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LM5023: Relationship between IQR and VCS Offset voltage and Skip cycle operation

Kang
Prodigy 30 points
Part Number: LM5023

Hello. I am Mr. Kang

I have a question while testing SMPS using LM5023MMX-2/NOPB.

I know you are busy, but I would really appreciate your reply.

The existing smps product operated in skip cycle mode at light load, so by changing the QR pin distribution resistor and capacitors R1, R2, Cd and CS pin resistor Rexternal, it escaped skip cycle mode and operated normally at the same load.

Iqr and cs offset voltages have increased. Will this have any effect?

Before change
Iqr: 0.5mA
VCS_OFFSET: 0.039V
QR pin voltage: 6.88V

After change
Iqr: 1.1mA
VCS_OFFSET: 0.12V
QR pin voltage: 6.66V

The datasheet recommends an Iqr value of 1~4mA. Will there be a problem if I deviate from this value?

Thank you.

  • 0
    TI__Mastermind 21835 points

    Hi Mr. Kang,

    Thank you for the query on LM5023 and we are glad to help you.

    To answer your querry, the value of IQR is typically chosen at high line and it is required to be between 1mA and 4mA.  This means IQR will decrease as the input voltage decreases. So you will have to ensure this condition is met. Even the design example shown in the application note ensures that this value is 1.75mA at high line and at low line it typically falls below 1mA (I think it is 0.6mA). 

    The observation you have makes sense as with increase in Iqr the CS offset is expected to increase and this will limit the peak power delivered at high line by providing the required feed forward voltage.

    Regards,

    Harish