Rainbow Electronics MAX6649 - Manual (Page 12)

MAX6646/MAX6647/MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensors with Overtemperature Alarms
12 ______________________________________________________________________________________
Applications Information
Remote-Diode Selection
The MAX6646/MAX6647/MAX6649 can directly measure
the die temperature of CPUs and other ICs that have
on-board temperature-sensing diodes (see Typical
Operating Circuit), or they can measure the tempera-
ture of a discrete diode-connected transistor.
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 MAX6646/MAX6647/MAX6649
are optimized 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
MAX6646/MAX6647/MAX6649 is 1.008. The following
example uses the MAX6646/MAX6647/MAX6649 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.
Discrete Remote Diodes
When the remote-sensing diode is a discrete transistor,
short the collector to the base. Table 10 lists examples
of discrete transistors that are appropriate for use with
the MAX6646/MAX6647/MAX6649.
Avoid violating the A/D input voltage range by using a
small-signal transistor with a relatively high forward volt-
age. The forward voltage at the highest expected tem-
perature must be greater than 0.25V at 10µA, and the
forward voltage at the lowest expected temperature
must be less than 0.95V at 100µA. Do not use large
power transistors. Ensure that the base resistance is
less than 100
Ω. Tight specifications for forward current
gain (50 < ß < 150, for example) indicate that the man-
ufacturer has good process controls and that the
devices have consistent V
BE characteristics.
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.
AMD and Athlon are trademarks of Advanced Micro Devices, Inc.
MANUFACTURER MODEL NO.
Central Semiconductor (USA) CMPT3904
Rohm Semiconductor (USA) SST3904
Samsung (Korea) KST3904-TF
Siemens (Germany) SMBT3904
Zetex (England) FMMT3904CT-ND
Table 10. Remote-Sensor Transistor
Manufactures
Note: Discrete transistors must be diode connected (base
shorted to collector).