LM90

SNIS126A – MAY 2004 – REVISED MARCH 2013

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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 LM90'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 reading. Keeping the printed circuit board as clean

as possible will minimize leakage current.

Noise coupling into the digital lines greater than 400mVp-p (typical hysteresis) and undershoot less than 500mV

below GND, may prevent successful SMBus communication with the LM90. 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 termination within a

system with multiple parts on the bus and long printed circuit board traces. An RC lowpass filter with a 3db

corner frequency of about 40MHz is included on the LM90'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.

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