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TPS25942A: Statistics of V(REVTH)

Antenna_Head
Genius 11745 points
Part Number: TPS25942A

Team-

Page 8 of the TPS25942 data sheet provides the min/max thresholds for V(REVTH) as -15 mV and -3 mV respectively, and a typical value of -9.3 mV.

How many standard deviations are implied between these min/max values? +/- 3 sigma? +/- 4.5 Sigma?  +/- 6 Sigma?

We have a problem where the back-feed current necessary to turn off the device is getting too high and we need to better understand the statistics around this parameter to solve the problem.

Essentially, given that Rds,on can be as low as 26 mOhm, this implies that the device will not isolate until a current of 15 mV/26 mOhm = 0.576A flows backwards through the device, worst case. 

Using typical values, we would expect 9.3 mV/42 mOhm = 0.221A to be sufficient.

We need to understand the probability of currents exceeding about 350 mA; obviously, the lower the back-fed current, the better.

  • 0
    TI__Guru** 102651 points
    Hi,

    The V(REVTH) variation is calculated based on +/- 6 Sigma. 26mohm corresponds to case at lower temperatures. what is the operating temperature range?
    I agree, it is better to have lower back-fed current.
    Do you have any loads to draw current at input when the input power is removed ?
    May I know the reason to limit at 350mA?

    Regards,
    Rakesh
  • 0 in reply to Rakesh
    TI__Genius 11745 points

    Thanks; understood that 26 mOhm occurs at lower temperatures.  Unfortunately, the application is rated down to -40C, so I believe we need to use the min Rds,on for this analysis.

    This is part of a power OR'ing solution; a variable 5V-12V backup rail and a 5V rail are being OR'ed.  If the 5V is on first and then 12V switches on, it can back feed onto the 5V before that e-fuse can shut off, potentially forcing it high enough to cause voltage stress on other components on that 5V bus. There is only so much capacitance on the bus, so the amount of over-voltage is dependant on the level of current that backfeeds through.

    Please call me for more specific details.  The 350 mA limit is a SWAG about where the application is probably safe.  This is on an existing design; we can't change the PCB.  A future revision will have an architectural fix to remedy this.

  • 0 in reply to Antenna_Head
    TI__Guru** 102651 points
    Hi,

    One solution, I can think of is to use in priority power MUX configuration as shown in figure 79 in datasheet. Slight modification is required based on which rail is prioritized.
    I will setup call to understand more and design specifics.

    Regards
    Rakesh
  • 0 in reply to Rakesh
    TI__Guru** 102651 points
    Can you please share your contact e-mail ID.

    Regards
    Rakesh
  • 0 in reply to Rakesh
    TI__Genius 11745 points
    As I mentioned, we can not change the board at this time. We can only change the dvdt timing of the e-fuses.

    S. Dunbar, AFA Colorado (please look up my email in internal directory) Thanks!