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MSP430F5519: Is the USB LDO protected from reverse-voltage?

Matt Bruensteiner67
Prodigy 220 points
Part Number: MSP430F5519
Other Parts Discussed in Thread: MSP-FET

It seems to be implied that it is, because the Family User's Guide says 

Some applications may be self-powered, in that the VUSB power is supplied externally. In these cases, the 3.3-V LDO would be disabled (VUSBEN = 0).

But I don't see that it's explicitly stated that there is reverse protection.

Question 1: If my design is normally bus-powered (So VUSB is connected to DVCC and AVCC), but should be powered by the programming tool during device programming, do I need to provide protection against current flowing back in to the USB LDO during programming?

Question 2: VUSBEN resets to 1. If my design is self-powered, and I'm providing an external 3.3 V to VUSB, is there a limit on how quickly I must set VUSBEN to 0 after power-up to avoid problems? (My draft design does not enable the external power source until the USB 5 V is present) Is there a typical application circuit documented for this situation?

  • 0
    TI__Guru 56685 points
    Hello Matt,

    you will need to set VUSBEN = 0 in order to allow the device to be self powered. You will need to do this when you setup your USB connection during device startup. The capacitor hanging off of VUSB should help you out in this way. If your intent is to always be self powered, then i would not plug in a USB cable before the device is setup properly (device not on or USB setup). Please see section 4.2 Selecting a Power Configuration in the following application note for more information. http://www.ti.com/lit/slaa457

    Also, for additional ideas about potential power configurations for MSP430 USB, please see the following white paper. Please note that these drawing are not comprehensive, but rather give general power configuration ideas. http://www.ti.com/lit/slay026
  • 0 in reply to Jace H
    Prodigy 220 points
    Thanks, I think that addresses my second question.

    But what about the first one? Another way I could ask it is, Is there a recommended circuit to allow programming by JTAG on a device that will be deployed as a USB Bus-powered device?
  • 0 in reply to Jace H
    Prodigy 220 points
    > If your intent is to always be self powered, then i would not plug in a USB cable before the device is setup properly

    I should point out, I don't really have control over whether my users plug in one cable first or the other.
    Though I can put a FET in so that VBUS doesn't get connected until the external power is present.
  • 0 in reply to Matt Bruensteiner67
    Guru 34323 points

    Matt Bruensteiner67 said:
    Thanks, I think that addresses my second question.

    But what about the first one? Another way I could ask it is, Is there a recommended circuit to allow programming by JTAG on a device that will be deployed as a USB Bus-powered device?

    I don't see any reason for make simple things more complicated.
    Your device is preloaded with USB BSL, and code upload can be done without extra FET.
    If using (like) MSP-FET tool is must, then FET can sense target device supply and work on same voltage level.
    If eZ-FET Lite is used than it is working on 3.3V, same as your target device.
  • 0 in reply to Matt Bruensteiner67
    TI__Guru 56685 points
    Matt,

    The FET would be a good idea to prevent a reverse of voltage before you set up the device correctly for self powered mode, independent of what cable is being plugged in first. If your scenario dictates that you have a power cable and a USB cable, then I would configure the USB module to run off the internal LDO powered by VBUS. For if VBUS is not present, you will not have a USB connection and do not need the USB module active. This way your power to the chip is separated from USB power. You can use the VBUS detection interrupt to turn the USB module on/off to save power on the device depending if you use it in USB mode or not.

    As far as programming the device via MSP-FET goes, the USB section of the chip would not be used in this case. This means you will need the recommended external capacitor, and that upon power up, the VUSB will be set for output voltage, but no voltage will be output due to the fact that VBUS is not powered.

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