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Old August 4th, 2008   #3
Senior Member
Join Date: Jan 2008
Posts: 102

Hi Eric and thanks for the reply.

To meet the spec, The pullup resistor is actually 390R for a 3V3 supply.
If the dataline is now pulled to +12V (example but perfectly legal) by the responder driver, we get (12-3V3 / 390 ) = 22mA into the power supply.

That is quite high in my opinion!

the power dissipation on the pullup resistor is then 0.194W !

Worse, if the power of the master happens to be off for whatever reason, it is quite possible to power up the master throughthis single pullup.

Yes, we are building lower power devices but not that low and it is still a problem.

ESD protection does not help here because that only kicks in at +12 and -7V, whch are the ~ allowable limits according to the spec.

Kind regards
berntd is offline   Reply With Quote