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TPS51285B Enable inputs

Naveen Kumar S
Intellectual 625 points
Other Parts Discussed in Thread: TPS51285B

Hi All,

We are using TPS51285B as a primary power supply for our design. 

Can the enable pins EN1 and EN2 be connected to VREG3 output to enable 5V and 3.3V outputs?

Any precautionary measures to be taken while doing this?

Thanks & Regards,

Naveen

  • 0
    Intellectual 625 points

    Eagerly waiting for a reply.

    Thanks,

    Naveen

  • 0 in reply to Naveen Kumar S
    TI__Guru**** 191445 points

    For TPS51285B, the VREG3 and VREG5 outputs are present regardless of the EN states.  VREG3 starts up when the VIN UVLO threshold is reached.  VREG5 starts up when the VREG3 UVLO threshold is reached.  Using VREG3 to enable both 3.3 and 5V rails may cause some timing issues.  You may enable the 5 V output before there is sufficient input voltage to allow the 5 V line to regulate.  It will depend on your specific supply voltage and slow start timing characteristics.

  • 0 in reply to JohnTucker
    Intellectual 625 points

    Dear John,

    Thanks for the reply.

    In our design, the supply to TPS51285B is either 12V DC jack or 12V through PD(POE). Since we are using TPS51285B as the primary source, what would be the ideal way to enable these supplies in our case? 

    Your suggestions would be valuable to us.

    Thanks & Regards,

    Naveen

  • 0 in reply to Naveen Kumar S
    TI__Guru**** 191445 points

    Like I said, it really depends on your own specific requirements and how your 12V ramps up.  Ideally you could use a power monitor circuit to ensure that 12V, VREG5 and VREG3 are all in regulation before enabling the two outputs.  If you were going to use one of the internal LDOs, I would use VREG5 as it comes up last.  You just need to ensure that your 12V is up above about 8V, before the end of the 5V slow start time to ensure monotonic, linear start up.