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UCC2801: Switching frequency per datasheet

dridgway
Intellectual 410 points
Part Number: UCC2801
Other Parts Discussed in Thread: UCC2800, UCC2803

Hello, I have an old datasheet for the UCC2801 dated 2010 and the latest on from the website. The datasheet from 2010 states that the switching frequency is set with a R and C and the formula to calculate that frequency is 1.5/(R*C). The newer datasheet on the website does not have a formula for the UCC2801, but for all of the other versions. Can someone confirm what the formula is supposed to be? Thank you for your time,

David

  • 0
    TI__Genius 9130 points
    Dridgway,

    I will contact the product line to clarify this, and respond to you within the next day or two with a reponse.

    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    TI__Genius 9130 points
    David,

    I can confirm that this is an oversight in the latest updated datasheet.

    It should include all 4 devices with 5-V REF (UCC2800/1/2/4), all use the same osc eqn. f = 1.5 / (R * C)

    The 2 devices in the family with 4-V REF (UCC2803/5) use the eqn. f = 1 / (R * C)


    The datasheet will be corrected in the near future to correct this error.

    Thank you for highlighting the issue.


    If this addresses your question, please click the "verify answer" button to close this thread.

    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    Intellectual 410 points

    Thank you Bernard, this does answer my question. For some reason, I cannot see the "Verify Answer" button to click....

    I do have another question regarding this part...

    The design that I am debugging has a 10.7K resistor and 470pF cap on the RC pin. Per the equation that should be an oscillator freq of ~300kHz and an output of 150kHz, but what I am seeing is a constant 125kHz output. The datasheet specifies that no less than a 10K resistor should be used. Is 10.7K too small, should it be slightly larger? Will this effect the output frequency?

    Thank you again!

    David

  • 0 in reply to dridgway
    Intellectual 410 points
    Had to refresh the web page and then the "verify answer" button showed up!
  • 0 in reply to dridgway
    TI__Genius 9130 points
    David,

    Do you have another cct possibly loading the CT pin? Can you post your schematic? Maybe your PCB layout is causing extra parasitic cap on the CT pin?

    Are you measuring the freq from OUT rising edge to rising edge?

    It might be worth trying to double the R and halve the C to see if the freq is the same or not, this could help indicate if there is extra parasitic C that is slowing down the osc.


    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    Intellectual 410 points
    Thank you, I will try your suggestion. Would having additional parasitic capacitance actually slow down the clock instead of making it faster? I don't believe the layout is optimal and parasitic capacitance is probably playing a factor. The R is near pin 8 (Ref) and the C is near pin 4 (RC).
  • 0 in reply to dridgway
    TI__Genius 9130 points
    David,

    Since the osc is inversely proportional to the R*C product (f = 1.5 / (R * C)), an increase in either R or C will slow down the osc. So parasitic cap will shift the freq, although if you are using 470 pF, there would need to be a lot of extra cap to shift the freq down by 16% (75 pF)

    Is there any slope comp or similar circuitry connected to the CT pin?

    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    Intellectual 410 points

    Bernard, there is a 0.1uF cap in parallel with the 10.7K and 470pF cap. 

    Thanks!

    David

  • 0 in reply to dridgway
    TI__Genius 9130 points
    David,

    I assume that the 0.1-uF cap is a decoupling cap from REF pin to GND?

    If so, then this will not be loading or affecting the CT pin, and should not affect the freq. In fact, a decoupling cap on the REF pin is always a good idea.


    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    Intellectual 410 points
    Yes, you are correct, this cap is from Ref to GND.