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TPS40170-Q1: High frequency noise

Kevin SHIN
Prodigy 495 points
Part Number: TPS40170-Q1
Other Parts Discussed in Thread: TPS40170, LM5145

Hello team,

My customer have some problems with high frequency noise.

They used 1200pF of C21(compensation capacitor) because TI EVM used 820pF.

Is it okay to compensate the circuit?

And I have a inquiry.

Is there any material that suggest phase margin standard about high frequency noise?

If so, could you give me some materials?

Thank you in advance.

Best Regards,

Kevin Shin

  • 0
    TI__Guru 53312 points

    Hi Kevin,

    Please use the Excel calculator file in the TPS40170 product folder to assist with component slection. The LM5145 calculator also works for this as the voltage-mode feedforward gain is the same for both parts.

    In general, the cap from COMP to FB sets the high-frequency pole - the pole frequency should be set at Fsw/2.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    TI__Prodigy 495 points

    Hello Tim,

    Thank you for your reply.

    Is there any material that explain setting the cap as the pole frequency should be set at Fsw/2?

    Could you support that meterial?

    Thank you.

    Best Regards,

    Kevin Shin

  • 0 in reply to Kevin SHIN
    TI__Guru 53312 points

    Hi Kevin,

    Take a look at the compensation section in the LM5145 datasheet.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    TI__Prodigy 495 points

    Hello Tim,

    Thank you for your support.

    The customer wanted to know the phase margin standard that high frequency noise isn't occurred in TPS40170.

    Is there any exact value of phase margin that high frequency noise isn't occurred such as 45 degree of phase margin make no high frequency?

    Or if you don't have that exact value, when phase margin is 24 degree, this situation can make high frequency?

    The reason why I wanted to know exact phase margin is when the customer did in the bench, there is no noise but when the customer applied the same board to the car, high frequency noise is occurred. 

    Phase margin of that board is 24 degree.

    Is this 24 degree come to this problem?

    Thank you.

    Best Regards,

    Kevin Shin

  • 0 in reply to Kevin SHIN
    TI__Guru 53312 points

    Hi Kevin,

    For a general stability standpoint, the phase margin should be 45 degrees or more (24 degrees is way too low).

    As mentioned, high-frequency noise is attenuated by the cap from COMP to FB. Also important is to keep the FB trace as short as possible. Please send a schematic and completed quickstart calculator for this application and I will review the values.

    Regards,

    Tim