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I'm designing a device which uses an FT201X USB I2C bridge for driving a series of LED controllers. The total LED current would far exceed the few hundred milliamps that a USB port will allow me to consume.

Figure 8.1 in the datasheet shows a typical usage diagram. If I wish to use a separate switch-mode supply to power the entire device, am I correct in thinking that I should just ignore the +5VDC rail on the USB device, and instead feed the 5VDC from my switch-mode supply into the VCC of the FT201X, and connect the switch-mode supply's ground to the GND pin on the USB connector?

My concern is that, since the USB port is going to be connected to an SMPS internally on the motherboard/PSU, I'd be bridging the grounds of two switch-mode supplies. I've read that this can be problematic, causing ground loop issues.

Is this likely to be a problem? If so, is there a standard way to implement galvanic isolation of the USB data lines?

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Look at page 23 (figure 7.2) of the datasheet. This shows the wiring for a "self-powered" device. Indeed, the chip is powered from the separate power supply (not USB +5V).

The grounds must be common, though. You may want to consider using an isolated PS for your device to avoid ground loops. That would be the easiest way to avoid potential problems.

Also note the wiring from USB +5V to VBUS_sense.

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  • \$\begingroup\$ From what I interpret, the circuit shown there demonstrates how to provide an external enable signal to a dedicated power supply for the device. To quote, it says "Figure 7.3 shows an example of using a discrete P-Channel MOSFET to control the power to external logic." The design of the circuit also implies that the USB port is providing the power (CBUS3 turns the MOSFET on, allowing to current to flow from pin1 of the USB port to the rest of the circuit), rather than a secondary supply driving all circuitry. \$\endgroup\$ – Polynomial Oct 2 '16 at 22:10
  • \$\begingroup\$ @Polynomial Note my edits. Look at 7.2 not 7.3. \$\endgroup\$ – DoxyLover Oct 2 '16 at 22:11
  • \$\begingroup\$ Ah, that makes more sense now. I don't know how I missed that in the datasheet, feel a bit silly now. Should an isolated SMPS be sufficient? Does the fact that I'm using 5V-to-3.3V POL regs on the board make any difference? \$\endgroup\$ – Polynomial Oct 2 '16 at 22:13
  • \$\begingroup\$ @Polynomial Unless the POL itself is isolated(unlikely/expensive), you will have the ground connected to both the PC's earth connection and your PS's earth connection, allowing for a ground loop. Easiest way to get isolation is probably in your PS. \$\endgroup\$ – DoxyLover Oct 2 '16 at 22:17

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