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UCC2813-1: Oscillator frequency calculation

lev nemets1
Intellectual 475 points
Part Number: UCC2813-1

We got a problem with accurate calculation/simulation of UCC2813-1 oscillator frequency.

  1. We found that the frequency from formula 1 (datasheet) is perfectly coincide with simulation but does not coincide with Fig.18 (datasheet). Simulation/calculation gives lower frequency than Fig.18. For the condition given in Table 7.5 (Datasheet: Rt=100k, Ct=330pF) simulation/calculation gives 45.04kHz but datasheet gives typical 46kHz frequency.
  2. We thought that Fig.16 (datasheet) will give us more accurate oscillator frequency but simulated/calculated Fig.16 (datasheet) gives even lower frequency 42.05kHz
  3. We thought that we do not consider some delay in oscillator circuitry, but any delay only makes frequency lower. The only way to have frequency for circuit Fig.16 the same as the typical value (datasheet 46kHz) is to use to use 2.5V threshold voltage instead of 2.65V. We noticed that in UCC2813-1 Spice model this threshold is given as 0.5*Vref. So, it is 2.5V

Could you, please, explain these discrepancies in datasheet.

 

Regards

Lev

  • 0
    TI__Genius 15286 points

    Hi Lev

    1. Fig. 18 plots shows approximate values of switching frequency (fsw) values for different RT and CT values. For the case of RT=100kohms and CT=330pF we can have an idea of fsw but it is difficult to get an exact value. Please note that the Datasheet is given Minimum, Typical and Maximum values. Typical value of oscillator frequency is 46KHz but the real value can be within the range between minimum and maximum.

    2. As mentioned above, Fig 18 (Fig. 16 is oscillator equivalent circuit) gives you an idea of fsw. If you are very concerned about fsw value I recommend measuring in the lab the oscillator pin referred to GND and measure it in the scope.

    3.  As mentioned above, Fig. 18 just gives you an idea of fsw. 

    Thank you.