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MSP430F5508 USB Short Circuit behavior

Other Parts Discussed in Thread: MSP430F5508

Hello, 

As commented on THIS post i got a product that currently is isolated with a push pull transformer, but i am planning to change my design. I am planing to use MSP430F5508 and expose USB pins with GND and a PIN used for charging a battery (and also power the board while the battery is being charged). I  considering using tpd4s014 to protect  USB and VCC lines. I got a question. Is there any problem if USB lines get short circuited to GND, with each other, or with a line that power a battery charger? That probably is going to happen daily and continuously as the pads are going to be exposed and used during shower bath and inside swiming pool. The circuit i am planning to use is the same used at MSP-EXP430F5529 as it already proved to be reliable on other projects.

What else can i add to my circuit to protect USB lines?

Regards,

LR

  • In addition to my quetion. Considering the described situation, is there any problem on not powering VUSB when USB is not used? And is there any problem if MSP430 is powered by 1.8V and VUSB by 3.3V?

    Regards,

    LR

  • Protection IC you mention seems to be good choice for your app. VBUS disconnecting FET is very useful either. According to datasheet your uC USB data pins become i/o pins so you can control their state and voltage. Before asking further question, please read whole USB peripheral chapter of 6xx series user manual. There's "40.2.2 USB Power System" which possibly answer your questions about power too.

  • Thank you for your help again,

    But no, Port U is powered by VUSB pin, so if i am not powering it as commented on my second POST the pins are not operational, so i would be able to control it. The answer about short circuit i cold find at the User Manual after creating this post. Thank you for pointing me that. 

    One question that i got i could not find  either on Manual or Datasheet. During programming phase of my project there was a 25uA current that was being consumed more than i was expecting. After a few tests the USB LDO proved to be the problem so i wont be able to use it while the system is not being charged. In current situation my system is powered by 3V LDO. What is the minimum voltage the USB PHY would need to operate? I could either leave VUSB powered off when not used or to create a switching circuit that changes the USB power source to LDO when the USB is attached.

    Regards,

    LR

  • Manual says: "Alternatively, the [USB LDO] power system can supply power only to the USB module". When not attached, USB does not need power, when attached - it does get power by own LDO regulator. Where's problem?

  • I think i was not enough clear. LDO itself consumes 25uA from MSP430 VCC even if not powered. This and this thread talks about that problem. I noticed the same problem with my product, So when battery is not charging i cant use internal LDO so i will have to either power pins from an external LDO or leave it floating without any power. I am not sure how safe is the last option. I can use my own LDO that is 3.0V (TPS78230 for instance) right now (i was considering using a lower voltage but keeping it is not a big deal)to power VUSB pin while there is no external supply. I was considering using my own LDO to power USB when it gets connected but think the most reliable option is to switch to internal LDO when it get powered (using a diode) as it will limit the current and the system wont get unstable even if the USB cable is shorted. What do you think about that idea?

    Thank you for all the help. I really appreciate your patience.

    Regards,

    LR

  • When USB cable is shortened then this is the same as USB is not connected. You just need to make USB VBUS voltage monitor (using pin change interrupt and voltage measurement using ADC sampling) to detect when to switch on USB LDO and enable peripheral. Or still I don't get it?

  • Yes you got it. I was just commenting that even when LDO is not ON i must supply power to VUSB pin in order to use that PORT as standard IO as explained at the User Manual section 40.2.1.3. But i think i just realized the solution powering it with my external LDO and using a diode to switch between both power sources.

    Do you see any pitfall on that idea?

    Regards,

    LR

  • Luis Filipe Rossi said:
    in order to use that PORT as standard IO

    Check what happens when you don't. If USB peripheral have no power, then IMHO this is safe to leave pins floating. I just use logic here because to be honest - have no experience with particular peripheral.

  • It is not simple to check. Right now i got a board with the pins non powered .. can't figure out what sort of tests i can make. Actually they are not easy to test as i left no Test point over there...

    In general they would be in a high impedance state i belive... probably the same behavior as if PUOPE and PUIPE are cleared, but some TI guy could help us here with this doubt.

    Regards,

    LR

  • Don't you have any high impedance voltmeter around?

  • Sorry think lost your message announce on my e-mail. 

    Did not test yet, probably i will open an ticket asking TI guys either. 

    Thank you!

    Regards,

    LR

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