JTAG-SMT3-NC powered from USB

[jmcexx]

I am considering powering the VDD input to the JTAG-SMT3-NC module from the USB supply instead of the device's power supply.  Of course this will require a 3.3V LDO to bring VDD down.  Do you see any issue with doing that?

In my case, I would have:

  VDD - 3.3V from LDO from USB V+ (when cable plugged in)

  VREF_UART = 3.3V from system power supply (on when board powered up)

  VREF_JTAG = 1.8V from system power supply (on when board powered up)

 

My intention is to allow the UART connection over USB to stay connected even when power cycling the board.  I ask because all of the documents show VDD being supplied from the system, not USB.

Thanks for any thoughts!

 

 

[JColvin]

Hi @jmcexx,

I asked one of our design engineers about this and they let me know that from a hardware perspective that this would work.

However, they also let me know that the USB configuration probably requests 0 mA of current and with the configuration the you are describing the current draw from VBUS will be non-zero so there's a small chance the PC will complain but they imagine it will work in 95% of the cases.

Thanks,
JColvin