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Getting 900mA for USB 2.0 using TUSB8020

Al Clark
Prodigy 90 points

A common problem with USB is the 500mA current limit in 2.0 for bus power applications.  This is a very common problem with high performance audio devices since high quality data converters tend to require high power. Adding DSPs makes the problem even more difficult. 

The USB 900mA limit for 3.0 is very helpful but unfortunately not available when operating in high speed (480Mb/s). 

I was thinking that there might be a legitimate trick. What if we use an internal two port USB 3.0 hub such as a TI TUSB8020 and use 1 port as a self powered 2.0 interface. The remaining port could be used as a 3.0 charging port or some kind of USB 3.0 power supply circuit. This would in essence, function as the device's  power supply. This would make the composite a 3.0 device (hub) that would draw no more than 900mA. 

Has anyone explored this idea and willing to provide a schematic?

Would this arrangement technically meet USB requirements and if not, would it work reliably regardless of the logo certification.

  • 0
    TI__Guru* 76130 points

    Hello,

    Does your USB device supports battery charging? 

    If the USB logo is not a requirement, you could use a power switch to provide current to your device and simply don't connect the overcurrent signals to the hub.

    Regards.

  • 0 in reply to Elias Villegas M.
    Prodigy 90 points

    My current device can work at 100mA before enumeration and then controls a FET switch for maximum power. My concern is that since I am not operating in superspeed that the host may not allow me to pull 900mA. 

    The idea of the 2 port hub is to fool the host into treating my complete device as a 3.0 composite and therefore allocate 900mA reliably.

    I might use a USB 3.0 B connector (and a 3.0 cable) so that the cable IR drops are lower.  

    I understand that CDP is not a logo compliant option since I don't have a battery. I can probably live with that.

    In theory, USB 2.0 devices cannot get 900mA from a 3.0 port even though the port clearly has the capability.

    I think a compliant method would be to have a simple USB 3.0 peripheral on one port and self power my USB 2.0 port. The 3.0 would just enumerate , ask for 900mA and then switch on the power supply. I haven't found a simple USB 3.0 part that can do this.  A perfect part would be a single upstream port and downstream port with the USB 3.0 enumeration inside.  

    From what I can see, the USB 3.0 committee guys thought that the only people who needed more power were disk drive people who obviously needed more speed as well.  I can tell you that anyone who builds bus powered instruments or high quality audio products have this problem.

    Thanks

    Al

  • 0 in reply to Al Clark
    TI__Guru 60870 points

    Hi Al,

    Your idea of reporting as a USB 3.0 composite device would work, but you would need to use a USB 3.0 hub that supports bus-powered operation (the current TI hubs do not) and accommodate the power consumption of the hub into your power budget.   Even if your device reports as a USB 3.0 composite device it is required to consume only 500 mA if connected to a USB 2.0 host.

    Regards,

    JMMN