ADM1021A
Rev. 7 | Page 16 of 19 | www.onsemi.com
APPLICATIONS INFORMATION
FACTORS AFFECTING ACCURACY
Remote Sensing Diode
The ADM1021A 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,
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+.
The user has no choice in the case of substrate transistors, but
if a discrete transistor is used, the best accuracy is obtained by
choosing devices according to the following criteria:
1.
Base-emitter voltage greater than 0.25 V at 6 μA, at the
highest operating temperature.
2.
Base-emitter voltage less than 0.95 V at 100 μA, at the
lowest operating temperature.
3.
Base resistance less than 100 Ω.
4.
Small variation in h
FE
(such as 50 to 150), which indicates
tight control of V
BE
characteristics.
Transistors, such as 2N3904, 2N3906, or equivalents, in SOT-23
package are suitable devices to use.
Thermal Inertia and Self-Heating
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. For the remote sensor, this should not be a
problem, because it is either a substrate transistor in the
processor or a small package device, such as SOT-23, placed in
close proximity to it.
The on-chip sensor is, however, often remote from the proc-
essor and only monitors the general ambient temperature
around the package. The thermal time constant
of the QSOP-16 package is approximately 10 seconds.
In practice, the package will have an electrical, and hence a
thermal, connection to the printed circuit board, so the
temperature rise due to self-heating is negligible.
