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TPS61094: Auto buck or boost mode - chip keeps on switching although supercapacitor is full

Jouko Korhonen
Prodigy 30 points
Part Number: TPS61094

Configuration: Input voltage 3.85V,
* MODE and EN pins are tied together and connected to output via 100k pull-up resistor.
* supercapacitor (SC) charging termination voltage set to 3.6V
* output voltage option set to 3.6V
* charging output current set to 100mA

Phenomena seen by oscilloscope:
SC voltage is slowly rising. Switching node (pin 5) switch 4-5 times and rests for 1.3s, then repeats.
When SC voltage reaches 3.6V, the voltage starts sinking and the switching node remains (occasionally) switching.
When voltage sinks ~75mV the phenomena repeats.
All the time the input voltage 3.85V at Vin is steadily connected to Vout.
There is practically no load.

Why the chip keeps on switching when SC is full?
This obviously causes high leakage: 5F capacitor droops 75mV in less than a minute!

  • 0
    TI__Genius 9565 points

    Hi Jouko,

    When SC voltage is close to charging voltage, there is still a small charging current ICHG_TERM, it is about 10mA, so the device is still switching.

    But I think the SC voltage drop doesn't cause by the switching, it should be caused by the internal resistance of SC. The SC voltage you measured is equal to voltage drop of internal resistance plus real SC voltage, and the voltage drop of internal resistance is related to charging current, Vdrop = ICHG*R.

    When SC voltage < charging terminal voltage, the charging is 100mA, while the charging current is 10mA when SC voltage is close to charging terminal voltage, so in this condition the voltage drop becomes lower, and that is the ~75mV voltage sinks you measured.

    I have a test in our lab, the SC voltage will finally keep stable over time, maybe you can also try with longer time.

    Regards,

    Nathan