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TPS82084: Output discharge function

Alan Hoehn
Intellectual 460 points
Part Number: TPS82084
Other Parts Discussed in Thread: LM3880

I am using two of the TPS82084's to power two different rails on a part.  The rails are 1.8V and 0.9V.  I have a requirement where I need to power up the 0.9V first and then the 1.8V.  However on power down it is the opposite of 1.8V first and then 0.9V.   How do I use the output discharge feature of the TPS82084 to have the 1.8V rail discharge before the 0.9V?   The 0.9V rail is drawing an average of 1A of current the and 1.8V is drawing and average of 26mA.  Thanks.

  • 0
    TI__Guru*** 149360 points

    Hi Alan,

    Would you consider using a dedicated sequencing IC to handle this reverse sequencing for you?

    If not and if you have a spare GPIO pin, an MCU can help.  One idea would be to diode OR the 0.9V's PG output and the MCU output together into the 1.8V rail's EN input.  For startup, the MCU would enable the 0.9V rail and enable its GPIO output.  The 0.9V rail's PG would keep the 1.8V rail off until the 0.9V is ready.  For power down, the MCU would first disable the 1.8V rail.  After waiting sufficiently long for 1.8V to decay to an appropriately low voltage level, from I = C dv/dt, the MCU would turn off the 0.9V rail as well.

  • 0 in reply to Chris Glaser
    Intellectual 460 points

    Hi Chris,

    Actually right now I am using the LM3880 for sequencing but it is not fast enough on an uncontrolled power.  I like your idea about using the logic gates and the PG good pin. Not sure if using an MCU pin is going to be adequate in an uncontrolled power down.  Perhaps I could use a voltage supervisor that monitors the main power and then AND that with the PG good of the 0.9V into the enable of the 1.8V.  Do you think that is good way to satisfy the power up and power down requirement?  

  • 0 in reply to Alan Hoehn
    TI__Guru*** 149360 points

    Thank you very much for explaining your use case--a removal of the input voltage.  I had wrongly assumed that you were asking about a 'normal' shutdown situation.

    Yes, an SVS on Vin combined with the 0.9V PG pin is a solution.  The challenge is then to keep the 0.9V rail up and running until the 1.8V rail is discharged.  You need to add sufficient input capacitance to supply the system's power during shutdown.  For the delta V on the input, you can take the voltage at which the SVS will trip down to the UVLO level of the TPS82084 (2.3V worst case).  This is the amount of stored energy which should supply the system for as long as it takes the 1.8V rail to discharge.

  • 0 in reply to Chris Glaser
    Intellectual 460 points

    Your suggestions gives me a path to a solution.  Thanks.