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ADM1023 Datasheet(PDF) 6 Page - Analog Devices

Part No. ADM1023
Description  ACPI-Compliant High-Accuracy Microprocessor System Temperature Monitor
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADM1023 Datasheet(HTML) 6 Page - Analog Devices

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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
may be obtained from a direct measurement of VBE where,
V
nKT
q
I
I
BE
C
S
=× ln
()
(1)
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 ADM1023 is to measure the change
in VBE when the device is operated at two different collector
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
× 10–19 Coulombs)
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
BE, the sensor is switched between operating cur-
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
BE. This
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:
nMIN = 1.0057 < nTYPICAL = 1.008 < nMAX = 1.0125
The ADM1023 takes this ideality factor into consideration
when calculating temperature TTD of the thermal diode. The
ADM1023 is optimized for nTYPICAL = 1.008; any deviation
on n from this typical value causes a temperature error that is
calculated below for the nMIN and nMAX of a Pentium III Processor
at TTD = 100
°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
IBIAS
VDD
VOUT+
TO ADC
VOUT–
BIAS
DIODE
LOW-PASS FILTER
fC = 65kHz
CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.
C1 = 2.2nF TYPICAL, 3nF MAX.
*
Figure 12. Input Signal Conditioning


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