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POE - maintain power signature

Steve Robalino
Prodigy 110 points
Other Parts Discussed in Thread: TPS2375, TPS23752EVM-145, TPS23752

how is MPS (maintain power signature) implemented in POE designs?

the spec says to have a 10mA load at a certain interval - which how it is done with Class 0 or 1

I do not see how its being done on any designs or sample boards I have seen (like the TPS 2375 eval board)

but these boards work even with no load connected

how does a class 0 POE device support a 10ma MPS?...if it draws 4mA only

 

  • 0
    TI__Mastermind 42795 points

    The basic PD controller like TPS2375 cannot meet the MPS current when it is unloaded. On the EVM there is a jumper (JP7) which is installed at the factory and this meets the MPS level (see section 3.2 in the EVM user guide).

    In designs using a dc/dc converter, the unloaded converter may by itself use enough quiescent current to meet MPS. An example is TPS23753EVM-004. But TPS23753-001 is an example where you need to load the converter output or it will not meet MPS. So, basically it is up to the equipment to properly load the port to meet MPS.

    TPS23752 is a new TI POE device with "sleep mode". The user can put the converter to sleep and the PD controller can maintain MPS automatically (in either dc or pulsed mode) until the user wakes up the converter. TPS23752EVM-145 can demonstrate this feature and there is a good explanation of the mechanism's in the product datasheet.

  • 0 in reply to Eric Wright
    Prodigy 110 points

    please another clarification, a class 0 device drawing only 5mA for example, needs to load to 10mA to meet MPS?

     

  • 0 in reply to Steve Robalino
    TI__Mastermind 42795 points

    Yes, any class of PD must meet 10mA dc or pulsed MPS after the operating dc voltage is applied ("48V"). But, I think I see where you confusion may lie. In TPS2375 DS, Table 1 lists class 0 current (in the 802.3af LIMITS (mA) column) as 0-4mA. This current is not the operating current, but is the current that TPS2375 presents to the PSE during classification (when the port voltage is between 13V and 21V). TPS2375 does this by enabling the 4.42kohm load. After classification, then 48V is applied and this is when the PD must maintain the 10mA load (see the PD Power (W) column in Table 1).

    Hope this helps.