3.

Ideally, the LM82 should be placed within 10cm of the

Processor diode pins with the traces being as straight,

short and identical as possible. Trace resistance of 1

Ω

can cause as much as 1˚C of error.

4.

Diode traces should be surrounded by a GND guard ring

to either side, above and below if possible. This GND

guard should not be between the D+ and D− lines. In the

event that noise does couple to the diode lines it would

be ideal if it is coupled common mode. That is equally to

the D+ and D− lines.(See Figure 10)

5.

Avoid routing diode traces in close proximity to power

supply switching signals or filtering inductors.

6.

Avoid running diode traces close to or parallel to high

speed digital and bus lines. Diode traces should be kept

at least 2cm. apart from the high speed digital traces.

7.

If it is necessary to cross high speed digital traces, the

diode traces and the high speed digital traces should

cross at a 90 degree angle.

8.

The ideal place to connect the LM82’s GND pin is as

close as possible to the Processors GND associated

with the sense diode.

9.

Leakage current between D+ and GND should be kept

to a minimum. One nano-ampere of leakage can cause

as much as 1˚C of error in the diode temperature read-

ing. Keeping the printed circuit board as clean as pos-

sible will minimize leakage current.

Noise coupling into the digital lines greater than 300mVp-p

(typical hysteresis), overshoot greater than 500mV above

V

prevent successful SMBus communication with the LM82.

SMBus no acknowledge is the most common symptom,

causing unnecessary traffic on the bus. Although, the SMBus

maximum frequency of communication is rather low (100kHz

max) care still needs to be taken to ensure proper termina-

tion within a system with multiple parts on the bus and long

printed circuit board traces. An R/C lowpass filter with a 3db

corner frequency of about 40MHz has been included on the

LM82’s SMBCLK input. Additional resistance can be added

in series with the SMBData and SMBCLK lines to further

help filter noise and ringing. Minimize noise coupling by

keeping digital traces out of switching power supply areas as

well as ensuring that digital lines containing high speed data

communications cross at right angles to the SMBData and

SMBCLK lines.

10129717

FIGURE 10. Ideal Diode Trace Layout

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