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PMP40871: Variable 20-48V low-noise power supply

danielb
Intellectual 270 points
Part Number: PMP40871
Other Parts Discussed in Thread: TPS61391, TPS7A4101

Tool/software:

Can the PMP40871 reference design be adapted to provide a variable 20-48V low-noise power supply? The output voltage would be adjusted using a DAC signal ranging from 0 to 3V.

  • 0
    TI__Expert 4760 points

    Yes, you should be able to replace the feedback resistor dividers with DAC scaling circuit to get the variable output voltage range you want.

  • 0 in reply to Sheng-yang Yu1
    Intellectual 270 points

    Hi Sheng. How to implement it? Should I replace the feedback resistors with DAC scaling circuits in both ICs?

    Why this design doesn't use the APD output of the TPS61391? 

  • 0 in reply to danielb
    TI__Expert 4760 points

    If you can allow DAC output 0V to represent 48V output, and DAC output 3V to represent 20V output. Then you could keep R1=392kohm, change R2 to 13.3kohm, and add a 41.7kohm resistor from DAC output to FB pin. That will allow you to adjust output voltage from 20V to 48V.

  • 0 in reply to Sheng-yang Yu1
    Intellectual 270 points

    Dear Sheng, you mean I should change just the resistors of the TPS61391? How the TPS7A4101 could also vary the voltage without adjusting its resistors?

  • 0 in reply to danielb
    TI__Expert 4760 points

    Sorry I missed the 2nd stage. You could apply the feedback resistors values to TPS7A4101 FB instead.

  • 0 in reply to Sheng-yang Yu1
    Intellectual 270 points

    How to you deduce this formula to calculate the resistor values?

    Could I apply the same DAC signal to adjust both stages at the same time to increase the efficiency?

  • 0 in reply to danielb
    TI__Expert 4760 points

    Yes, if you can change output voltages on both stages can definitely improve the efficiency. I think you will likely need two DAC signal to feed into both stages.

    The way I calculate the required resistance is described below:

    1. Pre-select R1 (top-feedback resistor) to assign a fixed value. Ex: set R1=392kohm.

    2. When DAC output=0V, R3 (resistor from DAC output to feedback pin) and R2 (bottom-feedback resistor) can be considered parallel. And we will have maximum output voltage at this point. So I have equation 1:

    Equation 1: Vout_max * R2//R3 / (R1+R2//R3) = VFB. In our case, Vout_max=48V, VFB=1.2V.

    3. When DAC output=3V, we will have minimum output voltage at this point. So I have equation 2:

    Equation 2: Vout_min / R1 + (3V - VFB) / R3 = VFB / R2. In our case, Vout_min=20V.

    By solving equations 1 and 2, you can get the resistance of R2 and R3.

    Regards,

    Sheng-Yang Yu