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TPS54302: Feedback Resistor Network

David Buszmann
Prodigy 80 points
Part Number: TPS54302
Other Parts Discussed in Thread: , TPS54335A

Hello,

I am looking to have Vout be adjustable from ~0V to 17V.  To do this I have set R3 to 4k and placed a resistor (Rdac) from a DAC to the FB pin.  Using the following equation I generated the table below.  

Vout = Vref * (1+ (R2/R3)) + ((Vref - Vdac)*(R2/Rdac))

Vdac Rdac R3 R2 Vref Vout
5 27000 4000 100000 0.6 -0.6963
0 17.822

I am using the TPS54302EVM-716 with the appropriate resistor changes to test this out.  With a Vdac of 0V to ~3.3V the output voltage is what I expect.  However, at higher Vdac voltages, Vout at times becomes unstable, and/or the lowest it goes is ~1.5V.  Vin is 24V in this case. 

Any suggestions what I am doing wrong, or on how to make this work?

Thanks,

David

  • 0
    TI__Expert 5745 points

    Would you mind to shoe me your schematic? During the Vdac of 0V~3.3V,  is the Vout=5.6V~17Vout? And the Vout will be unstable with the Vdac more than 3.3V?

    May be the min-on time of the internal MOS limit the Vout. You can try to decrease the Vin to check the range of Vout.

  • 0 in reply to Vental Mao
    Prodigy 80 points

    Yes, during Vdac of 0V to 3.3V, Vout is correct.  When Vdac is above 3.3V then Vout starts to get unstable or noisy (it's jumping around ~100mV - 200mV).  

    Decreasing Vin down to 7V did not allow Vout to go further below 1.5V. Vin = 8V, Vdac = 5V, Vout = 1.5V.

  • 0 in reply to David Buszmann
    TI__Expert 5745 points

    It maybe the loop isue, increase the Rdac to 40K and add a low pass filter(R=1K, C=100pF) in series of the Rdac. the High Rdac can move the pole to high frequency to make loop more stable.

  • 0 in reply to Vental Mao
    Prodigy 80 points

    With Rdac = 40k + filter, to keep the same output range, I also adjusted R2 = 140k and R3 = 6k.  In these conditions it is more stable, however Vout bottoms out at 1.9V (Vin  = 8V, Vdac = 5V).

  • 0 in reply to David Buszmann
    TI__Expert 5745 points

    Try R2 = 168k and R3 = 8.4k, your equation is not correct, because some current will flow into FB pin

  • 0 in reply to Vental Mao
    Prodigy 80 points
    I switched the values as you suggested. Using the method as I was testing above (no load) Vout still bottomed out above 0V. However, I decided to connect a load and the output went to 0V. When the load was removed, Vout floated back up. Even with the EN pin JP1 installed, Vout floated up. Is this behavior expected?
  • 0 in reply to David Buszmann
    TI__Expert 4875 points
    Hi David,

    For TPS54302, we found it is challenging to deal with the application when Vout is lower than 1.2V. It needs big output cap value to lower the bandwidth for good loop response. For your application, the output voltage range is too wide and it is difficult for a stable loop response under all circumstance. I think you could try to add more output cap and a feedforward cap for more phase margin.

    Best,
    Anthony
  • 0 in reply to Anthony Lang
    Prodigy 80 points

    Thanks for the info Anthony.  I will try your suggestion.  In the meantime, is there another part TI would suggest using for this application?

    Regards,

    David

  • 0 in reply to David Buszmann
    TI__Expert 4875 points

    Hi David,

    For you application, Vout is from 0V to 17V. What I could figure out is TPS54335A. It is not internal compensated. You could tune the control loop by yourself. However, it is not pin to pin with TPS54302 and the FB reference is 0.8V. I am not sure whether the circuit could work under output lower than 0.8V. It is also challenge to tune the control loop to work under full output range you need. You could use Webench to check on bode plot to see the loop response under different output voltage. Thanks!

    Best,

    Anthony