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TPS61071 ripple when Vin is near Vout

TonyMCC
Intellectual 760 points
Other Parts Discussed in Thread: TPS61071, TPS61220, TPS61070

I have started testing the TPS61071 evaluation board which is set up for an output voltage of 5.05V as the default. Everything seems to work fine except when I increase the input voltage to a value >4.7V, the input/output voltage ripple increases to 50mV p-p for the output voltage and 100mV p-p on the input voltage rail. I have included images for the Vin of 4.7V that has minimal ripple and 4.8V which has the increased ripple. The scope CH1 is TPS61071 Vout and the CH2 is TPS61071 VIn, using short scope probe leads. The Vin to the evaluation board is being supplied from a E3631A Agilent linear power supply. There is no load applied to the TPS61071 output.

Vin=4.7V

Vin=4.8V


  • 0
    TI__Guru**** 150950 points

    Welcome to the forums.  I would ask that you read the ground rules post at the top of this forum for tips on using the forum, especially in using the Wikis and in titling your posts.  I have re-titled your post to make it more meaningful to others.

    Here is a Wiki entry that is relevant: http://e2e.ti.com/support/power_management/non-isolated_dcdc/w/design_notes/870.aspx

    With Vin very near Vout and no load, too much energy is transferred to the output and so the ripple increases.  It is important to test a given device under operating conditions that your system will see.  So, if you know you will have a minimum load, then test with that load.  

  • 0 in reply to Chris Glaser
    Intellectual 760 points

    Thanks for fixing my initial post...I'll review the best practices for future posting as you suggested.

    I have tested the evaluation board in the configuration I plan to use the TPS61071 in which is a fairly light loading of 1mA typical to 10mA maximum. Thaty amount of loading did not affect the ripple issue I was seeing with Vin >4.7V. I can certainly try to increase the loading further to see its affect on the ripple amplitude.

    Are there any other recommendations for reducing the ripple amplitude besides increasing the loading ? I have tried increasing the size of the input/output caps and had moderate success (< 30% ripple reduction).

    Or is there a different part I should be using instead of the TPS61071 ? My application has a 4.5V to 5.2V Vin range and needs a 5.35V Vout.

  • 0 in reply to TonyMCC
    TI__Guru**** 150950 points

    For that light a load, the TPS61220 is usually selected as it is a lower power device.

    There is another wiki entry about that device.  It behaves similarly as the TPS61070.  But with 150 mV between Vin and Vout for your case, it might work.  You could also just add a diode in series with the input to drop the voltage a little.

  • 0 in reply to Chris Glaser
    TI__Guru**** 150950 points

    A quick test of the TPS61220 in the lab shows that your Vin that close to Vout does increase the ripple from about 20 mV to 80 mV.  This may be ok for your application being less than 2% of the output voltage.  A diode in series with the input would drop the input voltage enough to give low ripple.  As well, an RC filter on the output could be used.

    Finally, we have TPS630xx buck/boost devices that could work for you.

  • 0 in reply to Chris Glaser
    Intellectual 760 points

    Thanks for the feedback.

    I am getting fairly good ripple results (< 25mV p-p, 1mA-10mA loading) after installing 47uf capacitors at the input and output locations provided on the eval. board.

    The ripple did increase to ~50mV p-p when I installed a 3.9pf Cf across the R1 (909k) as suggested in the TPS61071 datasheet (per equation 2). The datasheet mentions if R2 is "significantly lower than 200k" consider installing the Cf capacitor...can the "significantly lower" be quantified further, I plan to use R1=1Meg and R2=105k for my application and would like to know if the Cf is needed.

  • 0 in reply to TonyMCC
    TI__Guru**** 150950 points

    The larger capacitors are the best way to reduce it, though the Cff should help further.  It's never required but does help improve the transient response.  I would just leave it off if your calculations show that less than 10pF is needed.  There is some capacitance just from the pads of R1 that sums to some pF.