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ADS1255 with only DVDD turned on

Charles P
Prodigy 150 points
Other Parts Discussed in Thread: ADS1255

Hello.  Will there be greater than normal current or damage to the ADS1255 if I leave 3.3V on DVDD but turn off the analog section?  Thank you.

  • 0
    TI__Mastermind 44550 points

    Hi Charles,

    There is no required order to powering up the analog and digital supplies, and only having one supply powered should not be an issue. You will not see higher current from only providing one supply.

    With that said, other analog/digital device pins should NOT exceed the analog/digital supplies (respectively). Therefore, if the analog supply is off, all analog inputs and reference inputs need to be at 0V. This may require switches to disconnect the analog inputs from the device.

    In my opinion, it is safer to provide the 5V AVDD supply and operate the device in power-down mode.

    Is there a specific reason why you want to turn off the analog section?

    Best Regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 150 points

    Thanks for that, Chris.  To answer your question, yes, there is a reason.  The system is nominally powered by a 24V input, buck converter to 6V, and the 5VA and 3.3VD are on linear regulators from the 6V bus.  When USB is plugged in, the 24V may not be available, so the 6V and USB5V are diode OR'ed together at the input to the 3.3VD linear regulator to provide 3.3VD power.  It is possible to diode OR the 5VUSB to the 5VA bus (the linear regulator does block current in the reverse direction) but I'd rather not use the extra part or worry about extra USB power, especially if an unpowered USB hub, etc. is used.

  • 0 in reply to Charles P
    TI__Mastermind 44550 points

    Hi Charles,

    5V USB is a tricky power source. USB voltage is not very reliable and may only be 4 to 4.5V and source up to 500mA maximum. Also be aware that when you connect your system to a USB cable, your system ground becomes earth ground referenced. Depending on your application, you may not want to be earth ground referenced.

    A good quality powered USB hub may include a stable 5V supply that you can rely on; however, you'll still need to consider the 5V LDO's dropout voltage, which will give your system a supply <5V. If that is tolerable, then you may be fine. Otherwise, I would recommend using a boost converter to boost up your 5V USB supply to 6V. Then use a switch, or your  diode ORing circuit (with low forward voltage drop Schottky diodes) to select between the input supply.

    Best Regards,
    Chris