LM5021: Skip cycle bypass

Sankalp Gitaje

Part Number: LM5021

I want fixed frequency over the load variation from 0 to 1.5A (Bypass skip cycle) . So I added resistor from VCC pin to CS pin as per datasheet Figure 14 Typical Application Circuit and equation (24), R22 resistor is 10Kohm and R21 is 620k (Also checked with 100ohm, 6.2Kohm combination). But still I am not getting fixed frequency refer attached waveform. CH1 gate waveform, CH2-CS pin, CH4-VCC.

*FYI cap connected to CS pin is 150pF.

Can you please confirm the waveform attached is correct. Why I am not getting fixed offset voltage at CS pin even VCC is fixed ~8V.

Vin: 88 to 280Vdc,

Vout: 13.5Vdc,

Load: 20W,

Fsw: 100KHz.

over 2 years ago

0 Claire Chang over 2 years ago

TI__Intellectual 2760 points

Hi Sankalp,

Thanks for reaching out, I am looking into this issue with my team and I will get back to you in 1-2business days.

Thanks for your patience.

0 Claire Chang over 2 years ago

TI__Intellectual 2760 points

Hi Sankalp,

Can you try adjusting your R21 and R22 to have VCS offset at 200mV? Datasheet mention to keep voltage on CS pin minimum 125mVdc in order to disable skip cycle mode (refer to 7.3.7. of datasheet, and there is 5mV hysteresis so 125mv+-5mV).

0 Claire Chang over 2 years ago

TI__Intellectual 2760 points

Hi Sankalp,

Were you able to resolve this issue? If there are no more questions, I would like to close this thread.

Thanks.

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LM5021: Skip cycle bypass