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UCC3801: Frequency operation equation missing for UCC3801

Jorge Castro46
Prodigy 20 points
Part Number: UCC3801
Other Parts Discussed in Thread: UCC2801

Hi,

I am looking for the correct "frequency operation" equation of the UCC3801.

According to 9.3.1.4 RC section in the datasheet, there are a couple of possible equations:

For the UCCx802 and UCCx804, use Equation 4.

f = 1.5 / (R*C)         [Eq.4]

and for the UCCx803 and UCCx805, use Equation 5.

f = 1.0 / (R*C)         [Eq.5]

However, the equation applicable for UCC3801 is not indicated.

Which equation should I use in this case?

Thank you,

Jorge

  • 0
    TI__Mastermind 40560 points
    Hello Jorge,

    The correct frequency equation for UCC3801 is [Eq.4], f = 1.5 / (R*C).
    A nearly identical question was asked about this about a month ago.
    To view it, please search on "UCC2801: Switching frequency per datasheet" in the search box at the top of the forum page.
    Apparently, the datasheet needs to be corrected to add this missing information.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Prodigy 20 points

    Thank you Urlich,

    Just to confirm one additional thing, in table 8.5 Electrical Characteristics, there is a Note 4:

    Note (4) Oscillator frequency for the UCCx800, UCCx802, and UCCx803 is the output frequency. Oscillator frequency for the UCCx801, UCCx804, and UCCx805 is twice the output frequency.

    So, in my case, if I apply the [Eq. 4] and I get f = 50kHz, does it mean that the output frequency is 25kHz, right?

  • 0 in reply to Jorge Castro46
    TI__Mastermind 40560 points
    Hello Jorge,

    Yes, that is correct, sir, because the UCCx801 (and -04, -05) contains a flip-flip to limit the maximum duty-cycle to 50%. This has a side-effect of dividing the oscillator frequency in half to become the output switching frequency.

    Stated another way, to obtain a desired output frequency with the UCCx801 (-04, -05) you need to program the oscillator to twice that frequency.

    Regards,
    Ulrich