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Linear Regulators

ryck blanchard
Prodigy 20 points

I have a question to ask of your design or application engineers.

Is there any reason for putting a diode across a linear regulator as shown?

We have a lessons learned case where it says this is required on all linear regulators. What it the opinion of TI on this recommendation. It is said that if the output would stay up longer than the input damage could occur.

I would really like TI’s opinion on this, I have never seen an app note recommending this.

 

Ryck Blanchard

Uninson Industries  (GE Aviation)

Ryck.blanchard@unisonindustries.com

  • 0
    TI__Mastermind 22610 points

    Yes, this is real, but is not applicable for most linear regulators. For some linear regulators the external diode is needed to protect the internal structure should the input voltage fall faster than the output voltage. Otherwise excessive reverse current will flow through an internal parasitic diode that exists in the bipolar pass element.

    The existence of this parasitic diode is usually a by-product of the fabrication process used, not so much a design feature.

    See the LP2980 datasheet  section "REVERSE CURRENT PATH" for an example (there are other devices) ...

    http://www.ti.com/lit/gpn/lp2980-n

  • 0 in reply to Donald Jones
    Prodigy 20 points

    How would this apply to both the 78L00 series or 79L00 ? Also I new mosfets had an intrinsic diode as part of the fab process but I have never heard of bipolar devices having a parasitic diode. Thats why when using bipolar's or  IGBT's you have to use a freewheeling diode if you need a diode.

    Ryck

  • 0 in reply to ryck blanchard
    TI__Mastermind 22610 points

    Perhaps " ... internal parasitic diode ..." is not correct term ... "Internal parasitic PN junctions" might be better.

    There is no intrinsic diode across the pass transistor array. The output stage is not a simple one transistor item. There are numerous transistors, resistors, all in a tub that is expected to be biased by the input voltage, with the design expectation that the input voltage will be always higher than the regulated output voltage.

    These junctions are not capable of carrying any high level of current for any period of time without being damaged, The external diode provides an alternate, parallel, path for the reverse current in order to prevent internal damage to the device

    If the application will never see a condition that will have the input voltage pulled rapidly below the output voltage, the reverse current protection diode would not be needed.

     

  • 0 in reply to Donald Jones
    Prodigy 90 points

    Is this diode needed for an LM3480IM3-5.0 with 0.47uf of output capacitance?