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TPS7A20: TPS7A2033PDBVR

Jeevan Sidharth G
Prodigy 210 points
Part Number: TPS7A20

Tool/software:

Hi,

I am using TPS7A2033PDBVR in my design. During evaluation it was noted that placing a 220Ω ferrite bead gives an output equivalent to that of the VCC. Placing a 0Ω resister gives the expected output of 3.3V. Could you please explain on why does this happen?

Thank you

Regards,

Jeevan

  • 0
    TI__Genius 10880 points

    HI Jeevan,

    TPS7A20 requires a 1µF to 200µF output capacitor with low ESR for stability. Adding a ferrite bead between the LDO and the output capacitor effectively increases the ESR and ESL of the capacitor (as seen by the LDO), causing the LDO to become unstable. That instability can manifest in a number of ways, including oscillations or complete loss of regulation.

    I recommend moving C102 to the LDO output or removing the ferrite bead. 

    Best Regards,

    Alex Davis

  • 0
    Prodigy 210 points

    Hi Alex,

    Fine but I have added the ferrite bead as a noise filter. In that case is there any specific value for the ferrite bead to function properly. We tried moving c101 to output side but the result observed was still the same.

    Regards,

    Jeevan

  • 0 in reply to Jeevan Sidharth G
    TI__Genius 10880 points

    Hi Jeevan,

    If you add a ferrite bead between the LDO output and your load, you must ensure that the 1µF minimum output capacitance requirement is met by adding a capacitor directly at the LDO output before the ferrite bead.

    Ensuring that the resonant frequency created by your selected ferrite bead inductance and any downstream capacitance lies well outside the typical 10kHz-100kHz bandwidth of most LDO's error amplifiers will also help minimize the likelihood of stability issues.

    Moving the ferrite bead to the LDO input while ensuring you have enough input capacitance to support your application's load transients can also help provide additional high-frequency filtering while reducing the risk of stability issues.

    Best Regards,

    Alex Davis