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TPS55340: Step up controller stops switching after short circuit on the output

Dimitris Kaikas
Prodigy 30 points
Part Number: TPS55340
Other Parts Discussed in Thread: PMP9779

We use TSP55340 as as step up controller with the following specifications:

- Vin: 21 to 27V, 24V nominal

- Vout: 28.5V

- Iout max: 400mA

If we apply a short circuit on the output, then most of the times the controller stops switching and does not recover, so we get on the output 24V minus the voltage drop on D5. If we cycle the power on the input, then the controller restarts operation.

Also, some times the controller gets burned out after the application of the short circuit on the output.

Oddly, the same schematic on another PCB with almost the same layout does not exhibit any of the above behaviors (neither stops switching nor gets burned out).

Could you please advise what is going wrong ?

Thank you very much. 

  • 0
    TI__Mastermind 40080 points

    Dear Sir,

    TPS55340 doesn't has short circuit protection function. Current limit function and the short circuit is a different concept. for the TPS55340, when the load current is very big, the low side FET on time is very short.,  like around 50ns.  But this short on time still exist even in the short circuit condition.  So in the short circuit condition, the IC may damaged.

    But actually, if the current rating of the inductor and the output diode is not big enough, then the diode or the inductor may damaged before the IC. 

    Which component damaged in your test? Diode, inductor or the IC?

  • 0 in reply to Helen chen
    Prodigy 30 points
    Hi Helen,

    When a damage happens, this is always on the IC. During the tests, 50% of the times the IC recovers it's operation normally, 40-45% stops switching until a power cycle is applied and 5-10% is damaged. The applicable short circuit duration is in the range of 70->120ms.

    What do you propose to limit the current ? Shall we try an inductor with bigger inductance and higher DCR ?
    Something else ?

    Best regards,
    Dimitris Kaikas
  • 0 in reply to Dimitris Kaikas
    TI__Mastermind 40080 points

    Dear Sir,

    As TPS55340 doesn't has short circuit function, so extra component must be added. Please refer to the TI design PMP9779. The concept will be similar.

  • 0 in reply to Helen chen
    Prodigy 30 points
    Dear Helen,

    I understand that interrupting short circuit is important and indeed we 'll add protection such that the one described in PMP9779 or software controlled with shorter duration (we now have software controlled at 170ms short circuit duration).

    But this does not explain why the same design works without any problem (neither IC damage, nor switching function disruption) on another pcb and we have to find the exact cause so as to be assured that this problem will not arise again on the future.

    Could you please advise on other factors that might cause the switching regulator to stop working after short circuit condition is recovered ?


    Regards,
    Dimitris Kaikas
  • 0 in reply to Dimitris Kaikas
    TI__Mastermind 40080 points

    Dear Kaikas,

    A possible reason is that minimum on time of the TPS55340 is a little bit different. As you can see, the Ton_min defined on the datasheet is 77ns typical. But actually, during the short circuit condition, for some part, the Ton_min is only around 30ns, which is pretty short.  And the part with shorter minimum Ton will more likely survive from the short circuit condition. Another important reason is the trace impedance, as the minimum Ton is very short, so  a  little bit impedance will helps the IC survive from the load short test.