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Original question:

TPS63020: Driving a supercapacitor (1F) load

TPS63020: Driving a supercapacitor (1F+) load

Martin Clemons
Prodigy 80 points
Part Number: TPS63020

The datasheet states that the minimum duty cycle in buck mode is 20% (8.5 Electrical Characteristics.)

Let's say that the TPS63020 has a large supercap/ultracap on the output, and the circuit is being powered on for the first time in a while (the supercap is discharged to ~0V). +4V is applied to the input of the TPS63020 and the converter starts up. Since the output voltage is very low, the average inductor current setpoint is limited to somewhere around 400ma by the soft start circuit (9.4.1 Softstart and Short Circuit Protection.) The inductor current quickly ramps up and the converter is operating in CCM. Once the average inductor current exceeds the 400ma setpoint, the duty cycle will need to be decreased significantly below 20%, since Vout is still very near 0V, so either:

1. The duty cycle will be limited to 20% minimum and the average inductor current will increase without limit (quickly exceeding the 4A switch ratings), or

2. The duty cycle will be significantly below 20% until the capacitor voltage ramps up to ~0.8V -> M(D) = 0.2. If the voltage on the supercap is 50mV, M(D) = 1/80 D ~ 1.25%. Assuming a constant current of 400mA, it will take ~2s to charge the supercap to 0.8V, and during this time the duty cycle must be less than the minimum specified in the datasheet.

So which of these scenarios will happen?

Thank you!

  • 0
    TI__Expert 7925 points
    Hello Martin,

    Thank you for the question. I will look into it and get back to you by the end of the day.

    Best regards,
    Milos
  • 0
    TI__Expert 7925 points

    Hello Martin,

    I am sorry for the delay.

    Note that the 400 mA average current limit refers to the input side (the high side switch of the buck part), therefore not the inductor when the device operates in buck mode. This limit increases linearly as the output voltage increases from 0 V (short circuit) to somewhere above 1 V. On top of that, there is a 4 A limit for any switch. The 20% limit on duty cycle is for normal operation, not when a limit is reached.

    Therefore, the second scenario will happen, however, not with a constant D since there will be current limiting somewhere.

    As an example, these are the curves when charging a ~2 F supercap to 2.1 V:



    Input voltage: 4 V
    Output voltage: green
    Input current: red
    Output current: blue

    In this case the limit is dictated by the output current reaching 4 A limit. The input current ramps from a value higher than 400 mA because the output voltage does not start from zero since there is some resistance in the leads and supercap's ESR. In other cases the input current limit could also prevail.

    Best regards,
    Milos

  • 0 in reply to Milos
    Prodigy 80 points

    Thanks Milos, the scope screen capture is especially helpful!I was concerned that the 20% minimum duty cycle limit was a hard limit (for bootstrapping the gate drive or something like that), but apparently the converter operates fine at a lower duty cycle on start-up.