (TD)

To measure

∆V

rents of I and NI. The resulting waveform is passed through a

low-pass filter to remove noise, then to a chopper-stabilized ampli-

fier that performs the functions of amplification and rectification of

the waveform to produce a dc voltage proportional to

∆V

voltage is measured by the ADC, which gives a temperature output

in binary format. To further reduce the effects of noise, digital

filtering is performed by averaging the results of 16 measurement

cycles. Signal conditioning and measurement of the internal

temperature sensor is performed in a similar manner.

Figure 12 shows the input signal conditioning used to measure

the output of an external temperature sensor. This figure shows

the external sensor as a substrate PNP transistor, provided for

temperature monitoring on some microprocessors, but it could

equally well be a discrete transistor. If a discrete transistor is

used, the collector will not be grounded and should be linked to

the base. 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 inter-

nal diode at the D– input. If the sensor is operating in a noisy

environment, C1 may optionally be added as a noise filter. Its value

is typically 2200 pF, but should be no more than 3000 pF. See the

section on Layout Considerations for more information on C1.

SOURCES OF ERRORS ON THERMAL

TRANSISTOR MEASUREMENT METHOD

EFFECT OF IDEALITY FACTOR (n)

The effects of ideality factor (n) and beta (Beta) of the temperature

measured by a thermal transistor are discussed below. For a ther-

mal transistor implemented on a submicron process, such as the

substrate PNP used on a Pentium III processor, the temperature

errors due to the combined effect of the ideality factor and beta are

shown to be less than 3

°C. Equation 2 is optimized for a sub-

strate PNP transistor (used as a thermal diode) usually found on

CPUs designed on submicron CMOS processes such as the

Pentium III Processor. There is a thermal diode on board each of

these processors. The n in the Equation 2 represents the ideality

factor of this thermal diode. This ideality factor is a measure of the

deviation of the thermal diode from ideal behavior.

According to Pentium III Processor manufacturing specifica-

tions, measured values of n at 100

°C are:

The ADM1023 takes this ideality factor into consideration

on n from this typical value causes a temperature error that is

°C,

∆T

Kelvin

C

C

MIN

=×

+

°

=

°

1 0057

1 008

1 008

273 15

100

0 85

.

– .

.

(.

)

– .

∆T

Kelvin

C

C

MAX

=×

+

°

= +

°

1 0125 1 008

1 008

273 15

100

1 67

.

– .

.

(.

)

.

Thus, the temperature error due variation on n of the thermal

diode for Pentium III Processor is about 2.5

°C.

C1*

D+

D–

REMOTE

SENSING

TRANSISTOR

IN

I

TO ADC

BIAS

DIODE

LOW-PASS FILTER

CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.

C1 = 2.2nF TYPICAL, 3nF MAX.

*

Figure 12. Input Signal Conditioning