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UCC2801: Issues with incorrect PWM frequency output in simulation (LTSpice) compared to our hardware implementation

Santos Castillo
Prodigy 20 points
Part Number: UCC2801
Other Parts Discussed in Thread: UCC2800, UCC2802, UCC2804

UCC2801

Ct=220pF

Rt=36.5k

I am expecting an output frequency between 75kHz and 87 kHz, but I am getting 91.88 kHz. Confirmed in time domain and via FFT.

However, in our hardware implementation, it does work within our range. 

This seems to be a fidelity issue with the model; I am using the unencrypted SPICE model provided in LTSpice.

I have also tried the circuit in SIMetrix and the same result

Additionally, it seems like there is no RC design equation in the datasheet for the 2801 model.

  • 0
    TI__Expert 7900 points

    Hi Santos,

    I will look into this and get back to you today.

    Eric

  • 0
    TI__Expert 7900 points

    Hello again Santos,

    The UCC2800, UCC2801, UCC2802, and UCC2804 are specified together in the electrical tables.

    So, they should all follow Equation 4 for the oscillator frequency (at pin 4).

    Notice Note 2 for the oscillator frequency specifications;

    With 220pF and 36.5kohms the oscillator frequency (pin 4) should be 186.8kHz and the PWM output frequency (pin 6) should be 186.8kHz/2 = 93.3kHz.

    The datasheet only specifies one condition: 46kHz (330pF, 100kohms) where the tolerance is +/- 13%.  At other frequencies I expect it to be a bit higher tolerance.

    I don't see how you calculated 75-87kHz for 220pF and 36.5kohms.

    Regards,

    Eric