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BQ24640: Backup Power Solution Using Supercap

Wendy Wang1
Genius 13570 points
Part Number: BQ24640

Hi,

I have just read this document http://www.ti.com/lit/ug/tidu594/tidu594.pdf.

In the document, it introduces a block diagram (attached below) to charge the super-cap.

Its says that this kind of charging circuit benefits the cost-effective, but results in low efficiency and long charging time. 

My questions: 1. Why this kind of charging circuit results in low efficiency and long charging time?

2. The pros and cons between this kind of charging circuit and the super-cap chargers like BQ24640.

Thanks&Best regards

  • 0
    TI__Mastermind 24075 points

    Hi Wenxiu,

       According to equation 1 of tidu594: 

    VSYS is your input voltage, D2 is forward voltage drop of the schottky diode, and Vsup is voltage of super cap. The downside with this, is as voltage of supercap increase by being charged (Vsup increases), the charge current would decrease linearly. From the CC-CV charging profile you can see that during CC phase charge current remains constant until supercap is charged to the charge regulation voltage, which is why the method mentioned in the reference design is not efficient and has relatively long charging time because charge current decreases throughout the charge process.

     

    CC-CV phase helps in accurately charging your super cap to the set charge regulation voltage. If you have impedance between supercap and where IC senses output voltage, IC will measure charge current*impedance + actual supercap voltage and terminate early if just using CC phase. In CV phase IC will regulate output voltage and taper down charge current till termination is hit. In BQ24640 the charge current, and charge regulation voltage are configurable, but you cannot disable CV phase of charging.