THMC50 Datasheet, PDF (17/32 Pages) Texas Instruments – REMOTE/LOCAL TEMPERATURE MONITOR AND FAN CONTROLLER WITH SMBus INTERFACE

THMC50
REMOTE/LOCAL TEMPERATURE MONITOR AND
FAN CONTROLLER WITH SMBus INTERFACE
SLIS090 – JULY 1999
PRINCIPLES OF OPERATION
PCB layout checklist
D Place the THMC50 as close as possible to the remote diode.
D Keep the remote diode traces away from high voltages (12-V bus).
D Keep the remote diode traces away from fast data/memory buses and CRTs.
D Use recommended trace widths and spacings.
D Place a ground plane under the traces.
D Use guard traces flanking the remote diode traces and connect them to ground.
D Place the 0.1-µF VCC3AUX bypass capacitor as close to the THMC50 as possible.
GND
10 MILS
10 MILS
REMOTE_DIODE+
REMOTE_DIODE–
GND
10 MILS
MINIMUM
10 MILS
Figure 13. Recommended PC Board Layout for Remote Diode Traces
use of twisted pair and shielded cables for remote thermal sensor
For remote-sensor distances longer than 8 inches or in particularly noisy environments, a twisted pair is
recommended. Its practical length is 6 feet to 12 feet (typical) before noise becomes a problem, as tested in
a noisy electronics laboratory. For longer distances, the best solution is a shielded twisted pair like that used
for audio microphones. For example, Belden^® #8451 works well for distances up to 100 feet in a noisy
environment. Connect the twisted pair to the remote diode terminals and the shield to ground, and leave the
shield’s remote end unterminated. Excess capacitance at REMOTE_DIODE– limits practical remote sensor
distances. For very long cable runs, the cable’s parasitic capacitance often provides noise filtering; hence, the
2200-pF capacitor can often be removed or reduced in value. Cable resistance also affects remote-sensor
accuracy. A 1-Ω series resistance introduces about 0.5°C error.
temperature measurement system
The remote thermal diode sensor monitors a remotely placed diode such as those found in some
microprocessors. A 2N3904 transistor can also be used for remote thermal sensing. The THMC50 also has an
internal thermal diode sensor which can be used to monitor the ambient temperature of the environment around
the device. Figure 14 shows the block diagram of the analog front end for the THMC50 temperature-to-digital
converter.
A digital 8-bit comparator is used to compare the temperature readings to the user-programmable high/low, to
over-temperature limit values, and to generate interrupts accordingly.
Belden is a registered trademark of Belden Corporation.
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