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TPS62290 3.3V output ripple 500mV for 1A max load

prakash venugopal
Prodigy 110 points
Other Parts Discussed in Thread: TPS62290, TPS62080, TPS54218, LP38513-ADJ

Hi all,

      We had tested the Texas TPS 62290 as per the recommended layout. We had tested the output voltage with Max load 1A We are trying to arrive 3.3V  and 1.2V from two different TPS 62290 with the input voltage of 5V.  

For 3.3V,  When i connect 1A max load --> 3.2V output with 500mV ac ripple For 1.2V , when i connect 1A max load --> 1.17V output with 100mV ac ripple

1.2V regulator with 100mV seems to be ok. But in case of 3.3V, the output ripple of 500mV is huge.  

My Question is, Please refer the Page No.11 of Pdf and schematic attached.   In case of 1.2V, 33pf (C10) is placed parallely with Resistor (R18) whereas in case of 3.3V, there is no 33pf capacitor placed  parallel with resistor(R11). This makes the difference in the ripple factor for 3.3V?

Please let me know and help on this. 

Thanks,

V. Prakash

8037.xlx_s3a_evl-sch.rar

  • 0
    TI__Guru**** 151860 points

    Thank you for attaching a full schematic and a clear description of your issue.  I believe what you are seeing is not output ripple but instability.

    Output ripple is an AC signal of the output voltage due to the switching action of the converter.  Bu definition, it occurs at the switching frequency (~2.25 MHz in this case).  I believe what you are seeing is control loop instability, which occurs at a much lower frequency.  I would need to see a waveform of the ripple to say for sure.

    From your schematic, you are violating the required output filter that this part was internally compensated for.  Per page 15 of the datasheet, the IC is compensated for a given range of output filter.  Note that Cout is the total Cout on the output bus, not just what is on the TPS62290 page of the schematic.  The IC sees the load and its total Cout, so this must be taken into account.  The input capacitance at your processor violates the upper limit of this capacitance.

    I recommend that you either remove downstream Cout to remain within the datasheet limits or switch to the TPS62080 family which supports a larger output capaitance.

    The feedforward capacitor, C10, can help you support higher Couts but not as high as you have.  The app note SLVA289 explains this in more detail.

  • 0 in reply to Chris Glaser
    Prodigy 110 points

    Hi Sir,

            Yes. you are right.  From the page 15 of datasheet, output capacitor range is 4.7 uf to 22 uf. 

    I am feeding this 3.3V to the FPGA (XC3S50AN). This FPGA has VCCIO and VCCAux supplied by 3.3V (TPS62290)

    There are 4 VCCIO and 4 VCCAux on this FPGA. I had added the decoupling of 10 uf to the each pin of VCCIO and VCCAux.

    so 8 x 10 uf capacitor added as decoupling for 3.3V of  FPGA.

    As you told, the total output bus capacitance will be 10 x 10 uf capacitor. This will damage the IC? Please recommend?

    Please refer the Page 7 of pdf attached. In that capacitor (C23) 470 uf capacitor is added as decoupling for VCCIO (3.3 v) with same TPS 62290 used.

    Please let me know what to do with this?

    5518.xlx_s3a_evl-sch.rar

  • 0 in reply to prakash venugopal
    TI__Guru**** 151860 points

    You need to operate the TPS62290 within the recommended output filter configuration.  It might be damaged if it is unstable and it will possibly reduce the performance of your system.  You need to calculate how much of that capacitance is actually required for your system and remove the excess, beyond what the TPS62290 can support.

  • 0 in reply to Chris Glaser
    Prodigy 110 points

    Hi sir,

          Xilinx FPGA is recommending to use 5 x 10 uf capacitort for decoupling VCCIO and VCCAux. so 5x10uf = 50 uf capacitor is beyond the limit of the recommendation

    of powersupply. In this case, i have to go for another supply or any other way to resolve this? Please help.

    Thanks,

    V. Prakash

  • 0 in reply to prakash venugopal
    TI__Guru**** 151860 points

    I recommend following both your FPGA's recommendations and the power supply's requirements for capacitance.  You could use the TPS62080 or the TPS54218.

  • 0 in reply to Chris Glaser
    Prodigy 110 points

    Hi Sir,

             Can i use LP38513-adj to generate 3.3V and 1.2V? Can i add the 8 x 10 uf capacitor in the output bus (decoupling) using this regulator. please let me know.

    Thanks,

    V. Prakash

  • 0 in reply to prakash venugopal
    TI__Guru**** 151860 points

    Yes.  But keep in mind that this is a linear regulator.  It will dissipate significant heat and get very hot, depending on your load current.  See page 9 in its datasheet.

  • 0 in reply to Chris Glaser
    Prodigy 110 points

    Hi sir,

          Thanks for suggestion. I will try to go for Linear regulator or switching regulator with High output capacitance.

    Thanks,

    V. Prakash