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Bus-powered TUSB2046B and Max current draw?

Brandon Brodhead
Prodigy 90 points
Other Parts Discussed in Thread: TPS2041, TUSB2046B

Hi all,

I am making some modifications to my design and would like to pattern it after the bus-powered, ganged example provided in the datasheet.  The only difference in my circuit is that I am only using 3 of the 4 downstream ports.  However, I would still like to be able to pull the maximum 100mA per port for a total of 400mA to the device.  Will using the TPS2041 as recommended in the datasheet allow me to do this, or will the hub detect that only 3 ports are used and request only 300mA?

If the second case is true, is there a way that I can "trick" the system into providing the maximum current to the device?  I have additional circuitry (mainly LEDs) on the board that is not related to any of the downstream USB devices (controller, mouse, keyboard) that I need to power as well.

Thanks,

Brandon

  • 0
    Prodigy 90 points

    Based on an answer to a similar question that I found on another site (http://electronics.stackexchange.com/questions/31347/usb-bus-powered-hub-more-than-100-ma-per-device), and the description for Figure 5 in the datasheet, it appears that the hub will automatically enumerate as a high-power device so that it can provide the maximum 100mA to each downstream device.  Using the ganged-port management method, I can provide overcurrent protection while still ensuring that all of the devices and additional circuitry on my board are getting sufficient power.  The only risk I run going this route is that something will get greedy and cause an overcurrent condition which will shut down all of the devices.  (If I design my circuit right, that should not happen.)

    Anyone out there willing to concur with my analysis?

  • 0 in reply to Brandon Brodhead
    Prodigy 90 points

    One other question - Figure 11 (Bus-powered, Ganged-Port Configuration) shows the 3.3V LDO coming directly off of the +5V USB line.  My circuit will be drawing a significant amount of current from the +3.3V supply, so would it make more sense to have it tied to the OUT line of the TPS2041? Will this cause any problems during startup with providing power to the hub?

  • 0 in reply to Brandon Brodhead
    TI__Guru* 76130 points

    Hello Brandon,

    Per USB spec, a bus-powered device can only draw up to 100mA and a self-powered device up to 500mA.

    If you are configuring the TUSB2046B as bus-powered, then it doesn't matter how many downstream devices you connect, the hub will be allowed to draw only up to 100mA.

    If you configure the hub as self-powered then it will be allowed to draw up to 500mA from VBUS.

    The downstream devices will draw up to 100mA if they are bus-power or up to 500mA if they are self-power, these devices will not draw any current from the hub, the hub has no ways to "supply current", the current is drawn from the power management device (in this case TPS2041).

    Hope this clarify something. You can send your schematics for review.

    Regards.

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

    Hi Elias,

    Thank you for your response, but I am not sure I completely follow.  If I understand correctly, you are saying that each device reports for itself how much power it requires, including the hub.  In other words, the BUSPWR pin on the hub only tells the host about how much power the hub needs for itself, not for the devices attached to it.  If that is the case, what is it that limits how much current can be drawn from VBUS?  Does the host regulate it, or is the designer of the downstream device (here 'device' would mean my circuit board with 3 USB ports) that is responsible for regulating the current draw (using a power management device like the TPS2041)?

    Thanks for your patience with my questions.

  • 0 in reply to Brandon Brodhead
    TI__Guru* 76130 points

    A USB device should be designed to be compliant with the USB Spec, that is it can't draw more than 100mA when it is bus-powered.

    At the process of enumeration, a USB Device will "tell" to the host if it is bus-powered or self-powered.

    Typically a USB Host will have means to determine if a downstream device is drawing more current and it will suspend that device in such case, a windows OS will show a pop-op message like "Power surge..." or so.

    So, if you are designing a "system" which will be connected to a USB host, you have to ensure that your system will not draw more current than the specified in the USB Spec, otherwise the USB Host will not work with your system.

    Regards.

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

    So in this case, where I have multiple devices behind a hub, the host determines how much current to provide based on the reports (self- or bus-powered) of each of the devices individually.  So the bus-powered pin on the TUSB2046B is just for the hub, and not for the devices attached to it, correct?  I think that is where my confusion has been. 

    In my design, where I have the hub and three downstream devices attached (if each device reports as bus-powered) then the hub will provide a maximum of 400mA on VBUS, which, since I am using the TPS2041, will be shared among the devices.  If this is the case, is there any way that I can cause the host to provide 500mA?

  • 0 in reply to Brandon Brodhead
    Prodigy 90 points

    Hi Elias,

    Can I send you my schematic so that you can take a look at it and let me know if there are any problem areas with how I have the TUSB2046B and TPS2041 implemented?

    Thanks,

    Brandon

  • 0 in reply to Brandon Brodhead
    TI__Guru* 76130 points

    Sure.

    You can post it here or you can send it to elias.villegas@ti.com

    Regards.