RE: [802.3af] 45Kmin instead of 70Kmin ?
Yair & John,
The change makes sense and TI would support the change.
From: Yair Darshan [mailto:YairD@xxxxxxxxxxxxxx]
Sent: Tuesday, January 08, 2002 6:55 AM
To: 'Jetzt, John J'
Subject: [802.3af] 45Kmin instead of 70Kmin ?
We specified that the high impedance option for the detection circuit
requires 70K min.
We also specified that the voltage across the port is 2.8V to 10V.
Now, if we want to get the 10V when we have the following conditions: 70K
series resistance, Rsig=23.75K (25k - 5%)
the probe source voltage need to be 10*(70k+23.75k)/23,75k= 10V*3.94=39.4V.
We have limited the open circuit voltage to 30V max.
So to get the 10V max we need to reduce the 70k min value to (30V*23.75K -
10V*23.75K)/10V = 47.5K.
Having 5% margin we get: 45K min.
In case of PSE to PSE connection the worst case power dissipation over this
resistor would be: 57^2/45k=72mW which is <125mW.
Thus to keep all numbers consistent with the intentions I suggest to change
from 70K min to 45K min.
Do you see problems with this change?
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