UCC256303: converter finetune

Hi Chentsu,

Thanks for reaching out. Are you referring to ripple noise on the output? Do you have waveforms you can share?

VCR should be evaluated on the following criteria:
1. startup behavior
2. maximum peak to peak voltage
3. loop response

The VCR waveform affects the initial switching frequency profile during startup so it is important to make sure there is sufficient margin to avoid tripping OCP when starting into maximum load. The initial switching frequency also impacts the startup timing. Section 2 of the practical design guidelines gives more detail regarding the impact of VCR on startup behavior:www.ti.com/.../slua836a.pdf

VCR has a common mode voltage of 3V and the peak to peak voltage should be less than 6V at your minimum operating input voltage with maximum load. This will influence how to size you capacitance to keep the maximum peak to peak voltage within 6V

The VCR waveform is composed of two components: the sampled resonant capacitor voltage and the internal frequency compensation current. How you set the "mix" between the sampled voltage and the compensation current impacts the loop response. More frequency compensation will make the LLC transfer function look more like direct frequency control. More sampled voltage will make the transfer function look more like a 1st order system. While there are many other parameters that will impact the loop response, VCR does have some influence on the control loop so it is important to look at the loop response before finalizing your VCR values.

The LL/SS pin is used for two items: soft start timing and burst mode programming. The LL/SS capacitor should be sized to meet your design's startup timing requirements. For burst mode programming, I would suggest following the burst mode tuning instructions in section 3 of the practical design guidelines. I would suggest finalizing your VCR capacitance values first before tuning the burst threshold because the VCR peak to peak will have some influence on the load current at which you transition to burst mode.

Please let me know if this addresses your questions on VCR and LL/SS.

Best Regards,
Ben Lough

Hi C.T.

Thanks. Please feel free to email me the waveforms whenever they are ready.

Best Regards,
Ben Lough