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LM3150 working frequency

Michal Kiczka
Prodigy 50 points
Other Parts Discussed in Thread: LM3150

Hi,

I have a problem with LM3150 IC. During normal operation there is everything ok - I have stable output voltage, and the working freq. is about 250kHz (the input voltage is about 30V and output voltage is 15V).

When I'm making the ESD tests (8kV) to an output of the converter then after the test (discharge) the LM3150 rapidly changes operating freq. to about few hundred Hz, and the IC burn off. 

Does anyone knows the reason why the operating freq. decreases so much ? How to secure the circuit against his kind of behaviour ?

  • 0
    TI__Genius 16025 points

    Hi Michal,

    I'm looking into this and will get back to you shortly.

    Thanks,

    Anston

  • 0
    TI__Genius 16025 points

    Hi Michal,

    You are probably exceeding the Abs Max on Enable and causing the part to operate in a mode it wasn't designed for. This causes the internal latches to malfunction and destroy the part. 

    To prevent this, please place a capacitor on the Enable pin to protect it from this over spec fluctuation.

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 50 points

    Hi Anston,

    Thank you very much for the answer, but unfortunately it is not that problem. I'm switching on and off the converter from a uC supplied from 3.3V so the EN input is protect with a body diode of the output of the uC.

    I think the problem is with the spike on the FB input. This kind of behaviour i can see also when I use external circuit for a current regulation (CC mode). When the current sensing circuit detects overcurrent state gives a signal to the FB input of the IC through serial resistor 1k and diode LL4148 (see attached file - 4572.Sch.pdf2465.Sch.pdfR218, D209). For the very short time the output voltage of this current sensing circuit (CSC) can be a few volts, and I'm sure that for the some time the overvoltage condition on the FB pin occures (by the way - can you in detail explain how the OVP works ?). When the voltage of the CSC drops down the IC goes to work again, but with different frequency. One thing I must explain - the IC does not blow up itself. The external transistors and the coil are heating very much and after some time they have fail.

    Best regards

    Michal

  • 0 in reply to Michal Kiczka
    TI__Genius 16025 points

    HI Michal,

    The Current Sensing Network could be feeding a FB pulse into the controller which disables the high side driver. I don't know how your sensing network works, so I encourage you to disable it and see if the discrepancies go away.

    The OVP for the FB pin is 0.72V. The current sensing network could be driving this pin to .7V and below .6V causing the transistors and inductor to see large ripples affecting their life and performance.

    If this doesn't fix the problem, please place a 10nF or 100nF capacitor on the Enable line and try the test again.

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 50 points

    Hi Anston,

    I didn't wanted to believe you, but I should. I've placed the 100nF capacitor on the EN input, and it solved the problem. Now I can make ESD tests up to 8kV, and everything is ok, and of course my current sensing circuit works fine too.

    Best regards, and many thanks

    Michal