UCC28C45: Highest frequency of the oscillator synchronization

Hi Wendy! 

Thank you for reaching out and for your support of our UCC28C45 device! The UCC28C45 is capable of supporting switching up to 1MHz even without a synchronization signal, a 400kHz PWM with sync should be absolutely fine. 

Since the max synchronization frequency isn't available in the datasheet, I would recommend reaching out to the TI customer support center and opening a ticket there. They should be able to quickly find that information for you. 

If your customer is curious about this because they are running into issues using the sync feature in their application, let us know and we can provide more focused support on that topic. 

Thanks again for getting in touch, I hope this information helps! 

Aidan Davidson

Hi Wendy! 

I did a little more research on this topic, I highly recommend checking out this application note. The IC covered is different, but the synchronization signal concepts described in detail are applicable to the UCC28C45 as well. 

In summary, the sync pulse must at the very least be long enough to allow for the timing capacitor to discharge fully each cycle. This can be calculated using the discharge current of the IC from the datasheet, the size of the timing capacitor, and the peak to peak oscillator ramp value from the datasheet with the equation described in the application note I pointed out above. For best results, I would recommend maintaining the 5:1 ratio of sync frequency to programmed RT/CT frequency as mentioned in the UCC28C45 datasheet. Since this device switches up to 1 MHz, this would mean a corresponding max sync frequency of 5MHz. 

Thanks again for getting in touch, I hope this helps! 

Aidan

Hi Wendy! 

I'm closing this thread for now, if you have any additional questions please open a new thread or post again here. 

Thanks again for getting in touch! 

Aidan

Hi Wendy, 

The UCC28C45 would generate a PWM output of 400kHz. 

The 5:1 ratio is a recommendation from our IC designers for obtaining the best results. However, if I do a quick ballpark with the equations featured in the Oscillator Discharge Ramp Equations section of the previously mentioned app-note, i get a ratio of ~5.9:1 but that would be an absolute maximum. The 5:1 recommendation is likely chosen to be fast enough to ensure that the sync pulse is properly triggering the Ct discharge ramp while slow enough to allow ample time for the IC to fully discharge the Ct cap.

Thanks again, I hope this helps! 

Aidan

Hi Wendy! 

The 5:1 ratio is specifically recommended for a sync pulse with an amplitude of 0.5V applied between the timing cap and a small resistor to ground as shown in the datasheet diagram. This configuration is summing the pulse with the existing RT/CT ramp to trigger the timing cap to discharge. The more relevant relationship is between the ratio and the amplitude of the sync pulse. The peak threshold for the RT/CT pin is on average ~2.52V, this is the value that has to be passed by the RT/CT pin voltage in order to trigger discharge. A higher frequency ratio is needed for a smaller amplitude and vice versa. In theory you could have a 1:1 ratio of programmed frequency to sync frequency as long as your sync pulse amplitude was > 2V. 

Aidan

Hi Wendy! 

Since we've moved our discussion on this topic to email, I am closing this thread. 

Thanks! 

Aidan