UCC28070-Q1: Frequency dithering rate adjust

Hello Gene,

Yes that is correct.

First the dither magnitude is set with the RDM resistor value and then the dither rate is set with the CDR capacitor value.

The data sheet has the equation for selecting these components.

Regards

John

Hi John,

From the datasheet equation, i understood the adjustment of dithering rate can be achieved by adjusting the capacity of CCDR when the magnitude of the dithering rate frequency is determined by FDM (RRDM resistance value).

At present, customer mainly want to know if the dithering rate can be achieved at 0.1KHZ or lower. Is there any limitation in the chip? If not, i would forward to customer to try.

Best Regards,

Gene

Hi John,

One more question,  through Page 16 on datasheet of UCC28070-Q1,  "This total range from minimum to maximum frequency is defined as the dither magnitude, and is centered around the nominal switching frequency fPWM set with RRT"

if the fpwm=65kHz, and dither magnitude =10kHz, dithering rate=8kHz,  would the fpwm traverses within 60 to 70 kHz, back and forth with 8kHz step?

Am I right?

Thank you for your time to remove my queries.

Best Regards,

Gene

Hello Gene,

There is no lower limit on the dither rate.
A 0.1kHz rate is equivalent to a time of 10mS between Fmax and Fmin.
So the value of CDR becomes proportionally larger.
Please bear in mind that the equations for CDR and RDM are approximations and they may not be so accurate if the operating conditions are far away from the characterised values in the datasheet. Then ther is also the tolerance associated with the resistor and capacitor values to account for.

For your second question/example there is no step change in switching frequency.
The frequency changes continuously with a linear variation from 60kHz to 70kHz and back to 60kHz in 0.125mS

0.125mS is the time period associated with the 8kHz

Regards

John

Hi John

Thank you for your detailed reply. I was mis-understood previously about the dithering rate. Now i am clear It is not a step pluse, it is linearly varies with the time.

Best Regards,

Gene