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LP3878-ADJ requires 2V dropout per 7.4.1, is this correct?

John Haraguchi
Prodigy 250 points
Other Parts Discussed in Thread: LP3878-ADJ

Paragraph 7.4.1 of the LP3878-ADJ datasheet implies the regulator requires a minimum 2V dropout to perform correctly.

This paragraph was not in the original National datasheet and is not consistent with Figures 5-7.

Is a 2V of dropout really required for this regulator?

  • 0
    TI__Expert 3550 points

    That is certainly how I read it as well; there is a footnote in the data sheet 'parametrics' section that specifically calls this out- tat is; stating the dropout voltage specification only applies if VIN is sufficient to achieve operation (bottom of page 5, footnote 1.  I have asked a colleague of mine to confirm my (and your) reading of this; but I believe that for the lowest voltage of say 1V; you'd need 3V input minimally (or 3.1 with some margin as you note).

  • 0 in reply to radar
    Prodigy 250 points
    I designed with this regulator years ago based on the old National datasheet which does indicate 2V of headroom is required.

    I use it in many instances where Vin - Vout does not meet this requirement.

    A colleague was going to use the regulator in similar fashion. During a peer review, this paragraph was flagged to indicate that it would not work properly.

    This in turn has brought suspicion to how I implemented the regulator on my circuit card assembly.

    So even though I have never had an issue with the hundreds of LP3878-ADJ regulators in use on multiple assemblies, I may be asked to design it out based on paragraph 7.4.1.
  • 0
    TI__Mastermind 22610 points

    Agreed that this point is not as clear as it should be.

    Depends on what your output voltage is set to.and the peak output current

    In the Electrical Characteristics table the Vin= 3.0V and Vout= 1.0V,, so for that particular situation you will have 2.0 from Vin to Vout, but that's not a dropout voltage limitation

    The 3.0V used for Vin in the EC table header comes from the last line item in the Vin(min) specification (Iout=750mA, and 40°C ≤ Tj ≤ 125°C)

    As for " ... 7.4.1 Operation With Vout+2V ≤ Vin ≤ 16 V ..."

    The LP3878-ADJ has been reformatted from the original National Semiconductor format into it's present form in two steps. The latest reformat required that new sections be added, including section 7.4.1, and new text was created to fill that section. I suspect that the technical writer used the information in the EC table header to create the 2V (i.e. 3V-1V) minimum.

    In short, for DC Vout purposes the minimum Vin is the HIGHER of either Vin(min) or (Vout + Vdo)