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Design equations for the capacitance going from VIN to -Vout in an inverting buck-boost converter

Chris Richardson
Prodigy 370 points
Other Parts Discussed in Thread: LM22670, TPS5430

Dear Non-Isolated Power Forum,

     Parts like the LM22670 and the TPS5430 have EVMs and component selection equations for the inverting buck-boost topology, and they always have capacitors (or at least footprints for some capacitors) going from VIN to -Vout. In these cases -Vout is the reference voltage for the control IC so it makes sense that some capacitance is needed to byass the VIN pins of the control ICs. But there are often other vague statements like "....to improve the stability" or "....to improve the startup" and then some hand waving, but I´ve never seen any actual design equations.

       Does anyone have any calculations and design philosphy for the required capacitance and the RMS current rating required for these capacitors?

Thanks,

chris

  • 0
    TI__Guru**** 150950 points

    If you have a question about a specific IC, it is best to put that part number at the beginning of the title of your post.

    But in general, for the TPS62k parts, the capacitor that decouples the IC (from Vin to -Vout) needs to be the same as the recommended input cap value for a buck converter.  So, the IC needs the same decoupling for any topology.  

  • 0 in reply to Chris Glaser
    TI__Mastermind 35185 points

    Hi Chris R,

    For the TPS5430 I have the same recommendation Chris G. gives. Most IC's will give a recommended minimum input cap value between the VIN pin and GND pin. This is what I recommend be used for the VIN to -VOUT decoupling capacitor in the inverting topology. For the TPS5430 this is 10µF.

    Regards,
    Anthony