ADT7467 Analog Devices, Inc., ADT7467 Datasheet - Page 18

• Manufacturer Part Number: ADT7467
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

ADT7467
Series Resistance Cancellation
Parasitic resistance to the ADT7467 D+ and D− inputs (seen in
series with the remote diode) is caused by a variety of factors,
including PCB track resistance and track length. This series
resistance appears as a temperature offset in the remote sensor’s
temperature measurement. This error typically causes a 0.5°C
offset per 1 Ω of parasitic resistance in series with the remote
diode.
The ADT7467 automatically cancels the effect of this series
resistance on the temperature reading, providing a more
accurate result without the need for user characterization of this
resistance. The ADT7467 is designed to automatically cancel,
typically, up to 3 kΩ of resistance. By using an advanced
temperature measurement method, this is transparent to the
user. This feature allows resistances to be added to the sensor
path to produce a filter, allowing the part to be used in noisy
environments. See the Noise Filtering section for details.
Factors Affecting Diode Accuracy
Remote Sensing Diode
The ADT7467 is designed to work with either substrate
transistors built into processors or 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 is connected to D−. If a PNP
transistor is used, the collector and base are connected to D−
and the emitter is connected to D+.
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To reduce the error due to variations in both substrate and
discrete transistors, a number of factors should be taken into
consideration:
•
•
If a discrete transistor is used with the ADT7467, the best
accuracy is obtained by choosing devices according to the
following criteria:
•
•
•
•
Transistors such as 2N3904, 2N3906, or equivalents in SOT-23
packages are suitable devices to use.
The ideality factor, n
deviation of the thermal diode from ideal behavior. The
ADT7467 is trimmed for an n
following equation to calculate the error introduced at a
temperature, T (°C), when using a transistor whose n
not equal 1.008. See the processor’s data sheet for the n
values.
To correct for this error, the user can write the ΔT value to
the offset register, and the ADT7467 will automatically add
it to or subtract 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 ADT7467, I
is 6 μA. If the ADT7467 current levels do not match the
current levels specified by the CPU manufacturer, it might
be necessary to remove an offset. The CPU’s data sheet
should provide information relating to n
differences. An offset can be programmed to the offset
register. It is important to note that if more than one offset
must be considered, the algebraic sum of these offsets must
be programmed to the offset register.
Base-emitter voltage is greater than 0.25 V at 6 μA with the
highest operating temperature.
Base-emitter voltage is less than 0.95 V at 100 μA with the
lowest operating temperature.
Base resistance is less than 100 Ω.
There is a small variation in h
that indicates tight control of V
ΔT = (n
f
− 1.008)/1.008 × (273.15 K + T)
HIGH
, is 96 μA, and the low level current, I
f
, of the transistor is a measure of the
FE
f
value of 1.008. Use the
BE
(for example, 50 to 150)
characteristics.
f
to compensate for
f
does
f
LOW
,