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TPS54560EVM-515: using DAC to dynamically control Vout

Part Number: TPS54560EVM-515

Hi,

I am trying to implement a variable buck to output 5A@0-55V at Vin=60V. I took TPS54560EVM by replacing the inductor and Cout based on Webbench design tool. To dynamically change Vout, I adopt a DAC with a R3 in series with it connected to FB pin.

As result, the Vout is a function of V_DAC as below:

Vout =  0.8*(1+R1/R2) + (V_DAC - 0.8)*R1/R3.  (0.8V is the reference voltage)

where V_DAC: [0, 2.048V] corresponds to [55V,0V]

Since on TPS54560EVM-515 the Vout was preset at 5V by R1= 53.6KOhm and R2=10.2KOhm resistor on FB pin where (53.6/10.2+1)*0.8 = 5.0V,   I fixed the existing R1=53.6KOhm value and calculated the required R2 and R3 based on above equation. The new R2 was achieved by connecting another R with the original R2=10.2KOhm in parallel. As a result, R1 = 53.6KOhm, R2 ~ 1.3KOhm, R3 ~ 1.8KOhm.

The above modification ran fine and I could smoothly change Vout by adjusting V_DAC.

After that, I used above design to make a new PCB by adding extra current & voltage sensing capabilities, the only difference is that I nearly doubled the values of R1, R2, and R3 (R1 = 105KOhm, R2 = 2.61KOhm, R3=3.79KOhm in new design) in order to further reduce power dissipation.

However, when I reduced V_DAC from 2.048V to get high Vout,  Vout become unstable when Vout >7V: I could see from DMM that AC Vout was more than 1V, also acoustic noises generated from the PCB. 

I tried increase the R3 value while keeping R1 and R2 unchanged, The instability of Vout delayed at higher Vout values. This somehow suggested the DAC impedance was too low.

So my question is how to select R1, R2, R3 values when using DAC to control Vout, even only R1/R2 and R1/R3 matter from the above equation.

 

  • Hi,

    Instead of using a DAC, could you also try to use an external adjustable power supply to change the output voltage with a given resistor?

    This is just to make sure that the design is stable with the selected value of resistor. 

    Normally we choose a lower value of feedback resistor for better noise noise immunity.

    In order to calculate the resistor value, i think you can use the superposition in order to calculate R3.

    Maybe this article will help

    http://www.ti.com/lit/an/slva861/slva861.pdf

    Thanks

    -Arief