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LTC4089 automatic switchover from USB to wall adapter using FTDI FT231XS


I am working on the design of device that can be charged either from USB, a 5V wall adapter or a 24V input and selected the LTC4089 to do the job. In addition, the device features a FT231XS USB to serial converter. The FT231XS has a few configurable outputs. In particular, BCD and SLEEP signals are of interest to my application.

  • nBCD is an open drain output shorted to ground when a 5V wall adapter (as opposed to 500 mA limited USB port) is detected.
  • nSLEEP is an open drain output internally pulled high to 3.3V by the FT231XS and shorted to ground when in suspend mode.

I would like to use these signals to increase the battery charge current when a wall adapter is detected and disable the charger when in suspend mode. The image below shows the relevant section of the (preliminary) design. the goal is to set the current limit for USB to somewhat below 500 mA (476 mA in this case) and for a wall adapter to somewhat below 900mA (876 mA in this case). Perhaps someone can take a quick look at the schematic and let me know if the design works as expected or if I am violating any current constraints of the LTC4089.

thanks in advance,



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