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LMR51420: LMR51420XDDCR bootsrtap circuit question

Adam
Expert 1195 points
Part Number: LMR51420

Im using LMR51420XDDCR to convert from 21V to 3.3V. I use 3.3uH inductor and 88uF output capacitance.

I wanted to reduce the switching ringing of output voltage.

To do that I add 50ohm resistor in series to bootstrap 100nF capacitor and I'm getting nice reduction of switching ringing on the output voltage.

My question is - can this device work with 50ohm resistor in series to bootstrap cap in terms of stability/control? (as small values like 3ohm do not make such a good job in reducing ringing)

I know it reduces overall efficiency, but that's ok for my design.

Adam

  • 0
    TI__Guru 71865 points

    Hello

    As you suspected, the value of 50Ohm is rather large.  Not only does the boot cap supply

    energy to the high-side FET, but also to control circuits that are "floating" along with the high-side FET.

    Therefore, any added resistance to this supply may cause a malfunction in the control circuit of the regulator.

    So, TI would not be able to recommend that your application would function correctly under all conditions, since we have not tested with this resistor in place.

    To help squash ringing on the SW node it is better to use an RC snubber.  

    I have attached an app note that will help you choose the RC components; you only need one RC from SW to GND.

    Thanks

    RC_snubber_design_revA.pdf