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TPS563200: Output Voltage Resistor Equation 0.768 constant seems wrong

Prodigy 140 points

Replies: 16

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Part Number: TPS563200

I used the Webench to come up with a 12V to 3.3V 2A regulator circuit.  It is the basic circuit in the TPS563200 datasheet on Figure 19 with the R1 = 33.2k Ohms and R2 = 10k Ohms but I'm getting a voltage output of 4.75 volts.

When I use these values the 0.768 constant in the Vout equation under section 8.2.2.1 actually turns out to be 1.131.  Is there something I'm missing or should I select R1 to be 19.5k Ohms with R2 = 10k Ohms?

Thank you

  • In reply to Shawn Nie:

    Hi

    I am thinking the system is unstable, could you share the waveform (VIN, VOUT,SW,IOUT)?

    Shawn
  • In reply to Shawn Nie:

    Ok Shawn, I see the recommended layout in Figure 36 of the datasheet.  I'm attaching pictures of the signal plots on a scope but I'm going back to the drawing board and redoing the layout.

    I have to do another reply to show you the SW signal

  • In reply to Robert Butts:

  • In reply to Robert Butts:

    I'm not sure how you want me to plot Iout
  • In reply to Robert Butts:

    So the first plot is Vin, the second is Vout (3V with R1 = 19.6K) and the third plot is SW
  • In reply to Robert Butts:

    Hi,

    The SW waveform is irregularity, it seems unstable. Please update the result after you redraw the layout.

    Shawn

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