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TINA/Spice/TPS79301EVM: TPS79301 output not correct

Mark Whittington
Intellectual 480 points
Part Number: TPS79301EVM
Other Parts Discussed in Thread: TINA-TI, TPS793

Tool/software: TINA-TI or Spice Models

I'm trying to generate a 1.25V output in my circuit.  I've followed the equation from the data sheet (equation 4) and set R2 = 30.1kohm.  This results in a value for R1 of 624 for R1.  When I measure the output I'm getting a value of ~1.5V.  I've set C1 to 100pF per the recommendation in the paragraph below equation 4. 

When I probe the Fb pin I measure the same ~1.5V.  Is there any special advice for voltages near the feedback reference level? 

Separately I tried to load a simulation of the LDO in my configuration using TINA-TI only to find that the TPS79301 is not available by default.  The spice model provided by TI on the part webpage is encrypted which means I can't upload the model into TINA-TI.

  • 0
    TI__Mastermind 23361 points

    Hi Mark,

    You have the right values for R1 and R2 resistor to generate the 1.25V output. In TPS793 datasheet, equation 4 is the one to calculate power dissipation, and I don't think that's the one you were referring to. Is C1 the feedforward capacitor between the output pin and feedback pin?

    Can you also provide the input and output capacitor values you used in your application? This device requires an output capacitor to be stable. If you don't have the output capacitor, the device may be oscillating. When the output is oscillating, if you use a regular DC voltage meter to measure the voltage, you may see a higher voltage than the voltage the device has been set to because the oscillating AC spikes will result in a higher averaged DC value. When the device is working normally, the feedback voltage should be near 1.22V. In order for me to confirm the issues on your board, will you post your complete schematic and provide a screenshot at least with your input voltage and output voltage?

    For your inquiry on TINA simulation model, please allow some time for me to investigate the issue.

    Regards,
    Jason Song

  • 0 in reply to Jason Song
    Intellectual 480 points

    Jason,

    Thank you for the reply.  Your questions had me go back and take a second look.  The schematic is correct but it looks like the layout has an error.  The output capacitor is on the opposite side from the LDO output.  I put the cap on top and the voltage is now correct.  I didn't see any oscillation on the oscope but it seems like the output capacitor fixes it.

    Mark

  • 0 in reply to Mark Whittington
    TI__Mastermind 23361 points
    Great. Thanks for the feedback. Do you still need the model for simulation?
  • 0 in reply to Jason Song
    Intellectual 480 points

    Not anymore.  Thanks.