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BF547 USB - Powering VDD_USB to ensure USB_VBUS input is safe

Question asked by draeman on Jul 30, 2014
Latest reply on Apr 20, 2017 by Usaghi2nd



I am working on a custom platform that uses a BF547 processor to support operation as a USB device.  This platform is not bus powered, but of course VBUS is brought into the USB_VBUS pin of the Blackfin and is used for some USB OTG functionality.  The Blackfin receives power from a dedicated power supply that can be switched off.  This supply provides both VDD_EXT and VDD_USB, which are tied together in the design.


If the platform is plugged into a USB host that is providing bus power, but the platform's power supply is switched off, then it will be in a state where the Blackfin's USB_VBUS pin is energized and the VDD_USB pin is not energized.  Can you please confirm this scenario violates the data sheet?


Looking at the ADSP-BF547 data sheet, Rev D, page 33.. Vihusb shows a maximum operating voltage of 5.25V and no minimum.  However, the associated footnote says to refer to page 39 specifically for the case of the USB_VBUS pin.  The footnote on page 39 says:


Pins USB_DP, USB_DM, and USB_VBUS are 5 V-tolerant when VDDUSB is powered according to the operating conditions table. If VDDUSB supply voltage does not meet the specification in the operating conditions table, these pins could suffer long-term damage when driven to +5 V. If this condition is seen in the application, it can be corrected with additional circuitry to use the external host to power only the VDDUSB pins. Contact factory for application detail and reliability information.


So I take this to mean that the design as described above is bad.  I need to separate the VDD_EXT and VDD_USB power domains.   Then I need to regulate USB_VBUS down to 3.3V, and diode-or that together with VDD_EXT to generate VDD_USB.  This way VDD_USB can be energized via either the normal power supply, or by the host-provided bus power.  This is necessary to ensure compliance if the platform is connected to a host while the Blackfin is unpowered.  Do I have this correct?


Thank you.