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# UCC256402: UCC256402 - Switching Frequency Question

Part Number: UCC256402

Question 1: During the soft start of this IC [UCC256402], if C1, the upper voltage divider capacitor, is adjusted from C1=47pF to C1=100pF, why does the soft start frequency decrease? If the C1=47pF then fs = 510kHz; C1=100pF then fs = 280kHz

Theoretically, the larger C1, it has the larger the voltage detected by VCR_PIN will accelerate the switching of the upper and lower Mosfet of the LLC, and the frequency should be increased. May I know that is correct?

Question 2: When the IC [UCC256402] is soft-starting, if C1=47pF, then adjust CL1 That we can have a large range to adjust SS frequency. If C1=100pF, then adjust CL1 that we just have small range to adjust SS frequency. May I know why is different?

C1=47pF, CL1=470nF then the fs is 415kHz; C1=47pF, CL1=1uF then the fs is 510kHz, C1=100pF, CL1=1uF then the fs is 280kHz; C1=47pF, CL1=2.2uF then the fs is 310kHz.

If the C1 is 100pF, is there any other way to make the soft-start frequency roughly around 400kHz?

BR, Gary

• Hi Gary,

When we reduce the upper VCR capacitor value, during starting, the frequency should be higher. So, 47pF should give higher startup frequency compared to 100pF for upper VCR cap.

For simple explanation, let's assume that charge control component is ignored. In that case, effective VCR cap will be (Cvcr upper + Cvcr lower) charged by the internal ramp currents.

When effective capacitance is lower, for the given internal ramp current, dv/dt will be higher as Iramp=(Cvcr upper + Cvcr lower)(dv/dt). This would generate high switching frequency pulse as VCR raises to internal threshold voltages much sooner.

You will have to adjust Vcr caps and C_LL/SS value in order to adjust the soft start frequency.

Regards

Manikanta P