• State
  • Locked Locked
  • Replies 8 replies
  • Subscribers 63 subscribers
  • Views 555 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS65217Cnon-portable application question - Fail safe power application using both AC and BAT pin

PSB
Prodigy 220 points
Other Parts Discussed in Thread: TPS65217, AM3352

Hi,

We are designing with the TPS65217C and the Sitara AM3352. Our system has two power sources coming from different supplies, Primary +5V and Secondary +5V. In reading the datasheet for the TPS65217 (Revised January 2015) it is not clear to me if I can use this device for the following use case.

Run the primary +5V to the AC input pin and run the secondary (Backup) +5V to the BAT pin. We would need the system power to be applied if EITHER OR BOTH of the +5V souces (Primary or secondary) is on. 

Our voltage outputs use case will be default as described in the device comparison table on page three of the datasheet for the TPS65217C device.

My concern comes  from the note on page 76 (Table 36 use case 3  for "Output Rail Status on initial power connection") of the TPS65217 datasheet  statement that LDO1 is off when BAT is connected to 5V supply and that PB_IN must be pulled low to enter the [WAIT PWR_EN] state.  This might be a problem for us if the primary power is off, and we try to apply power via the backup slave unit on the BAT pin.  We have no power switch, and the system must come up autonomously when plugged in.  Is there a solution where we can cold boot from either power source without pushing a button using the TPS65217C?

Will appreciate any feedback and help on this. Thanks in advance. 

  • 0
    TI__Expert 3740 points

    PSB,

    You could use the USB pin, as it powers up the device automatically if power is applied to the pin.

    Janice

  • 0 in reply to Janice Escobar
    Prodigy 220 points
    Hi Janice,

    Thanks for your reply. Reading the section 9.3.9.1 on page 27 "Shorted or Absent Battery (VBAT < 1.5 V)", it does not look like we can use both AC and USB inputs if battery is absent. Please see the following comments within the dashed lines directly from the datasheet. Any comments?
    --------------------------
    The linear charger periodically applies a 10-mA current source to the BAT pin to check for the presence of a
    battery. This will cause the BAT terminal to float up to > 3 V which may interfere with AC removal detection and
    the ability to switch from AC to USB input. For this reason, it is not recommended to use both AC and USB
    inputs when the battery is absent.
    -----------------------------

    Thanks,
    PSB
  • 0 in reply to PSB
    TI__Expert 3740 points
    PSB,

    Yes, this is our recommendation, but there is no other way besides supplying from AC or USB to start the device automatically. The BAT pin needs PB_IN to be asserted to start the device; unless, it is already on.

    Janice
  • 0 in reply to Janice Escobar
    Prodigy 220 points
    Thank you Janice. So since there is no other way to do this what would be the implication to the device for violating the recommendation. Has TI done any testing on the use case of using the AC and the USB inputs at the same time? what are the % chance that the device will fail to switch from AC to USB in this mode and what would be the recovery process from this condition?

    Thanks.
  • 0 in reply to PSB
    TI__Expert 3740 points
    PSB,

    There hasn't been any testing done with results that I know, the issue was just documented to make designers aware. I've seen other customers use this approach like BBB. This is a very widely used reference design, would check on the BBB forums to see if anyone has had issues, but I have not seen any. One thing I would caution though is that if there is no back up power (no BATT) the device is subject to a brownout condition. You always could short the batt pin to one of the inputs so there is always backup power.

    Janice
  • 0 in reply to Janice Escobar
    Prodigy 220 points

    Hi Janice,

    We are considering the USB input but are concerned about the 4.3V minimum input voltage.  The backup supply in the system is remotely located, so there is a possibility of the 5V backup power being more like 4.0V.  So what happens if the USB power input goes below 4.3V?  Does the PMIC shut down?

    Thanks,

    PSB 

  • 0 in reply to PSB
    Prodigy 220 points
    Hi Janice,

    I more question. If we tie the backup power to USB input, then can we ground the BAT input? This should take care of the warning note (datasheet para 9.3.9.1) for the BAT input from the BAT charger charge circuit. However if grounding is not allowed for the BAT input, can we simply switch off the charger? Wouldn’t that prevent the charger from periodically polling the BAT pin?

    Any feedback?

    Thanks
    PSB
  • 0 in reply to PSB
    TI__Expert 3740 points
    PSB,

    I would just turn off the charger on boot up, you can do that via I2C. That will disable the pulse.

    Janice