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[8023-POEP] 802.3af Minimum Signature Voltage

To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: [8023-POEP] 802.3af Minimum Signature Voltage
From: Joseph DeNicholas <Joseph.DeNicholas@xxxxxxx>
Date: Wed, 16 Mar 2005 08:56:39 -0700
Reply-To: "IEEE 802.3 Power over Ethernet plus"<stds-802-3-poep@xxxxxxxx>
Sender: owner-stds-802-3-poep@xxxxxxxx

Greetings -
I was wondering why the minimum signature voltage in 802.3af was placed so low, at 2.8V. With the input diode bridge drop of 1.5V or even 1.6V at cold, this doesn't leave a lot of head room (1.2V) for the PD ICs to turn a series high voltage MOSFET device on and get it into low RDS(ON). This MOSFET disables the 25k signature resistor at high voltage to conserve power. I noticed the majority of PSE ICs are supplying higher than that, around 3.5V or more, but I was wondering why the spec is so low when there doesn't seem to be a need for such low voltage operation when all we are trying to do is measure a resistor.

Has anyone proposed changing this in PoEplus?

Thanks,
Joe

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