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Part Number: TPS2372
I'm understand the concept of demotion but I don't understand the Table 3 on the datasheet for TPS2372. On page 16 we can see Table 3. It states that a PSE type 3-4 can just allocate 12.95w on a type 4-8 PD. If a Type 3 PSE can supply 60w ( 51w at the PD), I don't understand why the PSE can just allocate 12.95w according to the table. I would agree with Table 3 if on the PSE column the first position was for Type 1 PSE, second position for Type 2 PSE and third position for Type 3 PSE. I'm probably missing something and that is why I don't understand it , if somebody could explain it would be really appreciated.
PoE Applications & Marketing Manager
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In reply to Thomas Lewis:
In reply to Darwin Fernandez:
Thanks for your help in advance. I understand the concept of demotion ( or at least that is what I think), it basically is when a PD requests more power than the PSE can supply, then the PD can power up with power limitations and the TPH and TPL output a specific code to communicate to the PD application what to do. What I don't understand is why a PSE type 3-4 fall in demotion mode when PD class 4 is requesting power?, as a example. The minimum power available for a type 3 is 45w and a PD Class 4 requests 25.5w, so I don't understand why it is demotion?. I can see that the PSE send just 1 class cycle and it means that the PSE allocates as a maximum 12.95w, but why a type 3 PSE send just 1 class cycle?. If you could explain why this happens it would be very appreciated.
In reply to Daniel Campano:
A PSE can be operated in a power-limited mode. For example, most multi-port PSE switches will have some power port management such that a Type 2 PSE can be power-limited because it can only budget 15W on one of its port. Therefore, it will only send 1 class finger and demote a Class 4 PD to Class 3 power levels. This is an example for the current IEEE802.3at standard.
This behavior will extend to the new .bt standard in that a PSE switch with power management can power-limit a Type 4 PSE port to 60W if that's what it can budget in its current operating state. So a Class 8 PD will then get demoted to Class 6 when the port only delivers the 4 class cycles.
To clarify Table 3, it is showing that a Type 3 or 4 PSE can demote a PD to a certain power level depending on the number of class cycles it delivers to the PD.
Please let me know if this helps.
Great help Darwin, It resolves my doubt, thanks.
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