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LT8609A regulation when Vin < Vout

Dear ADI Engineering Support,

I'm planning to use the LT8609A in a battery application and have a simple question. What happens to the regulator when the input battery discharges close or below the programmed output voltage? Does the chip keeps regulation in this case as well, and so the output voltage follows the input supply with a minimum dropout? Thanks for your feedback in advance.

Best Regards:

Tom Ledeczi 

  • Hi Tom,

    I don't think it can do 100% duty because BST cap need to be refreshed to drive top FET. Please check this article that features LTC3639.  Also, I'm sure that ADI has several 100% duty capable buck converters.

  • Hi Fil,

    First of all thanks for taking your time and address my question. I know that the LT8609S ( I switched from A to S) doesn't support 100% duty cycle, but according to the datasheet:

    „The LT8609S is capable of maximum duty cycle approaching 100% and the Vin to Vout droput is limited by the Rds(on) of the top switch. In this mode the LT8609S skips switch cycles, resulting in a lower frequency than programmed by Rt.”

    The frequency foldback feature is also mentioned when saying "The oscillator reduces the LT8609S' operating frequency when the voltage at the FB pin is low".

    In my battery application it is valid scenario that the battery voltage discharges below the programmed Vout. In this case I need Vout to drop and track Vin with a certain dropout. Based on the above the LT8609S satisfies this requirement and keeps regulation when Vin falls below the programmed Vout (but Vin still above the UVLO threshold). I didn't use a buck converter in this way before and just would like to clarify if this is a valid operating scenario of the LT8609S.

    Thanks again:


  • Hi Tom,

    The Vin to Vout dropout is best seen in the two figures of page 10 in datasheet. I believe this chip can support your requirement. It it not fully 100% duty operation but very close. The separation between the two will depend on the load at the output. You should be able to check this performance too in LTspice.