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TPS61021A: TPS61021A Max Recommended Output Cap

ka88
Intellectual 275 points
Part Number: TPS61021A
Other Parts Discussed in Thread: TPS61022, TPS61023

Hello,

Is there a specific reason for the 200uF maximum recommended output capacitance for the TPS61021A? Is it related to loop stability? In-rush? Or is it just a matter of adjusting the FF compensation capacitor and thus not truly a hard limit?

I am interested in driving a speaker (w/ Class D Amp) downstream from this output, with a 470uF electrolytic on the TPS61021A output rail due to high source impedance battery supply. So preferably I would place it at the output of TPS61021A instead of the input.  Peak output current (non-inrush) from TPS61021A is likely about 1A and configured at 3.3V output.

  • 0
    TI__Expert 8605 points

    Hello,

    The 200uF maximum recommended output capacitance is not a hard limit. But a bigger capacitance than 200uF will affect loop stability and needs extra evaluating. 

    Best Regards,

    Travis

  • 0 in reply to Travis Wang
    Intellectual 275 points

    Hello Travis. Thanks for the answer.

    What is the suggested method of analyzing the effect on loop stability for >200uF output capacitor?  Do you mean looking at the output transient response and adjusting the feed forward capacitance as necessary?

  • 0 in reply to ka88
    TI__Expert 8605 points

    Hello,

    TPS61021A do not have loop analysis excel tool so we need to check it with frequency response analyzer on bench. If you do not need the Ilimit pin, you can switch to TPS61023 or TPS61022 and evaluate them with their loop stability by the excel tool:

    SLVC803 Calculation tool | TI.com

    If you cannot change your device and do not have a frequency response analyzer, you can give me your input voltage range, output current and BOM so that I can design you a reference feed forward capacitance.

    But I can only do test on one IC under room temperature. You'll need extra stability test on multiple ICs under all temperature range to ensure your product's reliability.

  • 0 in reply to Travis Wang
    Intellectual 275 points

    Hello Travis,

    I can switch to the TPS61022.  However, I need to evaluate the in-rush current during soft-start with 470uF effective output capacitance because my input source is an alkaline battery (relatively high source impedance).  The datasheet only shows soft start plots with a resistive load, do you have any simulation models of TPS61022 that I can use for soft start evaluation?

  • 0 in reply to ka88
    Intellectual 275 points

    Just one correction Travis, the question above about soft start evaluation applies to both TPS61022 and TPS61023.

  • 0 in reply to ka88
    TI__Expert 8605 points

    Hello,

    The inrush current during soft-start is limited by a current limit value lower than the current limit after start up. So if you cannot accept the 3A limit of TPS61023 or TPS61021A, you need to switch to another device with a lower current limit. Please tell me the maximum input current and output voltage you need so that I can advise you a more suitable part.

    Best Regards,

    Travis