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ADM1023
This is given by:
q
D V BE + nKT 1n (N)
where:
(eq. 2)
ADM1023 is optimized for n TYPICAL = 1.008; any deviation
on n from this typical value causes a temperature error that is
calculated below for the n MIN and n MAX of a Pentium III
processor at T TD = 100 ? C.
D T + n * 1.008 273.15 Kelvin ) TTD (eq. 5)
K is Boltzmann’s constant.
q is the charge on the electron (1.6 ? 10 –19 Coulombs).
T is the absolute temperature in Kelvins.
N is the ratio of the two collector currents.
n is the ideality factor of the thermal diode (TD).
To measure D V BE , the sensor is switched between
operating currents of I and NI. The resulting waveform is
passed through a low-pass filter to remove noise, then to a
chopper-stabilized amplifier that performs the functions of
amplification and rectification of the waveform to produce
a dc voltage proportional to D 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 are performed in a similar manner.
Figure 13 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 is not
grounded and should be connected 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 internal 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
*
D T MIN + 1.0057 1.008 1.008 273.15 Kelvin ) 100 ° C +
+ * 0.85 ° C (eq. 4)
*
D T MAX + 1.0125 1.008 1.008 273.15 Kelvin ) 100 ° C +
+ ) 1.67 ° C
Thus, the temperature error due to variation on n of the
thermal diode for a Pentium III processor is about 2.5 ? C.
In general, this additional temperature error of the thermal
diode measurement due to deviations on n from its typical
value is given by:
1.008
where T TD is in ? C.
Beta of Thermal Transistor ( b )
In Figure 13, the thermal diode is a substrate PNP
transistor where the emitter current is forced into the device.
The derivation of Equation 2 assumed that the collector
currents were scaled by N as the emitter currents were also
scaled by N. Thus, this assumes that beta ( b ) of the transistor
is constant for various collector currents. Figure 14 shows
typical b variation vs. collector current for Pentium III
processors at 100 ? C. The maximum b is 4.5 and varies less
than 1% over the collector current range from 7 m A to
300 m A.
b MAX < 4.5
1000 pF maximum. See the Layout Considerations section
for more information on C1.
n b
I E
Sources of Errors on Thermal Transistors
Measurement Method; The Effect of Ideality Factor (n)
b
I C =
b
b +1
I E
I C + I E b ( b ) 1)]
The effects of ideality factor (n) and beta ( b ) of the
temperature measured by a thermal transistor are described in
this section. For a thermal 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 substrate PNP
transistor (used as a thermal diode) usually found on CPUs
I C (mA)
7 300
Figure 14. Variation of b with Collector Currents
Expressing the collector current in terms of the emitter
current.
(eq. 6)
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 Equation 2 represents the ideality factor
of this thermal diode. This ideality factor is a measure of the
where:
b 300 m A + b 7 m A (1 ) e )
e + D b b and b + b (7 m A)
(eq. 7)
n MIN + 1.0057 t n TYPICAL + 1.008 t n MAX +
D V BE + nKT
(1 ) e ) b ) 1
q
(1 ) e ) b ) 1
deviation of the thermal diode from ideal behavior.
According to Pentium III processor manufacturing
specifications, measured values of n at 100 ? C are:
(eq. 3)
+ 1.0125
The ADM1023 takes this ideality factor into consideration
when calculating temperature T TD of the thermal diode. The
Rewriting the equation for D V BE , to include the ideality
factor, n, and beta, b yields:
1n N (eq. 8)
All b variations of less than 1% ( e < 0.01) contribute to
temperature errors of less than 0.4 ? C.
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