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TPS23750/TPS23770 UVLO

Toshio Ushikubo
Mastermind 9620 points
Other Parts Discussed in Thread: TPS23750, TPS23770, TPS23753, TPS23756, TPS23757

Hello,

TPS23750 and TPS23770 has different UVLO rising voltage as follows.

 TPS23750   39.3V typical (Standard)

 TPS23770   35.1V typical(Legacy)

UVLO rising voltage of TPS23753/TPS23756/TPS23757(released later) is 35V and same with Legacy of TPS23770.

I think PD input voltage minimum specification of IEEE802.3af is 36V so UVLO rising should be 35V(not 39.3V).

Q1:

Please explain the case “TPS23750  UVLO 39.3V typical” is required.

Q2:

About” TPS23770 UVLO 35.1V typical(Legacy)”, Legacy has a meaning of “old” but TPS23753/TPS23756/TPS23757(released later) has same UVLO voltage with Legacy.

Please explain why 39.3V is Standard and 35.1V is Legacy.

Beast Regards.

  • 0
    TI__Expert 7990 points

    The Legacy term referred to meeting a particular customer's pre-IEEE802.3af release product.  They had a particular set of  thresholds to match.  When we offered both thresholds, the split was about 50/50, however customers who used the preferred thresholds would also use the Legacy thresholds.

    Your assesment of the proper startup voltage is skewed by the assumption that the load is drawing full current before it has enabled.  The minimum source voltage is 42V, and given pre-enable current levels the PD voltage is just about the same, so the stanfard turnon thresholds will be met.  However, you should realize that the allowed maximum IR voltage drop in the feed is greater than the hysteresis of the Legacy thresholds.  So if a source voltage were to rise more slowly than the startup and inrush of the PD there could be a startup oscillation.  This is one reason the parts (and recommendations) all have a much lower inrush current.  In general, both offered solutions will work with a PSE voltage that rises to the required voltage in a reasonable time and with a PD design that draws reasonable inrush and operating currents.