Rainbow Electronics MAX6649 - Manual (Page 12)

MAX6649
+145°C Precision SMBus-Compatible Remote/
Local Sensor with Overtemperature Alarms
12 ______________________________________________________________________________________
As mentioned above, the nominal ideality factor of the
MAX6649 is 1.008. As an example, assume you want to
use the MAX6649 with a CPU that has an ideality factor
of 1.002. If the diode has no series resistance, the mea-
sured 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 MAX6649.
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 temperature,
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.
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 and DXN with
an external 2200pF capacitor. Larger capacitor values
can be used for added filtering, but do not exceed
3300pF because larger values can introduce errors due
to the rise time of the switched current source.
PC Board Layout
Follow these guidelines to reduce the measurement
error of the temperature sensors:
1) Place the MAX6649 as close as is practical to the
remote diode. In noisy environments, such as a
computer motherboard, this distance can be 4in to
8in (typ). This length can be increased if the worst
noise sources are avoided. Noise sources include
CRTs, clock generators, memory buses, and
ISA/PCI buses.
2) Do not route the DXP-DXN lines next to the deflec-
tion coils of a CRT. Also, do not route the traces
across fast digital signals, which can easily intro-
duce 30°C error, even with good filtering.
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
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).