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Burning one LM27402 after another

Hegyi Istvan
Prodigy 100 points
Other Parts Discussed in Thread: LM27402

Hi,

We have a circuit where we use LM27402 to produce 12V from ~18V.

At first we tried to control the enable pin directly, by changing between active low and active high (5V) but whenever the input voltage was lost while this enable 5V was present, the chip instantly died, taking the high side mosfet (AON7246) with it. 

Next we connected the enable pin to a microcontroller pin, to change the level on the enable pin by changing between active low and high z on the MCU pin. The enable pin has an internal pull-up of 2uA. This seemed to work for a couple of power cycles, but ultimately failed in the exact same way. This is the 5-th (and last) IC we had, and still have no idea what causes the damage.

As we plan to go in production with this part, and in prospect of buying several thousand, we would really like to see this working as soon as possible.

Any tips?

Thanks,

Istvan

  • 0
    TI__Mastermind 40160 points
    Hello Istvan, do you have a schematic you can share? thanks.
  • 0 in reply to David Baba
    TI__Mastermind 40160 points
    Hello,

    From your description it sounds as though V Enable is high When VDD and Vin Fall which cannot happen. Try Placing a 10k Resistor in series with your Enable signal to limit the current that sinks through the enable when this happens. I hope this helps?
  • 0 in reply to David Baba
    Prodigy 100 points

    Hi David,

    As we ran out of parts, we have to wait with the test you suggest until our new parts arrive, thanks for the pointer! 

    You are right, missing that line did indeed cost us two burnt parts. However, after we became aware of this VEN < VDD constraint, we modified our micro-controller code to switch between enabled/disabled through setting active low or high-z (and leave the rest to the internal 2uA pull-up) on our MCU pin (on the schematic this is labeled BUCK_ENABLE). Did we do something wrong by leaving it floating? The datasheet says that floating this pin automatically enables the output when the VDD and input voltage reach the UVLO thresholds. Is it illegal to float "enable" pin when VDD and/or input voltage are in UVLO?

    To be exact, we managed to do a single enable - disable cycle in our last test, the second time we tried we already had ourselves a burnt LM27402.

    Thanks,

    Istvan

  • 0 in reply to Hegyi Istvan
    TI__Mastermind 40160 points

    Hi,

     

    leaving the Enable Pin floating is OK.  Pulling Enable to a higher voltage than VDD (when Vin is not present) is what I think caused your issue?