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TPS61094: Supercap charge and discharge voltages on the TPS61094 are not what is expected

Scott Daniels
Prodigy 90 points

Part Number: TPS61094

Hi,

Below is my schematic for the TPS61094 supercap-based UPS. I am supplying a Vin=5.1v, and the configured backup output voltage is 4.8v. As you can also see, the charge voltage is set to 5.1V. I am using a 5.5V, 5.0F supercap attached to Vcap.

As I was testing the circuit, the backup time after Vin was removed was only a few seconds at a 500mA load; it is expected to be considerably longer. I measured the supercap voltage, and it is only charging to 4.8V, and only discharges to 4.5V. 

Can you provide what is causing these two charge and discharge voltages? One thing I considered was perhaps Mode & EN should be connected to Vout instead of Vin and that is why the supercap only discharges to 4.5V, but the datasheet shows them connected to Vin.

Please confirm the correct connection of Mode & En pins, and what is the probable cause of the premature shutoff of the charge and discharge of the supercap.

Thanks,
Scott

 

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  • +1
    TI__Mastermind 19800 points

    Hi Scott,

    Our device will work in automotive buck to charge the supercap. So the charger voltage should lower than Vout. Additionally, the EN and MODE pins of the chip must always be at a high level when the chip is working. However, in your schematic, they all be connected to VIN, and you also remove VIN during test. This can also cause abnormal chip operation. You can refer to our EVM user guide's connection method. I put it below. 

    Hope this can help. Thanks.

    Aurora

  • 0 in reply to Aurora Tie
    Prodigy 90 points

    Aurora,

    That explains things - thanks. Two more questions:

    1. I currently have my Vout voltage set to 4.8V, and that is what the cap charges to. So if I were to lower Vout to 4.6V, which I am considering, then am I to conclude that the supercap will likewise only charge to 4.6V?
    2. Do I need to tie my Mode & En to both Vin and Vout as shown in your schematic or can I just tie them to Vout? This application is meant to operate as a UPS for a Raspi, so Vout should always be on when operating.

    Thanks,
    Scott

  • +1 in reply to Scott Daniels
    TI__Mastermind 19800 points

    Hi,

    For your questions:

    1. Yes. The supercap will be charge by buck circuit, so its max voltage will not exceed than Vout.

    2. It's also ok to connect them to VOUT, just ensure they are all high under all condition. 

    Thanks.

    Aurora