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TPS54540: tps54540dda

johntse tse
Prodigy 40 points
Part Number: TPS54540

according to the spec, if the EN pin is pulled lower than 1.2V 

the switcher is turned off

i have turn off my tps54540 dda part  with the EN pin below 0.6v

but i still see at least 1.5v at the switcher output

why is that so?

i need to turn off the switcher completely

there are no other voltage on the pcb other than the incoming 12VDC and it is only going into the tps54540 input and to the EN pin via a 100k resistor

my email: johntse@asis-technologies.com

john

  • 0
    TI__Genius 11040 points
    Hi,

    Thanks for sharing the detailed information about your question. Could you help answer a couple clarifying questions?

    Could you share your TPS54540 schematic?

    What is the nominal output voltage and load current of the TPS54540?

    Is the device switching (probe SW node) after EN pin is pulled below 0.6V?

    Thank you,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    TI__Genius 11040 points
    Hello,

    I want to follow up with you on this e2e thread. Would you be able to answer a couple of clarifying questions?

    Could you share your TPS54540 schematic?

    What is the nominal output voltage and load current of the TPS54540?

    Is the device switching (probe SW node) after EN pin is pulled below 0.6V?

    Thank you,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    TI__Genius 11040 points
    Hello,

    Due to inactivity, I am going to close this thread.

    If your questions remain, please respond to this thread and it will reopen.

    Best Regards,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 40 points

    dear Katelyn

    sorry, i was not paying attention and has been deleting your email response until i realized that it is for me

    see attached schematic.

    i have put it into disable condition via a diode with < 0.6v forward voltage

    the spec says that as long as below 1.2v is enough to disable the IC

    input voltage is exactly 12v and output voltage is 3.3v

    during the test, the load current is small, about 60-100 ma, mainly by the cpu running at 120Mhz

    not sure if the device is still switching after disabled, did not have a scope with me, only an analog multimeter

    this circuit has only this input 12v source and no other source going in and only 3.3v to supply the boardPWR-test-disable.PDF

  • 0 in reply to johntse tse
    TI__Genius 11040 points
    Hello,

    Would you be able to verify that the EN is working as expected if you disconnect the diode, and turn the input voltage down to 1.2V? Then, you should see the output turn off. My expectation is that there must be leakage to the enable pin which is pulling this voltage above 1.2V. Did you also measure the EN pin voltage with the multimeter during your testing?

    Thanks,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    TI__Genius 11040 points
    Hello,

    Due to inactivity, I am going to close this thread. Please let me know if you have any remaining questions.

    Best Regards,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 40 points

    dear katelyn,

    yes, you are right,

    it turns out that some of the diode is not the correct part

    and so the voltage was around 1.2v

    and that is why some units worked and some units still have some voltages

    i did a total short to ground of the EN input and the SMPS did switched off completely

    so now, i will be doing some detective tests to determine at what voltage the SMPS will switch off completely in my circuit

    according to spec, it should switch off if voltage below 1.2v

    john