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TPS737: PSpice Simulation: Oscillation with 2.2uH Input Ferrite Bead

Kazuki Itoh
Genius 10770 points
Part Number: TPS737

Tool/software:

Hi team,

In PSpice simulation, my customer is seeing oscillation when inserting a 2.2uH ferrite bead at the input of TPS73701.
When the bead parameter is changed to 0Ohm, the oscillation is disappeared.
Please find attached.

TPS73701 PSpice with ferrite bead.pdf

Could you please comment why the oscillation is happening in this case?
Where is the FB path causing the oscillation?
How does the input-side ferrite bead cause the oscillation?

Best regards,
Kazuki Itoh

  • +1
    TI__Genius 12600 points

    Hi Kazuki,

    I suspect this is being caused by the lack of a damping resistance in the input filter circuit in simulation. Any transients (the LDO enabling, for example) can cause that LC structure to resonate. I'd suggest adding a small series resistance with both the input capacitors and the input inductor. This should be around 3-5mΩ for a typical ceramic capacitor. Most ferrite beads specify a DC resistance, which can be used as the series resistance value for the inductor. These resistors will help damp out input oscillations. 

    Best Regards,

    Alex Davis