ADM1032
In this respect, the ADM1032 differs from and improves
upon competitive devices that output zero if the external
sensor goes short-circuit. These devices can misinterpret a
genuine 0C measurement as a fault condition.
When the D+ and D− lines are shorted together, an
ALERT is always generated. This is because the remote
value register reports a temperature value of
−128C.
Since
the ADM1032 performs a less-than or equal-to comparison
with the low limit, an ALERT is generated even when the
low limit is set to its minimum of
−128C.
Applications Information
−
Factors Affecting
Accuracy
Remote Sensing Diode
If a discrete transistor is being used with the ADM1032,
the best accuracy is obtained by choosing devices according
to the following criteria:
Base-emitter Voltage Greater than 0.25 V at 6 mA, at
the Highest Operating Temperature
Base-emitter Voltage Less than 0.95 V at 100 mA, at
the Lowest Operating Temperature
Base Resistance Less than 100
W
Small Variation in h
FE
(say 50 to 150) that Indicates
Tight Control of V
BE
Characteristics
Transistors such as 2N3904, 2N3906, or equivalents in
SOT−23 packages are suitable devices to use.
Thermal Inertia and Self-heating
The ADM1032 is designed to work with substrate
transistors built into processors’ CPUs or with discrete
transistors. Substrate transistors are generally PNP types
with the collector connected to the substrate. Discrete types
can be either a PNP or an NPN transistor connected as a
diode (base shorted to 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 to D+. Substrate
transistors are found in a number of CPUs. To reduce the
error due to variations in these substrate and discrete
transistors, a number of factors should be taken into
consideration:
1. The ideality factor, n
f
, of the transistor. The
ideality factor is a measure of the deviation of the
thermal diode from the ideal behavior. The
ADM1032 is trimmed for an n
f
value of 1.008.
The following equation can be used to calculate
the error introduced at a temperature TC when
using a transistor whose n
f
does not equal 1.008.
Consult the processor data sheet for n
f
values.
DT
+
n
natural
*
1.008
1.008
273.15 Kelvin
)
T
(eq. 2)
This value can be written to the offset register and
is automatically added to or subtracted from the
temperature measurement.
2. Some CPU manufacturers specify the high and
low current levels of the substrate transistors. The
high current level of the ADM1032, I
HIGH
, is
230
mA
and the low level current, I
LOW
, is 13
mA.
If the ADM1032 current levels do not match the
levels of the CPU manufacturers, then it can
become 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 accounting for two or more offsets is
needed, then the algebraic sum of these offsets
must be programmed to the offset register.
Accuracy depends on the temperature of the
remote-sensing diode and/or the internal temperature sensor
being at the same temperature as that being measured, and
a number of factors can affect this. Ideally, the sensor should
be in good thermal contact with the part of the system being
measured, for example, the processor. If it is not, the thermal
inertia caused by the mass of the sensor causes a lag in the
response of the sensor to a temperature change. In the case
of the remote sensor, this should not be a problem, since it
is either a substrate transistor in the processor or a small
package device, such as the SOT−23, placed in close
proximity to it.
The on-chip sensor, however, is often remote from the
processor and is only monitoring the general ambient
temperature around the package. The thermal time constant
of the SOIC−8 package in still air is about 140 seconds, and
if the ambient air temperature quickly changed by 100, it
would take about 12 minutes (five time constants) for the
junction temperature of the ADM1032 to settle within 1 of
this. In practice, the ADM1032 package is in electrical and
therefore thermal contact with a printed circuit board and
can also be in a forced airflow. How accurately the
temperature of the board and/or the forced airflow reflect the
temperature to be measured also affects the accuracy.
Self-heating due to the power dissipated in the ADM1032
or the remote sensor causes the chip temperature of the
device or remote sensor to rise above ambient. However, the
current forced through the remote sensor is so small that
self-heating is negligible. In the case of the ADM1032, the
worst-case condition occurs when the device is converting
at 16 conversions per second while sinking the maximum
current of 1 mA at the ALERT and THERM output. In this
case, the total power dissipation in the device is about
11 mW. The thermal resistance,
q
JA
, of the SOIC−8 package
is about 121C/W.
In practice, the package has electrical and therefore
thermal connection to the printed circuit board, so the
temperature rise due to self-heating is negligible.
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