PoE++ (802.3bt) with LTC4291 and LTC4292, 90W or 71.3W

Question

Hi, 
 
 The 802.3bt standard is for PDs up to 71.3W and PSEs that can supply 90W (18.7W lost in cabling). These two chips list the maximum power draw can reach 71.3W, does that mean the chips can output 90W? Or are they only allowed to output 71.3W, meaning the PD will receive less than its rated maximum? 
 
 If these chips can only output 71.3W, are there any true 802.3bt offerings that can output 90W following the standard? 
 
 Thanks for your help!

EHorsma · Accepted Answer

Hello, 
 The LTC4291-1/LTC4292 specifies that it can support PD power up to 71.3W, not that it can "output (up to) 71.3W". This means the PSE will output at least 90W to insure delivery of up to 71.3W to the PD input over a 100m cable, per table 1 in the datasheet.

If you want to dig into the specifics, you can look at Table 4 which shows the current (2-pair, per channel) and power (4-pair, per port) limits for various PD classes in Auto mode.

Best Regards, 
 Eric)

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PoE++ (802.3bt) with LTC4291 and LTC4292, 90W or 71.3W

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