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TPS76630, Low Vin stability

Bryan Busacco
Expert 1215 points

The datasheet of the TPS76630 does not indicate what happens to the output when Vin goes below Vdo or the set Vo. Does the regulator oscillate, shut down, etc?

 

Thanks,

 

Bryan Busacco

  • 0
    TI__Mastermind 41780 points

    Hi Bryan,

    When the LDO is in dropout, the regulator tries to turn on the internal FET as hard as possible to basically the RDSON of the FET. From this, it will be a simple voltage divider Vout = Vin (RLOAD/(RLOAD+RDSON)). This article explains in great detail LDO's at dropout.

    http://focus.ti.com/download/trng/docs/seminar/Topic%209%20-%20Understanding%20LDO%20dropout.pdf

     

    Regards,

    Darwin

  • 0 in reply to Darwin Fernandez
    Expert 1215 points

     

    Also, what happens to the Iqwhen in  Vdo range and below?  If the IC is trying to "turn on the FET as hard as possible" does Iq go up?

     

     

     

    Thanks,

     

    Bryan Busacco

     

     

     

     

     

     

     

  • 0 in reply to Bryan Busacco
    TI__Mastermind 41780 points

    That is correct. Also, how much of the increase depends on the pass element. Since a BJT is current driven, it will have much more Iq at dropout than the voltage driven MOSFET. This application note decribes this Iq increase with VIN decrease and the "carrot" Iq characteristic on page 24.

    http://www.ti.com/lit/an/snva558/snva558.pdf

    Regards,

    Darwin

  • 0 in reply to Darwin Fernandez
    Expert 1215 points

     

     

    In my experience, I have seen many LDO’s work very differently when VIN drops below the programmed VOUT.  Some devices will shut down, some will oscillate, and in others VOUT will essentially follow VIN (minus the dropout of course) down to some point.  Often manufacturers will provide data showing VOUT vs VIN over a range above and below the fixed output, they also often show the quiescent current and and/or GND current in these conditions.

     

    What I would like to know is specific to the TPS76630 part.  This is the 3.0V fixed output version.  I would like to know for THIS part, what happens when VIN drops below 3.0V?  What will the output do on THIS part?  If the output follows the input (minus the dropout), how far down will it follow?  2.5V?  2.0V?  1.5V?  Could TI provide a graph?  Also, what will the quiescent current and GND current do in this condition?  How (if at all) will it change?  Again, can TI provide a graph?

     

    The data sheet is a bit confusing on what VIN min actually is.  There are a couple of statements that contradict each other, so I am not really sure how low I can go.

     

    Bryan Busacco

  • 0 in reply to Bryan Busacco
    TI__Mastermind 41780 points

    Hi Bryan,

    Unfortunately we do not have this data. However, I would expect the quiescent current to jump around an order of magnitude at drop out while the output voltage merely follows the input voltage (minus VDO) until the input voltage goes low enough that it can no longer power the device.

     

    Regards,

    Darwin