Rainbow Electronics MAX6642 - Manual (Page 10)

MAX6642
where temperature is measured in Kelvin and
n
NOMIMAL
for the MAX6642 is 1.008.
As an example, assume you want to use the MAX6642
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 +82.91°C (356.02K), an error of
-2.13°C.
Effect of Series Resistance
Series resistance in a sense diode contributes addition-
al errors. For nominal diode currents of 10µA and
100µA, the change in the measured voltage due to
series resistance is:
∆V
M = R S (100µA - 10µA) = 90µA ✕ R S
Since +1°C corresponds to 198.6µV, series resistance
contributes a temperature offset of:
Assume that the diode being measured has a series
resistance of 3
Ω. The series resistance contributes an
offset of:
The effects of the ideality factor and series resistance
are additive. If the diode has an ideality factor of 1.002
and series resistance of 3
Ω, the total offset can be cal-
culated by adding error due to series resistance with
error due to ideality factor:
1.36°C - 2.13°C = -0.77°C
for a diode temperature of +85°C.
In this example, the effect of the series resistance and
the ideality factor partially cancel each other.
Discrete Remote Diodes
When the remote-sensing diode is a discrete transistor,
its collector and base should be connected together.
Table 7 lists examples of discrete transistors that are
appropriate for use with the MAX6642.
The transistor must be a small-signal type with a rela-
tively 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 tempera-
ture, the forward voltage must be less than 0.95V at
100µA. Large power transistors must not be used. Also,
ensure that the base resistance is less than 100
Ω. 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. We have observed variations in remote tempera-
ture readings of less than
±2°C with a variety of dis-
crete transistors. Still, it is good design practice to
verify good consistency of temperature readings with
several discrete transistors from any manufacturer
under consideration.
ADC Noise Filtering
The integrating ADC used has good noise rejection for
low-frequency signals such as 60Hz/120Hz power-sup-
ply hum. In noisy environments, high-frequency noise
reduction is needed for high-accuracy remote mea-
surements. The noise can be reduced with careful PC
board layout and proper external noise filtering.
High-frequency EMI is best filtered at DXP with an
external 2200pF capacitor. Larger capacitor values can
be used for added filtering, but do not exceed 3300pF
because excessive capacitance can introduce errors
3 0 453 1 36
Ω ×
°
Ω
= + °
. .
C
C
90
198 6
0 453
µ
Ω
µ
°
=
°
Ω
V
V
C
C
.
.
T T
n
n
T
T
ACTUAL M
NOMINAL
M
M
=

 

=

 

=
1
1 008
1 002
1 00599
.
.
( . )
±1°C, SMBus-Compatible Remote/Local
Temperature Sensor with Overtemperature Alarm
10 ______________________________________________________________________________________
MANUFACTURER MODEL NO.
Central Semiconductor (USA) CMPT3906
Rohm Semiconductor (USA) SST3906
Samsung (Korea) KST3906-TF
Siemens (Germany) SMBT3906
Zetex (England) FMMT3906CT-ND
Table 7. Remote-Sensor Transistor
Manufacturers
Note: Discrete transistors must be diode connected (base short-
ed to collector).