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TPS7H4010-SEP: TPS7H4010 Output Capacitor Specifications

Michael Bins
Prodigy 20 points
Part Number: TPS7H4010-SEP

Tool/software:

1. The Cout formula (Eq28) in the TPS7H4010 datasheet is based on the ripple current. But the specified 330uF C4 capacitor in the evaluation board BOM (Table 6-1) for the 1.8V supply only has a resonant frequency of 180KHz (based on the graph in the Kemet datasheet for the 6V model). This is well below the 500KHz switching frequency used on the evaluation board. So it doesn't appear that this 330uF capacitor will affect the ripple current. Should capacitors with a resonant frequency below the switching frequency be counted as meeting the Cout formula result?

TPS7H4010-SEP Radiation Hardened 3.5-V to 32-V, 6-A Synchronous Step-Down Voltage Converter in Space Enhanced Plastic datasheet (Rev. A)

https://www.ti.com/lit/ug/snvu744/snvu744.pdf#page=10

https://content.kemet.com/datasheets/KEM_T2076_T52X-530.pdf

2. Should capacitors with a resonant frequency below the switching frequency be counted for use in the fx formula (Eq18) in the TPS7H4010 datasheet?

  • 0
    TI__Mastermind 28845 points

    Hey Michael,

    What paper/article are you seeing which says they wouldn't be considered part of the formula?

    The equation for voltage ripple used is admittedly simplistic, and there will be some increase in voltage ripple from the increased impedance/decrease in capacitance.
    I however have never heard of omitting the capacitor entirely from the equation.

    Thanks,
    Daniel

  • 0 in reply to Daniel Hartung
    Prodigy 20 points

    Thanks for your response. My question was only based on the capacitor resonance frequency being below that of the switching frequency.

    Above the resonant frequency, the LC low-pass filter essentially becomes an RL low-pass filter, where the R is the capacitor's ESR. So the
    capacitance value and reactance become negligible at the switching frequency. For example, Xc=1/(2PIx500KHzx330uF)<1mohm at 500KHz.

    So it seems like the Cout formula should not include capacitors that are above their resonance point at the ripple frequency. Instead, it seems the
    formula should include the capacitor's ESR. The datasheet Eq29 does cover maximum ESR, but that formula depends on the capacitance,
    which won't have any bearing if well above the resonant frequency.

  • 0 in reply to Michael Bins
    TI__Mastermind 28845 points

    Hey Michael,

    By the datasheets own graphs it shows there is still a capacitance associated with the capacitor.

    While it is less than the 330 uF at DC it would still be part of the equation.
    Even at 1 MHz the 009 capacitor still would have 100 uF of capacitance, which seems quite relevant to the equations.
    You are free to account for any frequency effects of capacitors you use in your applications when doing the calculations.

    Thanks,
    Daniel

  • +1 in reply to Daniel Hartung
    Prodigy 20 points

    Okay, that makes sense. Thank you.

  • +1 in reply to Michael Bins
    TI__Mastermind 28845 points

    Welcome

    Thanks,
    Daniel