ADM7170, ADM7171, ADM7172 soft start

Is the soft start cap used to bypass the reference?

Is there a max SS cap I can use?

The description says also that SS needs to be left open when not used, but could the SS pin be used also as a REF in or Track function to lower the output voltage? Is there a range of voltages that this can happen?

FormerMember Can you advise?

Thanks,

LucaV

  • 0
    •  Analog Employees 
    on Aug 12, 2014 9:11 PM

    Luca,

    The SS pin was never intended to work this way so we'd have to test it in the lab to see what happens.  Here's a brief explanation of how the SS pin works...

    The SS cap does not bypass the reference.  Instead the SS pin is a current source that charges an external capacitor and the voltage generated limits the output voltage of the LDO.

    To achieve the low output noise, the internal reference is heavily filtered (the pole is well below 1 Hz) which means during start up the filter must be disabled in order to keep the start up time short.  The filter is disabled once the voltage on the SS pin reaches about 0.8V.  In order for a tracking function to work, the voltage on the SS pin must be below 0.8V otherwise the internal reference will take over, also the response would be very non-linear.  Additionally, if the SS pin voltage is kept below 0.8V, the output noise would be determined by the noise of the voltage on the SS pin.