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TPS61094: Super capacitor voltage exceeds the setting termination Voltage

Part Number: TPS61094
Other Parts Discussed in Thread: TPS629210

Hello experts!

For an application, I am using TPS61094 in auto buck or boost mode having a supercap (1F, 3V ) as buck up power. I have set charging termination voltage to 2.5V and charging current to 10mA. I have also set the regulated output when in boost mode to 3.3V. This is the circuit I am using:

Everything works as expected so I added a small load (3-5mA) to the output for some longer term test. The load was driven from the 3.5V input so we never extract energy from the backup supply (super cap).
After some time (~4 hours later) I observed that the Voltage at SUP+ exceeded the termination voltage that I have set. I am using a scope and I am measuring 2.46V when the super cap stops charging. After 5 hours I am measuring 2.58V and after 7 hours I am measuring 2.64V. 
Do you have any clue what condition might cause this behavior. I had the feeling that in this case the super cap will discharge because of the leakage current for 100mV below the set termination voltage and then charge back again to 2.46 volts. I can not understand how it is possible to go above the set termination voltage. 

Thank you in advance!   

  • Hi Evangelos,

    It seems to be a strange behavior... Do you find this in our EVM or in the board you design by yourself ?



  • Hi! 
    I find this behavior on a custom board. I 'll setup the EVM with the same load and let you know. BTW I am using a 2 layer stack 1.6mm thickness in comparison to the EVM. 

  • I have tested with the same load on the EVM and I don't observe the same behavior. I have tested for 7 hours and Supercap voltage is 2.46V.

  • Hi Evangelos,

    So it seems to be related to customer design.

    Do you know is there any other devices in customer system? I mean it is possible that this is due to other reason not caused by TPS61094?

    By the way, I think the PCB layer difference has no impact on this. Maybe customer can also try a few more TPS61094 to see if it is repetitive behavior.




  • Hi Nathan, thanks for the support!
    I have the same feeling, so let me give some more information about the design. The input voltage is 6Vrms, so we are using a bridge rectifier  to make it DC and then we feed this voltage to TPS629210 to convert it to 3.5V. Then we feed 3.5V to the TPS61094. My load is an mcu blinking an LED. The Input power supply is very limited in terms of power (it can provide ~100mW). So when the LED turns on I can see a voltage drop of ~0.7-0.8V on the rectified input voltage. This does not effect the output of the TPS629210 which is always 3.5V. 
    We also have the SUP+ connected to an MCU pin (which for this experiment have not been set as input or ADC pin). Can this be the issue?

  • Hi Evangelos,

    Thanks for detailed information. 

    Yes I think maybe this is maybe caused by the connected MCU pin, maybe there is some leakge current from the pin, I think you can try to remove the connection between this pin and super cap to check if the phenomenon could reappear. If it doesn't appear, then it actually caused by this, maybe customer needs to check the configuration of the MCU pin.



  • Hi Nathan,
    The problem was indeed the configuration of the GPIO. It turned out that internally, there was a pullup and the Cap kept charging because of this. 
    Thanks for the feedback and the support.
    Issue is now resolved