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LM5060 short circuit issue

Andrew McAllister
Prodigy 30 points
Other Parts Discussed in Thread: LM5060, LM5069

Hi,

I have designed a PCB with the LM5060 to protect against over current and over voltage. During testing we have found that when we apply a short circuit the LM5060 does not turn OFF, then the FETs then over heat and blow up. The nPGD pin does show a fault when a short is present so my next approach was to see if turning OFF the EN pin if the nPGD pin goes high would be quick enough to save the FETs, unfortunately after performing this test the LM5060 blew up. I have repeated the test on another PCB and the same problem occurs.

I am using the datasheet recommended design described as "Application with Reverse Polarity Protection with Diodes for OUT Pin Protection" except Rsense is set at 16kOhm for a 20A protection based on using 2x IPB100N10S3-05 FETs. If apply a load of >20A then the LM5060 latches OFF and functions correctly, only when a hard short is present does this fault occur.

The datasheet suggests that the LM5060 is capable of protecting for shortcircuit conditions, can you help?

Kind regards,
Andy

  • 0
    TI__Mastermind 42795 points

    During the hard short case, the input line inductance effects will be more pronounced. Make sure the connection from input supply to the hotswap is short and that the input supply voltage is stable during the hard short.

  • 0 in reply to Eric Wright
    Prodigy 30 points

    Hi, thank you for your feedback.

    Im not sure this helps.. we will not have this level of control over how our customers use this device, the distance this PCB will be connected from the supply in a practical enviroment could be a number of meters and i am only using it at 300mm from the PSU. The PSU will be pulled down by the shortcircuit when it is connected and therefore not entirely stable.

    It doesnt explain why when a shortcircuit is present that the LM5060 does not turn OFF its output?

  • 0 in reply to Andrew McAllister
    TI__Prodigy 375 points

    Andrew,

    The LM5060 does not have a currentlimit or powerlimit a seen in a device designed for hot-swap applications.  If you need this level of protection the LM5069 would be a better choice.

    The device does feature over current proection. 

    When the voltage across the FET exceeds the  the voltage drop across the SENSE resistor the device will start charging the capacitor on the timer pin.  When voltage on the timer pin exceeds the timer pin fault threshold the FET will be quickly turned off.

    The first thing I would try is to reduce the size of the timer cap so it will more quickly turn off the FET.  The duration that can be tolerated depends on the FET you are using, its SOA and peak currents you see during a short. 

    Keep in mind during a hard short the FET will have the full input voltage across it.  The current could ramp up to a very high value before the timer cap reaches the fault thresold and turns of the FET. 

    Regards

    Dennis

  • 0 in reply to Andrew McAllister
    TI__Mastermind 42795 points

    Hi, could you send the LM5060 portion of your schematic so further internal review can occur? How did you confirm that LM5060 was not turning off it's output? Did you monitor GATE and GATE was not low or are you looking at OUT? It might be that the FET(s) "blow" shorted drain-source first which would not allow OUT to pull low.

  • 0 in reply to Eric Wright
    Prodigy 30 points

    The LM5060 portion of the schematic:

    When the shortcircuit is applied the OUT still has a voltage on it.

    There are two issue here, No.1 the LM5060 does not turn OFF during a short, No.2 when you use nPGD to determine if OUT is ok and if it isnt, turn OFF the LM5060 blows up.