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TUSB2077A - supply one unit load per port - battery self powered hub

MJ16032
Prodigy 240 points
Other Parts Discussed in Thread: TUSB2077A, TPS2044

Hi There,

I am designing a  7 port usb hub that will be used in a battery-self powered configuration.  In reading the documentation for the TUSB2077A, it states that in self powered mode that 500mA or 5 unit loads must be supplied to each of the downstream ports.  However, In the USB specification under section 7.2.1it states that "Battery-powered hubs may supply either one or five unit loads per port.".  

I would like to complete my design with by only being required to supply one unit load (100mA) to each of the downstream ports.  I am wondering if I will run into complication using this chip to do this?

A couple of further questions:

1- how can battery vs non-battery power be differentiated from in the USB spec?  I see no mention of this?

2- If a bus powered - high power device is connected to one of the hub ports, how will the transaction and functionality be handled by the TUSB2077A?  

3- Will this cause complications in the power budgeting from host to device?

 

TIA

 

  • 0
    TI__Guru* 76130 points

    Hello,

    The operation of the TUSB2077 is ruled by the USB Specification.

    When the hub is configured as a self-powered, it draws its current from an external supply source, in this way, the hub is able to supply 500mA to each of the downstream ports.

    When the hub is configured as bus-powered mode, it draws 500mA from the upstream port and can only provide 100mA to 4 downstream ports, also, the devices attached to the downstream ports cannot be high-power devices (because they will be drawing more than 100mA).

    There are not requirements for "current supply" but for "current draw" in the USB spec.

    So if you want the downstream ports only be allowed to "draw" 100mA, then you must configure the TUSB2077 as bus-powered mode.

     

    Regards.

  • 0 in reply to Elias Villegas M.
    Prodigy 240 points

     

    ELIAS VILLEGAS said:
    The operation of the TUSB2077 is ruled by the USB Specification.

    When the hub is configured as a self-powered, it draws its current from an external supply source, in this way, the hub is able to supply 500mA to each of the downstream ports.

    From USB Spec section 7.2.1:

    Self-powered hubs:  Power for the internal functions and downstream facing ports does not come from

    VBUS.  However, the USB interface of the hub may draw up to one unit load from VBUS on its upstream

    facing port to allow the interface to function when the remainder of the hub is powered down.  Hubs that

    obtain operating power externally (from the USB) must supply five unit loads to each port.  Battery-

    powered hubs may supply either one or five unit loads per port. 

     

    Based on the above USB spec I am not requiredd to supply 500mA to each port where I am using battery power to self power the hub. I am going to use this design in a Battery powered application.  In my case I can only  power 1 port with 500mA and other other 6 ports with 100mA, 1100 mA total.  Based on the above stated USB spec, this should be ok.  But, I am wondering how the TUSB2077A will handle this.  

    Example:  What if 3 high power devices are trying to draw 500mA each and there are 4 other ports @ 100mA each? This would require 1900mA for all of the devices to operate, but there is only 1100 mA available between all of the ports.  Can the TUSB2077A handle this power budgeting negotiation an let the 2nd and 3rd high power devices know that there is not enough current for all of them to run ( I don't think so)?  What happens when there is not enough current for all of them to run?  Am I required to state to the user that only 1 high power device can be handled at a time?  

     

     

     

  • 0 in reply to MJ16032
    TI__Guru* 76130 points

    The hub only "speaks" to one device at a time, so technically there is only one device active.

    You can configure the hub to allow the downstream devices to only draw 100mA(bus-powered mode) or to draw up to 500mA(self-powered mode).

    You can't only "supply" certain amount of current, the downstream device is the one that determines how much current it needs.

    What you can do, is to limit the maximum current that a device is allowed to draw from the hub (using the bus/self power feature)

    TI recommend the use of power management devices such the TPS2044.

     

    Regards.

  • 0 in reply to Elias Villegas M.
    Prodigy 240 points

     

    You can't only "supply" certain amount of current, the downstream device is the one that determines how much current it needs.

    Base on the USB specification that I listed in my previous quote, you can either supply one unit load or two unit loads.  As stated:

     Battery-powered hubs may supply either one or five unit loads per port. 

    My point is that I want to use all 7 ports @ 100mA.  The solution of using the "bus/self power feature" does not help with this.  If I configure the device as hub powered, only 4 ports can be powered from the bus, right?  

    Anyhow I think I may just use the 4 port device (TUSB2046B) as I am not seeing how to solve this problem using the 7 port device.