ADT7476ARQH 查看數據表（PDF） - ON Semiconductor
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ADT7476ARQH Datasheet PDF : 67 Pages
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ADT7476
2N3906
PNP
ADT7476
D+
D–
Figure 27. Measuring Temperature Using an
PNP Transistor
To measure DVBE, the sensor switches between operating
currents of I and N x I. The resulting waveform passes
through a 65 kHz low−pass filter to remove noise and through
a chopper−stabilized amplifier. The amplifier performs the
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 illustrated in
Table 10. 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 placed a capacitor across the D+ pin and
the D− pin to help combat the effects of noise. However,
large capacitances 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,
which makes using the sensor difficult in a very noisy
environment. In most cases, a capacitor is not required
because differential inputs by their very nature have a high
immunity to noise.
Factors Affecting Diode Accuracy
Remote Sensing Diode
The ADT7476 is designed to work with substrate
transistors built into processors or with discrete transistors.
Substrate transistors are generally PNP types with the
collector connected to the substrate. Discrete types can be
either PNP or NPN transistors connected as a diode
(base−shorted to the collector). If an NPN transistor is used,
the collector and base are connected to D+ and the emitter
to D−. If a PNP transistor is used, the collector and base are
connected to D− and the emitter is connected to D+.
To reduce the error due to variations in both substrate and
discrete transistors, a number of factors should be taken into
consideration:
• The ideality factor, nf, of the transistor is a measure of
the deviation of the thermal diode from ideal behavior.
The ADT7476 is trimmed for an nf value of 1.008. Use
the following equation to calculate the error introduced
at a temperature T (°C), when using a transistor whose
nf does not equal 1.008 (see the processor’s data sheet
for the nf values):
DT + (nf * 1.008) ǒ273.15 K ) TǓ
(eq. 2)
To factor this in, the user can write the DT value to the
offset register. The ADT7476 then automatically adds
it to or subtracts it from the temperature measurement.
• Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7476, IHIGH, is 180 mA, and
the low level current, ILOW, is 11 mA. If the ADT7476
current levels do not match the current levels specified
by the CPU manufacturer, it could be necessary to
remove an offset. The CPU’s data sheet advises
whether this offset needs to be removed and how to
calculate it. This offset can be programmed to the offset
register. It is important to note that if more than one
offset must be considered, then the algebraic sum of
these offsets must be programmed to the offset register.
If a discrete transistor is used with the ADT7476, the best
accuracy is obtained by choosing devices according to the
following criteria:
• Base−emitter voltage greater than 0.25 V at 11 mA, at
the highest operating temperature.
• Base−emitter voltage less than 0.95 V at 180 mA,
at the lowest operating temperature.
• Base resistance less than 100 W.
• Small variation in the current gain, hFE, (approximately
50 to 150) that indicates tight control of VBE
characteristics.
Transistors, such as 2N3904, 2N3906, or equivalents in
SOT−23 packages, are suitable devices to use.
Nulling Out Temperature Errors
As CPUs run faster, it is more difficult to avoid high
frequency clocks when routing the D+/D– traces around a
system board. Even when recommended layout guidelines
are followed, some temperature errors can still be
attributable to noise coupled onto the D+/D– lines. Constant
high frequency noise usually attenuates, or increases,
temperature measurements by a linear, constant value.
The ADT7476 has temperature offset registers (0x70 and
0x72) for the Remote 1 and Remote 2 temperature channels.
By doing a one−time calibration of the system, the user can
determine the offset caused by system board noise and null
it out using the offset registers. The offset registers
automatically add a twos complement 8−bit reading to every
temperature measurement.
Changing Bit 1 of Configuration Register 5 (0x7C)
changes the resolution and therefore, the range of the
temperature offset as either having a −63°C to +127°C range
with a resolution of 1°C or having a −63°C to +64°C range
with a resolution of 0.5°C. This temperature offset can be
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