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TPS3619: TPS3619 System Erratic behaviour during switchover

Omar Sultan1
Prodigy 40 points
Part Number: TPS3619

Hello all, I've been trying to make the system ( microcontroller mainly )  after a switchover from Main Vdd to Vbatt work seamlessly. As soon as the vdd is off and the Vbatt is the source of supply, the microcontroller acts up like there is a major noise issue with the system ie: unstable . What made the system work but out of pure luck without any logic explanation the system now works on Vbatt normally as soon as i put Vbat shorted to /RESET pin , i have an explanation behind this phenomena. Also after this modification , the PFO when Vdd is present is 3.3V (normal) , when vdd is gone and Vbatt is present the switchover is done no problem and the MCU works wihtout the erratic behaviour but the PFO is 2.4V which is supposed to be around 0V.

Note : before doing this action of shorting the /RST to Vbatt to fix the issue I may have applied 12V on the Vbatt rail for a few seconds  by accident so this may be the issue of PFO, i'll order more to verify the issue.

My system information: 

1. Vdd : 3.3V is a 3.3V regulator which is supplied by a 12V AC/DC PSU, so ina case of power outage this is gone.

2. Vbat  3.6V ( Lisocl2 battery ).

3. PFI : connected to 12V ( voltage divider configured to make threshold at 9.65V)

 

If you need any more information , kindly let me know.

Regards

Omar Sultan

  • 0
    TI__Mastermind 39575 points

    Hi Omar,

    Applying 12V to the VBAT pin could have damaged some internal circuitry and cause what you are seeing from the PFO output.

    Just to remind you, when the TPS3619 switches from VDD to VBAT rail, VBAT to Vout will have a 15ohm switch which may be the cause of your noise and instability issues.  VDD to Vout has only a 1ohm (max) switch.  Depending on your current consumption from the microprocessor, the voltage drop across the 15ohm switch maybe significant where the microprocessor is not receiving the correct operating voltage.  See below:

      

    Ben

  • 0 in reply to Ben C
    Prodigy 40 points

    Hello Ben,

    Thanks for sharing your thoughts on the subject. I have found the culprit and now system works without shorting the /RST pin . 

    Is it normal to have a leakage on to the Vdd pin , i'm seeing 1.4V(at Vdd) when Vbatt is chosen (3.3V) and selected to Vout ( having in mind that Vdd is completely off ) . Also would you recommend an alternative part which has a lower on resistance between Vbatt and Vout ?

    Regards

    OS

  • 0 in reply to Omar Sultan1
    TI__Mastermind 39575 points

    Hi Omar,

    Good question.  The /RESET pin appears to be a push-pull output.  If you tied VBAT to /RESET when /RESET goes low, you will create a short of VBAT to GND.  Not sure if the part is damaged when you had +12V on Vbatt/

    Ben