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TPS40304: Incorrect switching frequency observed

Gary Ellsworth
Prodigy 50 points
Part Number: TPS40304

We are using the TPS40304 for a 15V to 3.3V (0.5A to 5.5A) converter.  Our design analysis values closely match the webench design values shown in the schematic below.  I’ve included a scope capture of the M2 drain waveform.  Output is regulating at 3.3v over load range and line between 14 and 16v.  Ripple doesn’t look too bad at approximately 50mvpp but frequency at about 100khz, not 600khz as expected.  Can you give me an idea what may be causing this….or what mode of operation the chip may be in?  It looks like two 600kHz switch pulses, every 100kHz (about 7.5usec between cycles).

  • 0
    Prodigy 50 points

    Added reference schematic

  • 0 in reply to Gary Ellsworth
    TI__Genius 15047 points

    Hi Gary,

    Our US team will check it and reply you soon.

  • 0 in reply to Gary Ellsworth
    TI__Mastermind 49895 points

    Gary,

    I am out of the office for the US holiday, but what you describe is pulse skipping.  The converter is generation 1 full duty cycle pulse, a truncated pulse, and then waiting 4 switching cycles with no PWM pulses while the output voltage drops back below the reference, and the process repeats.

    Pulse skipping is generally a response to excessive gain in the regulation loop, either from improper compensation or from excessive parasitics, such as series resistance or inductance.

    If you can provide the following values from your actual design, I may be able to assist you:

    1) Inductor inductance and DCR, as well as it's saturation current rating

    2) Full list of output capacitors including their capacitance and ESR values

    3) Resistor value from VOUT to FB

    4) Value of series R-C between VOUT and FB (in parallel with the resistor in 3)

    5) Value of Capacitor from FB to COMP

    6) Value of series R-C between FB and COMP (in parallel with the capacitor in 5)

    Please fill that information into the TPS40k Type III Compensation design tool - https://www.ti.com/lit/zip/sluc263  and we can review your compensation design and look for design changes that can prevent the pulse skipping and get the TPS40304 operating at 600kHz like it should be.

  • 0
    TI__Mastermind 49895 points

    Gary,

    How are you doing?

    Were you able to make adjustments to the compensation to address your pulse skipping issue?

    Do you need further assistance?

  • +1 in reply to Peter James Miller
    Prodigy 50 points

    Hi Peter,

    We think we found an error with our compensation hookup.  It is not exactly as shown in schematic we provided.  When corrected the switching frequency behaves as expected.  We are fine tuning our compensation network now to optimize stability margins.  Will be in touch if we need more advice.

    Thanks, Gary

  • 0 in reply to Gary Ellsworth
    TI__Mastermind 49895 points

    Thank you for letting me know Gary.

    I'm going to close this thread as resolved.  You can create a new on if you have further questions.