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TPS62827: Output capacitance vs stability

Bart
Expert 6370 points
Part Number: TPS62827
Other Parts Discussed in Thread: TINA-TI

Team,

with TPS62827 we want to generate 0.9V from 5V, and output current between 2A and 4A.

Webench simulation suggests to use 470nH and 22uF at the output; whereas we want to have Cout=250uF effective -> how do we choose the L and Cff and the resistor feedback values? Is there any application note about it?

It would also be helpful to learn how to measure the stability on the EVM if we use more capacitance, and how this impacts on the selection of an inductor.

Thanks for your help.

TI Customer

  • 0
    TI__Expert 3645 points
    Hello,
    Regarding the choice of the resistors in the feedback divider and the value of Cff, please have a look at equation 4 and 5 in the section 9.2.2.2 of the datasheet (page 11). For the output filter LC, the value is reported in table 5 at section 9.2.2.3 (page 12), and we don't recommend the use of such a high output capacitor with your inductor. Anyway if you want to test your system checking the stability, you can follow this document that explains how to do this measure on such devices with DCS control topology: www.ti.com/.../snva364a.pdf
  • 0 in reply to Alfonso Furio
    Genius 4735 points
    Dear Alfonso,

    please can you tell me which control mode in buck converters is suitable for high output capacitor? :
    COT with emulated ripple
    Constant on-time (COT)
    Current mode
    D-CAP
    D-CAP+
    D-CAP2
    D-CAP3
    DCS-Control
    Voltage Mode

    Please can you describe the differences between the Buck control modes ? or does Ti has an app-note about this matter?

    Best Regards,
    D.
  • 0 in reply to d_zero
    TI__Expert 3645 points
    Hello,
    Since the devices have a fixed compensation network, to obtain the stability for different conditions, you can change the LC output filter. So, in principle, the control topology doesn't need to be selected based on the amount of the output capacitance. You should choose the control topology depending on your specific application. It is not available an application note with a comparison of all these topologies but, if you want, you have a look in this document on DCS-control: www.ti.com/.../slyt531.pdf
  • 0 in reply to Alfonso Furio
    Genius 4735 points
    Dear Alfonso,

    so i can choose the TPS62827 and use the 250uF output capacitor but then i need to recalculate the Inductor to obtain stability , is that correct?

    is there a way to calculate this or to simulate the circuit with TPS62827 and 250uF capacitor in TINA-TI? regarding to stability of the circuit?

    Or do i have to do measurements on a demo board with changed Cout and L ?

    Br,
    D.
  • 0 in reply to d_zero
    TI__Expert 3645 points
    Hello,
    Keeping the product LC fixed, you can evaluate which L can be used since the output capacitance is increased. To check how the system behaves is good to test the stability of a demo board (i.e., our EVM with your components).