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Original question:

TPS54340B: Introduce start up delay by using Cap strap at EN

TPS54340: VOUT Glitches - Follows glitch on undriven EN pin

Mari Ozolins
Prodigy 20 points
Part Number: TPS54340

I am using the TPS54340 in a design providing 5V @ 0.3A (MAX) with a VIN nominal of 24V. It is an automotive application. 
I have used WebBench PowerDesigner to select the correct components for this design. 

A PIC micro running off this rail and it is intolerant of glitches on its 5V supply rail (It RESETs)

I have an event (Touchscreen contract)  in my system that is causing the 5V supply to glitch. I don't know the causative path
(ground bounce?) but I do see that the EN pin will drop below 1.2V and thereby cut off VOUT momentarily.
 
I have used the UVLO feature and also left the pin floating. In the floating state I still see the glitch 
on the EN pin drop below 1.2V. If I simply touch the pin, or even adjacent pins, I can cause the same event
to occur. I have a second supply based on the same TPS54340 in the system- it does not have this same 
effect, EG: Touch the pins, Kills VOUT.






  • 0
    TI__Mastermind 27470 points

    HI:

    There is a weak pull up current source charging the EN to high so that DCDC can turn on. when you touch the IC, it like suddenly connect a cap to EN and GROUND. if the charing current larger than internal pull up current, DCDC will shutdown.

    can you try use small divide resistor at UVLO pin, see any help?

    THanks

    Daniel Li

  • 0 in reply to Daniel Li14
    Prodigy 20 points

    Hi Daniel,
    Thank you for your reply. 

    There is a resistor divider on the EN pin using the UVLO feature. It does not help. 
    My best solution is to pull-up the EN pin to +12V through a 100K resistor. 

    It is curious because I have another TPS54340 off the same supply rail and it does not
    have this same issue.

    If I touch other pins on the regulator I can also cause the Vout to glitch. It just seems very
    sensitive. There is some event in my system that is causing this regulator to glitch it's output
    and I'm suspecting the EN pin. Is there a better treatment of tis pin to eliminate this possibility?

    Thank you!


  • 0 in reply to Mari Ozolins
    TI__Mastermind 27470 points

    HI

    You can compare with the good TPS54340 with the one has issue, see any component or layout different, especially whether the output choke and capacitor are same as well as its compensation network. from en, you can try use small value like 10K or 1K, see whether you have improvement. 

    One more thing is you can check the layout, whether you separate AGND and PGND , tied them at single point.

    Thanks