Re: [8023-POEP] 802.3af Minimum Signature Voltage
Joe,
The signature voltage is only used to validate the signature, not to directly turn on the series MOSFET.
In other words, the signature voltage is used by the signature validation circuit, at a low level.
Once the signature is validated, and after classification (if used), the MOSFET is turned on.
Remember, there's always full supply voltage available within the PSE circuitry to turn on the MOSFET.
Regards
Jerry
--
Gerard E. Bachand
Power Systems Engineer
7 Atwood Terrace
Cherry Valley, MA 01611
508-756-7738
bachand@xxxxxxx
-------------- Original message from Joseph DeNicholas <Joseph.DeNicholas@xxxxxxx>: --------------
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 any!
one proposed changing this in PoEplus?
Thanks,
Joe