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TPS7a3701 at 1.26V - What resistors should I use?

Alex Triano
Genius 12621 points

Hi,

I am working with a customer, using the TPS7a3701 at 1.26V output. What resistor value should they use?

 

Customer question:

We’re using this part as follows:

  1. Vout = 1.26V; R1 = 20K; R2 = 430K.  This meets the rule that R1||R2 = 19K. 
  2. I’m worried that R2 = 430K is a very high impedance.  I’ve seen a case where a power supply feedback circuit could be disrupted during immunity testing and cause Vout to change.  Using smaller resistor values fixed the problem.

 

Can you check what your recommended feedback resistors should be for Vout = 1.26V?

  • 0
    TI__Guru 55940 points

    Hi Alex,

    The chosen resistors fit both design recommendations for best accuracy.  The parallel requirement for accuracy is to ensure that enough current is passing through R2 and take care of the impedance concerns.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    TI__Genius 12621 points

    Thank you Ryan,

    We are concerned that anything over 100k may fail immunity tests. Is there any problem with using smaller resistors such as 24k || 1k ?

     

    Thanks!

  • 0 in reply to Alex Triano
    TI__Guru 55940 points

    Hi Alex,

    24k || 1k does not meet the guidelines for best accuracy with R1 || R2 ~ 19k.  Can you accept the less accurate design?

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    TI__Genius 12621 points

    Hi Ryan,

    Perhaps I misunderstood. Did you say that requirement, R1 || R2 = 19k is to ensure enough current is passing through R2; and so wouldn't R1 || R2 ~= 1k allow even more current to pass through?

    I understand there will be more losses with using a 24k and 1k resistor, but does that in some way hinder accuracy?

     

    Overall our concern is that a 480k resistor is quite large and may fail immunity testing. Can you explain if or why this may actually be safe for immunity testing?

     

    Thanks!

    Alex

  • 0 in reply to Alex Triano
    TI__Guru 55940 points

    Hi Alex,

    For this device topology the parallel combination must be approximately equal to 19k. We go to more detail on page 11 of the datasheet.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 10 points

    Ryan,

    If I use 1K||24K => 0.96K, I could add 10K (external at pin 2) in series with internal 8K to make the error amp see 19K.  Do you see any problems with that.

    Pat

  • 0 in reply to Patrick Corder
    TI__Guru 55940 points

    Hi Pat,

    As long as Rd + (R1 || R2) ~ 19k, where Rd is the added resistor, this set up should work.  Rd will contribute a little more noise and of course its tolerance would need to be considered for any worst case calculations.

    Another thing to note is that Rd will have a voltage drop across it which would cause the device to "see" a lower voltage than if Rd was not present.  This would cause a higher voltage on the output.  As such R1 and R2 may need to be adjusted to compensate for the extra drop in the feedback loop.

    Very Respectfully,

    Ryan