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LMR33640: Problem with LMR33640ADDAR => When switched on, voltage peaks of up to 21V are maintained

Jochen Beißer
Prodigy 30 points
Part Number: LMR33640

Hello,

we use the LMR33640ADDAR to generate from 24V DC our used voltage of 3.3V DC.

We use this chip with the layout of the data sheet but now we have two times that all our chips which where connected to this power-chip are damaged.

We checked the voltage with an osziloskop and we found spikes in the moment of switching on up to 21V. And this spikes damage our chips.

Sometimes only spikes up to 10V, then 20V, or rarely 5V.
When the chip is ready then we can meassure 3.3V, but at the beginning there is a higher voltage. And this can damage our chips which where connected to this 3.3V.

What can we do that this voltage is only 3.3V.

  • 0
    TI__Mastermind 35190 points

    HI,

    Sorry to hear about your issue. In startup, the output voltage should ramp slow from 0V to 3.3V for this design.

    Reviewing your schematic:

    1. Device requires a minimum of 10uF (ceramic) input capacitance. C15 appears to be only rated for 16V and your nominal input is 24V. I recommend changing this capacitor to at least a 35V and ensuring it is ceramic.

    I understand that you reviewed the datasheet layout recomendation.

    Just to be certain it is all good, can you please also provide that and the ossiloscope captures showing the issue?

    Also, did the behavior change when removing the downstream load on the board?

    If so, it would be good to see the behavior with just a resistive load on the output of the dc/dc.