Rainbow Electronics MAX6692 - Manual (Page 12)

MAX6648/MAX6692
Precision SMBus-Compatible Remote/Local
Temperature Sensors with Overtemperature Alarms
12 ______________________________________________________________________________________
Effect of Ideality Factor
The accuracy of the remote temperature measurements
depends on the ideality factor (n) of the remote “diode”
(actually a transistor). The MAX6648/MAX6692 are opti-
mized for n = 1.008, which is the typical value for the
Intel
® Pentium ® III and the AMD Athlon MP model 6. If a
sense transistor with a different ideality factor is used,
the output data is different. Fortunately, the difference
is predictable.
Assume a remote-diode sensor designed for a nominal
ideality factor n
NOMINAL is used to measure the tem-
perature of a diode with a different ideality factor n
1
.
The measured temperature T M can be corrected using:
where temperature is measured in Kelvin.
As mentioned above, the nominal ideality factor of the
MAX6648/MAX6692 is 1.008. As an example, assume
you want to use the MAX6648/MAX6692 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 (358.15 K), the mea-
sured temperature is +82.91°C (356.02 K), which is 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, change in the measured voltage is:
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.
For best accuracy, the discrete transistor should be a
small-signal device with its collector and base connect-
ed together. Table 9 lists examples of discrete transis-
tors that are appropriate for use with the MAX6648/
MAX6692.
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 con-
sistent V
BE characteristics.
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-
3 0 453 1 36
Ω
Ω
×
° =
°
. .
C
C
90
198 6
0 453
µ
µ
°
=
°
V
V
C
C Ω
Ω
.
.
∆V
R A A A R
M S S
= µ µ = µ × − ( ) 100 10 90
T T
n
n
T T
ACTUAL M
NOMINAL
M M
=

 

=

 

=
.
.
( . )
1
1 008
1 002
1 00599
T T
n
n
M ACTUAL
NOMINAL
=






1
Intel and Pentium are registered trademarks of Intel Corp.
MANUFACTURER MODEL NO.
Central Semiconductor (USA) CMPT3904
Rohm Semiconductor (USA) SST3904
Samsung (Korea) KST3904-TF
Siemens (Germany) SMBT3904
Table 9. Remote-Sensor Transistor
Manufacturers
Note: Transistors must be diode connected (base shorted to
collector).