THMC10_08 Datasheet, PDF (14/22 Pages) Texas Instruments – REMOTE/LOCAL TEMPERATURE MONITOR WITH SMBus INTERFACE

THMC10
REMOTE/LOCAL TEMPERATURE MONITOR
WITH SMBus INTERFACE
SLIS089 − DECEMBER 1999
PRINCIPLES OF OPERATION
PC board layout considerations
D Place the THMC10 as close as practical to the remote diode. In a noisy environment, such as a computer
motherboard, this distance can be 4 inches to 8 inches (typical) or more, as long as the worst noise sources
(such as CRTs, clock generators, memory buses, and ISA/PCI buses) are avoided.
D Route the DXP–DXN lines away from the deflection coils of a CRT. Also, avoid routing 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.
D Route the DXP and DXN traces in parallel and in close proximity to each other, away from any high-voltage
traces such as 12 Vdc. Leakage currents from PC board contamination must be be taken into consideration,
since a 20-MΩ leakage path from DXP to ground causes about 1°C error.
D Connect guard traces to GND on either side of the DXP–DXN traces (Figure 7). With guard traces in place,
routing near high-voltage traces is not an issue.
D Route through as few vias and crossunders as possible to minimize copper/solder thermocouple effects.
D When introducing a thermocouple, insure that both the DXP and the DXN paths have matching
thermocouples. In general, PC board induced thermocouples are not a serious problem. A copper-solder
thermocouple exhibits 3 µV/°C, and it takes about 200 µV of voltage error at DXP–DXN to cause a 1°C
measurement error. Hence, most parasitic thermocouple errors are swamped out.
D Use wide traces. Narrow traces are more inductive and tend to pick up radiated noise. The 10-mil widths
and spacings recommended in Figure 7 are not absolutely necessary as they offer only a minor
improvement in leakage and noise, but usage is recommended where practical.
D Copper can not be used as an EMI shield and only ferrous materials such as steel work well. Placing a
copper ground plane between the DXP−DXN traces and traces carrying high-frequency noise signals does
not help reduce EMI.
PC board layout checklist
D Place the THMC10 as close as possible to the remote diode.
D Keep traces away from high voltages (12 V bus).
D Keep 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 DXP and DXN and connecting to GND.
D Place the noise filter and the 0.1-µF V_DD bypass capacitors close to the THMC10.
14
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265