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LM3480: LM3480 reverse current latching

Ulf Seifert
Prodigy 50 points
Part Number: LM3480

Hi, I have a LM3480 for supplying an µC from 12V. Because I want to use the 5V supply from the progger without any 12V supply I have an inverse Schottky diode across the (between in- and output) of the LM3480.

Unfortunately the LM3480 latches if the progger switches the 5V on. If I supply the circuit from my laboratory power supply the regulator does not latch. It also does not latch if I short in- and output during the switch-on procedure of the progger. The input C at the 12V side is 100nF. Is there any solution to fix my problem (larger diode...) or to replace the regulator with an other with better reverse protection?

  • 0
    TI__Mastermind 35425 points

    Hi Ulf,

    Can you please provide a schematic so we are certain we understand your design?

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 50 points

    Hi Stephen,

    its a simple schematic, at the 12V side there are some OP Amps, sinking not more than a few milliamperes together. If I had started the 5V from the progger with shorting the diode I measured 4.7V at the 12V side after opening the short and no latching occurs.

  • 0 in reply to Ulf Seifert
    TI__Mastermind 35425 points

    Hi Ulf,

    Is it possible to place a schottky diode on the output of the LDO?
    Anode on the LDO output voltage and cathode on the circuitry that requires +5V?

    If the current draw is very low, then the voltage drop would also be very low.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 50 points

    Hi Stephen,

    of couse I can add a diode between the LDO and the µC, but the 1st series PCB are populated and ready... I will do so for the next batches but the customer waits for the devices. The only solution would be to told him, that he must feed them with 12V before connecting the programming interface. I hoped that this is not necessary. With all these 78xx IC there is "good practice" for decades of years to put a diode across them and it does function also in this case.  A modern regulator IC should have an integrated reverse current protection, I think..

    Thanks again

    Ulf

  • 0 in reply to Ulf Seifert
    TI__Mastermind 35425 points

    Hi Ulf,

    Once the requirements are known for the linear regulator, the linear regulator can be chosen based on the architecture and internal features it provides.  Many customers do not need reverse current protection.  But for those that do, there are linear regulators out there which offer that protection inherent to the design, however there is usually a trade off.  There may be increased quiescent current, higher dropout voltage, larger package size, or other impacts.  For the linear regulator in your design, you will need the external diode as you have designed it for the reverse protection.

    When designing multiple power sources to power a single power net, it is common to 'diode OR' them into the shared net.
    There are multiple reasons for doing this.  One such reason is that many LDO's cannot handle more than a small voltage differential from their output to their input.  Or it may be known that a particular LDO cannot handle a prebias event, where it starts up into a charged output capacitor, etc.

    For this application I would recommend a 'diode OR' approach.  If you need reverse current protection, you may use TI's website to choose which component best fits your application needs.  Here is the link with the reverse current protection feature already selected.

    http://www.ti.com/power-management/linear-regulators-ldo/products.html#p2192=Reverse%20Current%20Protection

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 50 points

    Hi Stephen,

    thanks for the support and the helpful link.

    Ulf