THMC50

REMOTE/LOCAL TEMPERATURE MONITOR AND

FAN CONTROLLER WITH SMBus INTERFACE

SLIS090 – JULY 1999

17

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

PCB layout checklist

10 MILS

REMOTE_DIODE–

REMOTE_DIODE+

GND

GND

10 MILS

MINIMUM

10 MILS

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

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.