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THS4524: THS4524 powered down and /PD

Dan Solvin41
Prodigy 95 points
Part Number: THS4524
Other Parts Discussed in Thread: ADS1278

Hi,

If the THS4524 is unpowered, what is the maximum safe voltage that can be applied to it's /PD pins?   I'd like to be

able to allow 3.3V, but am worried that could damage the part.   Not sure if this pin has protection diodes and how they would react.    I

am able to limit current to well below 1mA (likely below 0.1mA) as the voltage on this pin is governed by a weak pull-up resistor to 3.3V.

Thanks, Dan

  • 0
    TI__Expert 8540 points
    Hi Dan,

    If you apply a voltage on the pin directly when the device is powered off, it will turn on internal ESD protection diodes that will draw current. I would suggest adding a series current limiting resistor on the pin of about 5k Ohms. the device only draws 1uA of bias current into the pin under normal operation, so it should only add a few mV of drop to the pin during operation.

    Regards,
  • 0 in reply to Jacob Freet
    Prodigy 95 points
    Jacob,

    Thanks for that. It would be impossible to add a series resistor without a board spin. Currently, there is a 100K pull-up resistor
    to 3.3V on the /PD pins. It appears that when the buffer is unpowered, it's pulling enough current to cause the voltage to be
    around 1.22V. I need this to be at least 2V for other circuitry on the board to behave. This signal is also tied to an ADS1278 with only 1.8V and 3.3V energized (5V not yet active).

    I can easily lower the pull-up resistor to 50K or less to force the voltage higher, but want to make sure that getting closer to 3.3V
    won't hurt the unpowered buffer. What is not clear about the ESD diodes is whether they'll pull even more current when
    the voltage rises. I would assume these diodes would be damaged not by voltage, but by excessive current, but don't know
    what that limit is.

    Dan
  • 0 in reply to Dan Solvin41
    TI__Expert 8540 points
    Hi Dan,

    Yes, I would expect that the ESD diodes will draw more current if you lower the resistor value. You may be able to get it to work by playing around with different resistor values and trying to see if it works, but I cannot guarantee the reliability of the devices in that scenario. Our ESD structures are designed to handle high voltage ESD events and are not characterized for sustained current.

    If you cannot add an series resistor to the power-down input on the device, the best option would likely be to power on the device and keep it in powered down using the pin.

    Regards,
    Jacob
  • 0 in reply to Jacob Freet
    Prodigy 95 points
    I tried adding additional pull-up resistors to 3.3V (already has a 100K), first a 51K, then another 51K in parallel. The

    voltage never got above about 1.50V. It seemed clear that the harder I tried to pull it up, the ESD diode in the unpowered

    THS4524 tried to pull it down. So, I gave up this battle as pulling up harder might have damaged the buffer. The

    solution I took was to just lift the /PD pins of the buffer off their pads, removing them from the circuit. Luckily an open /PD

    is still considered 'active' by this part. This worked, though I lost the ability to power down the buffers.


    Dan