THMC50 Datasheet, PDF (16/32 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
FAN_OFF output (continued)
+12 V
+12 V
FAN_SPD/NTEST_IN
+
_
LM324
Q1
2N2219A
FAN_OFF
+5 V
R3
1 kΩ
R1
36 kΩ
R2
10 kΩ
M1
MMFT3055EL
Figure 12. Example for Implementing the FAN_OFF Output Signal
layout and grounding
The power supply bypass, the parallel combination of 10 µF (electrolytic or tantalum), and 0.1 µF (ceramic)
bypass capacitors connected between terminals 5, 7, and ground should also be located as close as possible
to the THMC50.
The following are general guidelines for generating the PCB layout for the THMC50:
D Place the THMC50 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 Do not route the remote diode 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.
D Route the remote diode 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 taken into consideration,
since a 20 MΩ leakage path from REMOTE_DIODE+ to ground causes about 1°C error.
D Connect guard traces to GND on either side of the remote diode traces (Figure 13). 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 remote diode traces 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 the remote diode terminals to cause a 1°C
measurement error. Hence, most parasitic thermocouple errors are swamped out.
D Use wide traces, as narrow ones are more inductive and tend to pick up radiated noise. The 10-mil widths
and spacings recommended in Figure 13 are not absolutely necessary (as they offer only a minor
improvement in leakage and noise), but usage is recommended where practical.
D Do not use copper as an EMI shield as only ferrous materials such as steel work well. Placing a copper
ground plane between the remote diode traces and traces carrying high-frequency noise signals does not
minimize EMI.
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