Controlling USB Bus Voltage with the FT230X

For USB powered devices, controlling the power consumption is important. As documented in the USB2.0 specification, the maximum power draw possible for normal devices is 500mA at 5V. This level of current draw is only possible after enumeration with the host: before enumeration every device must be a low-power USB device and only draw a maximum of 100mA at 5V. The easiest way to handle this is to have the rest of the device off until enumeration can happen then turn the rest of the device on. The FT230X supports controlling a P-Channel mosfet for high side power control.

FT230X Bus Control Schematic

Before enumeration, the FT230X and the 2 powered LEDs will be pulling 26mA which is well under the limit of 100mA. After enumeration the PWREN signal gets pulled low opening up the P-Channel mosfet allowing power to flow to the rest of the system. Care should be taken to limit the system current to under the 500mA limit. A resettable PTC fuse with a rating of 500mA can be placed in series with the L1 inductor to limit the maximum current of the system.

The FT230X will require flashing via FT_Prog which I covered in a previous post. CBUS1 set to TX&RXLED#, CBUS3 set to PWREN#, and “Force Power Enable” setting needs to be enabled.

FT230X Bus Control Layout