SS8017TR SSC [Silicon Standard Corp.], SS8017TR Datasheet - Page 7

• Manufacturer Part Number: SS8017TR
• Description: Two Remote Temperature Sensors with SMBus Serial Interface and System Reset
• Manufacturer: SSC [Silicon Standard Corp.]

Rev.2.01 6/06/2003
A/D Conversion Sequence
If a Start command is written (or generated automati-
cally in the free-running auto-convert mode), both two
channels are converted, and the results of both meas-
urements are available after the end of conversion. A
BUSY status bit in the status byte shows that the de-
vice is actually performing a new conversion; however,
even if the ADC is busy, the results of the previous
conversion are always available.
Remote-Diode Selection
Temperature
good-quality, diode-connected small-signal transistor.
Accuracy has been experimentally verified for all of the
devices listed in Table 1. The 8017 can also directly
measure the die temperature of CPUs and other inte-
grated circuits having on-board temperature- sensing
diodes. 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
must be greater than 0.25V at 10µA; check to ensure
this is true at the highest e xpected temperature. The
forward voltage must be less than 0.95V at 200A;
check to e nsure this is true at the lowest expected
temperature. Large power transistors don't work at all.
Also, ensure that the base resistance is less than
100 . Tight specifications for forward current gain (+50
to +150, for example) indicate that the manufacturer
has good process controls and that the devices have
consistent VBE characteristics.
Thermal Mass and Self-Heating
Thermal mass can seriously degrade the 8017's effec-
tive accuracy. The thermal time constant of the
SSOP-16 package is about 140sec in still air. For the
8017 junction temperature to settle to within +1°C after
a sudden +100°C change requires about five time con-
stants or 12 minutes. The use of smaller packages for
remote sensors, such as SOT23s, improves the situa-
tion. Take care to account for thermal gradients b e-
tween the heat source and the sensor ,and ensure that
stray air current across the sensor package does not
interfere with measurement accuracy.
Table 1. Remote-Sensor Transistor Manufacturers
Philips
Motorola (USA)
National Semiconductor (USA)
Note:Transistors must be diode-connected (base short
ADC Noise Filtering
The ADC is an integrating type with inherently good
noise rejection, especially of low-frequency signals
MANUFACTURER
-ed to collector).
accuracy
depends
MODEL NUMBER
PMBS 3904
MMBT3904
MMBT3904
www.SiliconStandard.com
on
having
a
such as 60Hz/120Hz power-supply hum. Micro-power
operation places constraints on high-frequency noise
rejection; therefore, careful PC board layout and proper
external noise filtering are required for high- accuracy
remote measurements in electrically noisy environ-
ments.
High-frequency EMI is best filtered at DXP and DXN
with an external 2200pF capacitor. This value can be
increased to about 3300pF(max), including cable ca-
pacitance. Higher capacitance than 3300pF introduces
errors due to the rise time of the switched current
source.
Nearly all noise sources tested cause the ADC meas-
urements to be higher than the actual temperature,
typically by +1°C to 10°C, depending on the frequency
and amplitude (see Typical Operating Characteristics).
PC Board Layout
Place the 8017 as close as practical to the remote
diode. In a noisy environment, such as a computer
motherboard, this distance can be 4 in. to 8 in. (typical)
or more as long as the worst noise sources (such as
CRTs, clock generators, memory buses, and ISA/PCI
buses) are avoided.
Do not route the DXP -DXN lines next to the deflection
coils of a CRT. Also, do not route the traces across a
fast memory bus, which can easily introduce +30°C
error, even with good filtering, Otherwise, most noise
sources are fairly benign.
Route the DXP and DXN traces in parallel and in close
proximity to each other, away from any high-voltage
traces such as +12VDC. Leakage currents from PC
board contamination must be dealt with carefully, since
a 20M
about +1°C error.
Route the 2 pairs of DXP1-DXN and DXP2-DXN traces
independently (Figure 2a). Connect the common DXN
as close as possible to the DXN pin on IC (Figure 2a).
Connect guard traces to GND on either side of the
DXP-DXN traces (Figure 2b). With guard traces in
place, routing near high-voltage traces is no longer an
issue.
Route through as few vias and crossunders as possible
to minimize copper/solder thermocouple effects.
When introducing a thermocouple, make sure that both
the DXP and the DXN paths have matching thermocou-
ples. In general, PC board- induced thermocouples are
not a serious problem, A copper-solder thermocouple
exhibits 3 µV/°C, and it takes about 200µV of voltage
error at DXP -DXN to cause a +1°C measurement error.
leakage path from DXP to ground causes
SS8017
7 of 16