UC2825: Equation for output frequency

Hello Satishi,

The UC2825 follow the same equation as mentioned in the datasheet: fosc = 1.72/(Rt*Ct).

Also datasheet recommends that the timing resistor value to be between 5k and 100k. If you notice figure10 below the there is a linear relationship between this resistor value and freq however outside this range the this graph is much nonlinear and harder to predict (please refer to the table below).

I understand that they want to operate at 200kHz, if they decide to keep the Ct=2.2nF, following the equation thier Rt should be 4K.

I recomend using a Ct=1.5nF and Rt=5.7K.

Hopefully this answers your question.

Regards,

Sonal

Sonal-san

Thank you for reply,

Please let me reconfirm about your answer because it was not match by UC2825 datasheet information below.

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①UC2825 datasheet is described Rt recommended value is 3kΩ to 10kΩ.

②When your answered Rt and Ct value applied to datasheet graph, result was over 200kHz

③When calculated above condition by fosc = 1.72/(Rt*Ct), result was about 237kHz.

　In fact, measured fosc is 220kHz.

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Are UC2825 recommend value Rt: 5.7kΩ / Ct: 1.5ns or Rt: 4kΩ / Ct: 2.2ns, are these correct?

If there detailed UC2825 graph for relation of fosc and Rt and Ct, please let me know.

Best regards,

Satoshi

Hello Satoshi,

The deadtime calculation is not dependednt on the Rt value as you can see from the equation 8 of the UC2X4X datasheet:

As far as the timing resistance Vs frequency graph is concerned I see no diffrencer in the two plot.

The recomended values of Rt and Ct components are: 

Never use a timing resistor less than 5 kΩ. The recommended range of timing resistor values is between 5 kΩ and 100 kΩ; and the recommended range of timing capacitor values is between 1 nF and 100 nF.

Please use: Rt: 5.7kΩ / Ct: 1.5nF.

Regards,

Sonal