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LMG5200: Failure

Noel Neu
Prodigy 55 points
Part Number: LMG5200

Hi,

Im currently experimenting with the LMG5200 for use in a class D audio amplifier.

There is no finished circuit yet, I've only done some preliminary testing by feeding a sine wave and a sawtooth into a comparator with complimentary outputs. That then goes into two buffers with an RC element for dead time.

This works fine for some time -maybe a minute- before the LMG5200 fails. 

This happened FOUR times !

On three of these, VIN is permanently shorted to SW. On one, VIN, SW and PGND are shorted. 

I have no idea what went wrong. I also fail to understand why it always works for some amount of time and then fails suddenly. 

Searching online, this seems to be a common problem with the LMG5200, but I could figure out why.

Given the price of these parts, this is incredibly infuriating.

Any Ideas ?

  • 0
    TI__Intellectual 2255 points

    Hi Noel, 

    Thanks for reaching out- sorry to hear that you've been encountering some issues with this device. To gain a better understanding of the problem, could you please provide any scope screenshots that you may have of SW, PWML, PWHM? Additionally, could you try measuring the resistance between VIN and SW? This will help us to diagnose the type of failure.

    It may be the case that an abs. max rating is exceeded somewhere, and device failure does not occur instantaneously. Instead the device fails after some time due to a prolonged exposure. 

    Best Regards,

    John

  • 0 in reply to John Gomez
    Prodigy 55 points

    Hi John,

    Thanks for your quick response!

    Unfortunately I can not provide any scope screenshots right now, because all the LMGs I had are dead...
    From my memory though, they looked unsuspicious.
    PWML and PWHM were very clean with no significant under- or overshoots or ringing. Deadtime was approximately 10ns, switching frequency 500kHz.

    SW had some slight ringing, but that was to be expected.

    I did try to measure the resistance. 

    My meter showed 0.4Ohms for each of the nodes I described as shorted - so the VIN to SW on three devices and VIN, SW and PGND on the other one.

    That said, my meter also shows 0.4Ohms when I short the leads, so it is a very very low resistance short for sure.

    Anyways, it would be great to get some suggestions before I start frying more of these.

    Best,

    Noel

  • 0 in reply to Noel Neu
    TI__Intellectual 2255 points

    Hi Noel,

    There are a multitude of factors that could lead to this failure. Could you please provide some info for the questions below?

    1. What voltage is being used for the VDD supply?

    2. What is the high output of the comparator for the HS and LS signals?

    3. What voltage level is the audio being amplified to? Essentially, what is the voltage at the VIN pin?

    Could you also provide a schematic if possible?

    Best Regards,

    John 

  • 0 in reply to John Gomez
    Prodigy 55 points

    Hi John,

    I think I might have found the problem. 

    I was using this setup with an LC output filter but no load. This might have caused some undershoot on the SW node. 
    It didnt look so bad on the scope, but I didn't check how much exactly it was.

    After some some research I found that if the SW node surpasses -5V, damage will occur. 

    But then why would that short the high side transistor and not the low one ?

    I used 30V for testing, I can't imaging that the overshoots were so bad that this would be outside the specs.

    What do you recommend as a solution for this ? Clamping ?

  • 0 in reply to Noel Neu
    TI__Intellectual 2255 points

    Hi Noel, 

    Yes, using only the LC output filter with no load may be causing some additional unwanted resonance at the switch node. The best way to address the switch node ringing is by reducing the parasitic board inductance through careful layout. Special care must be taken to keep the switching loop small, preferably by placing the return path in an intermediate board layer. More information on key layout considerations can be found in section 11 of the data sheet. 

    We recommend to try running the amplifier again under load. If the issue persists, revising the switching loop layout should be considered with additional clamping circuitry.

    We don't have a clear answer on why the HS is shorted in some cases and both the HS and LS are shorted in others. However, the short between Vin to SW, and SW to GND can be attributed to high shoot-through currents, which can cause migration of metallized layers. 

    Best Regards,

    John