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TPS53353: Not enough magnitude margin

Nicole Han
Intellectual 2230 points
Part Number: TPS53353

Hi, team, 

My customer is using TPS53353 for 12V to 1.1V/15A output. Below pdf is their schematic. after test the loop, we found the magnitude is only 5.6dB, their end customer need >6dB. 

I have seem in one post, that if only ceramic capacitor is used, ripple injection RCC is needed, however if only bulk capacitor is used, then no RCC is needed. But doesn't suggest use ceramic together with bulk capacitors. 

I have suggested customer to remove all the ceramic capacitors, but they are afraid it may affect the performance, and now that  this project is almost pilot run, they don't have enough time to verify all the other performance. Is there any other ways that they can try to help with the magnitude margin and impact minimum at other performance? How? How?

 

S1252F-PWR.pdf

  • 0
    TI__Expert 5310 points

    Hello,

    As far as improving gain margin goes in this instance, we will need to reduce the system gain or provide a phase boost. As you mentioned, depending on the makeup of the output capacitance (all ceramic vs bulk capacitance), that will necessitate whether or not a ripple injection circuit is necessary. You can suggest removing the ceramic output capacitance, but removing capacitance will increase the gain of the system. If that recommendation is performed, please ensure that equation 6 in the datasheet is satisfied based on the effective ESR and output capacitance of the bulk capacitor. 

    In terms of modifying the current design/BOM, I have three recommendations to decrease the gain and/or provide better gain margin:

    1. Increase the ceramic output capacitance

    2. Increase inductance while maintaining ~0.1 inductor ripple/output current ratio at a minimum

    3. Consider adding a feed-forward capacitor in parallel with the upper feedback resistor

    Regards,

    Alec Biesterfeld