# RE: [802.3af] 45Kmin instead of 70Kmin ?

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Hi Yair,

Ok, so if the input voltage is limited to 30V, the PD voltage will be at
30V(23.75k/(23.75k+70k)) = 7.6V.  The detection voltage range is  reduced by
2.4V, but this would not cause at any problem at all.  You still have the
2.8V to 7.6V range to work with.

If the PSE uses a current source to probe the PD, then there is no problem
to pump the PD voltage up to 10V if it so choose.

Impedance rule does not apply to PSE probe using  first quadrant voltage
source.

Conclusion:  changing the spec. from 70k to 45k is unnecessary and I would
oppose it.  But thanks to Yair for many of his good comments and
contributions so far.

===========================================
Thong Huynh
Senior Corporate Application Engineer, Standard Products Business Unit
Maxim Integrated Products
Tel:  408-530-6375
Fax:  408-222-1737
Email:  Thong_Huynh@xxxxxxxxxxx

> -----Original Message-----
> From:	Yair Darshan [SMTP:YairD@xxxxxxxxxxxxxx]
> Sent:	Tuesday, January 08, 2002 3:55 AM
> To:	'Jetzt, John J'
> Cc:	'stds-802-3-pwrviamdi@xxxxxxxx'
> Subject:	[802.3af] 45Kmin instead of 70Kmin ?
>
>
> Hi guys,
> 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?
>
> Thanks
>
> Yair.
>
>
> Darshan Yair
> Chief  Engineer
> > PowerDsine Ltd.  -  Powering Converged Networks
> > 1 Hanagar St., P.O. Box 7220
> > Neve Ne'eman Industrial Zone
> > Hod Hasharon 45421, Israel
> Tel:  +972-9-775-5100, Cell: +972-54-893019
> Fax: +972-9-775-5111
> > E-mail: <mailto:yaird@xxxxxxxxxxxxxx>.
> > http://www.powerdsine.com
> >
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