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TPS65917-Q1: ADC IN2 causing latched condition with any input above 800mV on TPS65917-Q1

Walter Borntrager46
Prodigy 50 points
Part Number: TPS65917-Q1

In a design using the TPS65917-Q1, we have found on some boards that after a shutdown, if the voltage applied to ADC IN2 on the TPS65917-Q1 exceeds 800mV, a latched condition occurs where we have problems with power down/up.

ADC IN2 is set up in hardware to read a voltage that can be present when the PMIC is down, but the software is not yet reading this ADC.

This only occurs on some boards.  On the ones where it does occur,, the effect is dependent upon the voltage applied to ADC IN2 being above 800mV and this appears to be a precise threshold.  On boards where this does not occur, the unit consistently functions normally whether the voltage at ADC IN2 is above this threshold or not.

Does applying above 800mV to ADC IN2 on the TPS65917-Q1 cause a known problem?  Is there a specific area of the PCB design that I should look for manufacturing errors?  Our software is not yet using the ADC IN2.  Is it possible that using this ADC in software could solve this problem?

  • 0
    TI__Genius 16030 points

    Hello Walter,

    I haven't heard of this issue before. What is the actual issue you are seeing - is the voltage at ADCIN2 incorrect like it's clamping, or you're having a functional issue with the PMIC itself? What is the difference in behavior when ADCIN2 is greater than 800mV compared to less than 800mV?

    Note that the max recommended input voltage for ADCIN2 is 1.25V, and absolute max is 2.5V. It can withstand these voltages, but you should make sure the PMIC is not seeing higher than 1.25V, and that it is not supplied for an extended period of time when the PMIC is unpowered. This could cause reliability issues to the device.

    Regards,
    Karl

  • 0 in reply to Karl Wallinger
    Prodigy 50 points
    Hello Karl,

    The highest voltage this application would apply to ADC IN2 would be 1.15V, so we remain below that limit.

    We do have cases, though, where 1.09V is applied (through a voltage divider - 8.0V connected to a 56.2K series resistor and 8.87K is the pull-down value, for an applied voltage of 1.09V), and this can be applied with greater than a 30 second decay time lingering after shutdown.

    On some units, if we try to wake the PMIC back up when this 8.0V source has not fallen below 6.0V (which translates to 800mV at ADC IN2 after the resistor divider), then the PMIC does not power the processor back up.

    I verified that, on these units that exhibit this problem, if I short the 8.87K divider resistor to pull this voltage to ground, the problem goes away.

    Note that this happens consistently on some units and consistently does not happen on others. This might indicate a manufacturing error on the PCB Assembly, but we have not found it yet and were hoping that someone from TI could shed some light on where to look.

    The other question is whether it is problematic for this lingering voltage (even though it is low) to be applied to this input after power down. Is it possible that, because of this, we cannot use this ADC for this function?
  • 0 in reply to Walter Borntrager46
    TI__Genius 16030 points
    Walter,

    In general, if the PMIC is not powering up, it's due to one of the following:

    1. VCC < VSYS_HI (3.1V typical, but defined by OTP)
    2. POWERHOLD is low, and no negative edge on PWRON
    3. If your OTP is configured this way, RESET_IN (GPIO_1) low or PWRDOWN (GPIO_0) low.

    VCC must be greater than VSYS_HI in order to turn on the PMIC. And either POWERHOLD=high or negative PWRON edge will turn on the PMIC. I would check those first. If your OTP has GPIO_1 configured as RESET_IN or GPIO_0 as PWRDOWN, then check to make sure those are high as well.

    Is the ADCIN2 voltage used anywhere else on the PMIC? If not, is the 8V source used used somewhere else on the PMIC - maybe through some logic or through a power supply? I'm wondering if you can determine what other signals are different when your source is above or below 6V.

    One idea is to remove your 56K resistor, and use an external supply to ACDIN2. If you set it above and below 800mV externally, independently of your 8V supply, see if that causes the startup issue as well. If the issue happens in either case, maybe it's the 8V-rail residual voltage instead of the voltage seen at ADCIN2 pin.

    Regards,
    Karl
  • 0 in reply to Karl Wallinger
    TI__Genius 16030 points
    To your last question, I don't expect any issue when using a 56K series resistor, this should be large enough to limit current into the pin to an acceptable level. If possible, just measure on your system to make sure you see less than ~100uA while the PMIC is unpowered.

    Regards,
    Karl
  • 0 in reply to Karl Wallinger
    TI__Genius 16030 points
    Hi Walter,

    I haven’t heard back from you in the last 3 weeks, I’m assuming you were able to resolve your issue. If not, just post a reply below (or create a new thread if the thread has locked due to time-out).

    Regards,
    Karl
  • 0 in reply to Karl Wallinger
    Prodigy 50 points
    I've had to lower the voltage presented to this ADC to never exceed 800mV and move on to another project, so I won't be able to resolve it at this time.