UCC28C45 CT/RT

Raymundo Hernandez-Toscano

Other Parts Discussed in Thread: UCC28C45

Hi,

A customer of ours is reporting less than desired switching frequency performance. What is the internal capacitance on the RT/CT pin of the UCC28C45? Is it upwards of 50pF? if so, is there a need to compensate for this internal capacitance when setting the oscillator frequency? Also, I am not referring the external timing capacitance that is coupled with the timing resistor on the Vref pin that is used set the oscillator frequency and duty cycle.

over 8 years ago

Billy Long over 8 years ago

TI__Expert 4015 points

Hi Raymundo, what error is your customer seeing in the switching frequency and what capacitance value are they using? I'm not sure what the internal nodal capacitance is, but you should be able to calculate the frequency as 1.72/(Ct*Rt), or from the graphs in figure 1 of the datasheet. Theses devices have been widely used for many years with this method.
Thanks
Billy

Raymundo Hernandez-Toscano over 8 years ago in reply to Billy Long

TI__Intellectual 810 points

Hi Billy,

Correct, the customer is reporting a slower switching frequency than what he was expecting, based on the values for the timing resistor and capacitor. Can we get a measurement of the nodal capacitance?

Peter Meaney over 8 years ago in reply to Raymundo Hernandez-Toscano

TI__Mastermind 31580 points

Hi Raymundo,

Billy is OOO in holidays, what RC values is the customer using ?

Thanks

Peter

Raymundo Hernandez-Toscano over 8 years ago in reply to Peter Meaney

TI__Intellectual 810 points

Hi Peter,

I have already confirmed that there is less than 10pF of internal capacitance on this pin. Thanks for the support.

Regards,

-Ray

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UCC28C45 CT/RT