THMC50SSOPDBQ datasheet(16/32 Pages) TI

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THMC50

REMOTE/LOCAL TEMPERATURE MONITOR AND

FAN CONTROLLER WITH SMBus INTERFACE

SLIS090 – JULY 1999

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

PRINCIPLES OF OPERATION

FAN_OFF output (continued)

FAN_SPD/NTEST_IN

FAN_OFF

2N2219A

+12 V

MMFT3055EL

+5 V

1 k

Ω

36 k

Ω

10 k

Ω

LM324

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 M

Ω 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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