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OMAP-L138 in power off while USB0_VBUS is present

M.A.M
Expert 8805 points
Other Parts Discussed in Thread: OMAPL138

Hi,

Q1:   What is Texas Instruments design recommendations on the OMAPL138 for USB0_VBUS connected to a host (PC), when the OMAPL138 is powered off?

Q2:   What is Texas Instruments design recommendations on the OMAPL138 for USB0_DM, USB0_DP, connected to a host (PC), when the OMAPL138 is powered off?

If the OMAP USB0_BUS can NOT be connected to a HOST while powered off,  why is this not listed or described in the datasheets.
Please clarify in detail.

 

Background:
One can measure a voltage around 350mV on the DVDD3318/ USB0_VDDA33 power rails, when VBUS is +5V and present.
This thread indicate that the use-case is unsupported:   http://e2e.ti.com/support/dsp/omap_applications_processors/f/42/t/96557.aspx

/Magnus

  • 0
    TI__Guru* 84075 points

    Hi Magnus

    Sorry for the extreme delay in responding to this query. We discussed this use-case internally with the apps team and the device/USB design team, following is the recommendations

    Magnus Aman said:

    Q1:   What is Texas Instruments design recommendations on the OMAPL138 for USB0_VBUS connected to a host (PC), when the OMAPL138 is powered off?

    Essentially the USB OTG module for this design was designed for the use case where the device will be either enabled (powered up) or power applied to on VBUS attachment/application. However, for the case when VBUS being attached to a device that is powered-off, and can  remain  powered-off for an indefinite time, there is a risk of long-term fatigue on the device’s internal circuitry. Therefore previously this was indicated as an un supported use-case.

    FYI the 350 mV that you see on the 3.3V supply is due to the fact that inside the USB PHY there is a path from VBUS to 3.3V supply and this path will turn on if the 3.3V supply is not applied/on.

    TI therefore recommends customers employing this use-case to isolate the input USB0_VBUS from the VBUS during device being in power-off state until the device eventually is powered up, by implementing a discrete load switch such as the http://www.ti.com/product/tps22913c

    Note it is possible that a less then 1K resistor could have also worked , but given that some customers are sensitive to the stray 350mV on the 3.3V supply impacting other components on the board, a discrete load switch is more ideal.

    Magnus Aman said:
    Q2:   What is Texas Instruments design recommendations on the OMAPL138 for USB0_DM, USB0_DP, connected to a host (PC), when the OMAPL138 is powered off?

    You shouldn't need anything additional on the DP/DM pins , no known special requirement for these lines.

    Magnus Aman said:
    If the OMAP USB0_BUS can NOT be connected to a HOST while powered off,  why is this not listed or described in the datasheets.
    Please clarify in detail.

    I will document this in a hardware design wiki (essentially what I captured in the answer to Q1) for the device, and eventually see where in the datasheet/TRM collateral this might make sense to add/append.

    Hope this helps.

    Regards

    Mukul