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TPS784: TPS78401DBVR

Billy Parthipan
Prodigy 80 points
Part Number: TPS784

Tool/software:

Hi, I am using TPS78401DBVR to generate 3.1V. My supply is cannot be anything more than 3.6V, which is 0.5V more than Vout. Current requirement is about 100 mA ish (however one of the part suggest the use of 300 mA. Hence I am using it).

I am trying to workout the R1 and R2 values (and failed). Can you please help me with this.

If I use VFB = 1.2V and Ifb = 0.05 uA, then formula 2 do give values for R1, R2, which I cannot get hold of. The output needs to be precise. Hence asking your expert opinion.

And also I am assuming if my supply in is stable, I should be able to live with VIN = 3.1V +115mV worst case (not requiring Vout + 0.5V). Is my assumption correct? 

Thanks

Regards

Billy

  • 0
    Prodigy 80 points

    Just to expand on this I have calculated R1 = 379k (RT0603DRE07379KL 0.5%) and R2 = 240k (RT0603DRE07240KL 0.5%).

    Ideally I require LDO output  3.1V +/- 20mV.

    Thanks.

  • 0 in reply to Billy Parthipan
    TI__Expert 5555 points

    Hello Billy,

    Currently, it looks like your output should provide an output of 3.095V. 

    However, there are some tolerances associated with line regulation, load regulation, temperature and resistor tolerance. Fortunately, TPS784 is a pretty accurate device so you can expect the contribution from line, load and temperature to be 1% max. Since your resistors are 0.5%, that totals to 1.5% or 46mV.

    Dropout for this device is 115mV max so you should have plenty of headroom between VIN and VOUT.

    ~ Aaron

  • 0 in reply to Aaron Paxton
    Prodigy 80 points

    Hi Aaron,

    this sounds very promising. Thanks very much for this answer. I will see if I can tighten the tolerance a bit more by using 0.1% resistors to be on the safe side. Can I ask how you arrived the value 3.095V. What were the Vfb and Ifb values were you using? 

    Confirmation on the max. Vd and the headroom on the VIN is reassuring my thoughts.

    If I want to use the same device for 1.5V (though I only need less than 20mA on this rail) do you advise using this device? 

    Thanks

    Regards

    Billy

  • 0 in reply to Billy Parthipan
    TI__Expert 5555 points

    Hey Billy,

    I forgot to add that in. My mistake. Should be ±2.4mV (typ) or +12mV/-24mV (max) additional tolerance with that current bottom resistor value based on the feedback current. 

    You can reduce this impact by making the resistor values smaller. However, doing so increases the overall supply current of the device. Dropping down to a 37.9k + 24k divider would reduce this offset by a degree of magnitude. 

    To be frank, it's pretty unlikely you'll see beyond the typical values for feedback current. The currents are typically so low that it's difficult to measure at production test. 

    For 1.5V, you might consider the fixed output version TPS78415QDBVRQ1. It's automotive but it incorporates internal trimmed resistors to ensure greater accuracy vs. an external divider. 

    ~ Aaron

  • +1 in reply to Aaron Paxton
    Prodigy 80 points

    Hi Aaron, thanks for this. I guess I can live with the original values of resistors. Power saving is a key to this project. On the LDO, it is a good part. I have reserved it but not committed in the design. Design has now gone for review and if any bad comment comes on the 1V5 generation I will plug this in. For now I think you have resolved this matter. Thanks again. Regards Billy