sensor designed for a nominal ideality factor n
NOMINAL
is used to measure the temperature of a diode with a
different ideality factor n1. The measured temperature
T
M
can be corrected using:
where temperature is measured in Kelvin and
n
NOMIMAL
for the MAX6699 is 1.008. As an example,
assume you want to use the MAX6699 with a CPU that
has an ideality factor of 1.002. If the diode has no
series resistance, the measured data is related to the
real temperature as follows:
For a real temperature of +85°C (358.15K), the mea-
sured temperature is +82.87°C (356.02K), an error of
-2.13°C.
Series Resistance Cancellation
Some thermal diodes on high-power ICs can have
excessive series resistance, which can cause tempera-
ture-measurement errors with conventional remote tem-
perature sensors. Channel 1 of the MAX6699 has a
series resistance cancellation feature (enabled by bit 3
of the Configuration 1 register) that eliminates the effect
of diode series resistance. Set bit 3 to 1 if the series
resistance is large enough to affect the accuracy of
channel 1. The series resistance cancellation function
increases the conversion time for channel 1 by 125ms.
This feature cancels the bulk resistance of the sensor
and any other resistance in series (wire, contact resis-
tance, etc.). The cancellation range is from 0 to 100Ω.
Discrete Remote Diodes
When the remote-sensing diode is a discrete transistor,
its collector and base must be connected together.
Table 10 lists examples of discrete transistors that are
appropriate for use with the MAX6699. The transistor
must be a small-signal type with a relatively high forward
voltage; otherwise, the A/D input voltage range can be
violated. The forward voltage at the highest expected
temperature must be greater than 0.25V at 10µA, and at
the lowest expected temperature, the forward voltage
must be less than 0.95V at 100µA. Large power transis-
tors must not be used. Also, ensure that the base resis-
tance is less than 100Ω. Tight specifications for forward
current gain (50 < ß <150, for example) indicate that the
manufacturer has good process controls and that the
devices have consistent V
BE
characteristics.
Manufacturers of discrete transistors do not normally
specify or guarantee ideality factor. This is normally not
a problem since good-quality discrete transistors tend to
have ideality factors that fall within a relatively narrow
range. A variety of discrete transistors have variations in
remote temperature readings of less than ±2°C. Still, it is
good design practice to verify good consistency of tem-
perature readings with several discrete transistors from
any manufacturer under consideration.
