• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 2727 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5175: difference between average and cycle-by-cycle current limit

Ann_Lien
Genius 14430 points
Part Number: LM5175

Hi expert,

I'd like to know the difference between ISNS+/ISNS- current limit and over current limit.

As I know, these tow ways both senses current(input or output / LS-MOSFET ). Cycle-by-cycle current limit will occur when current hit current limit threshold then controller will smaller PWM pulse to limit current. And also we could only sense LS current.

In the other side, average current limit will sense current at input/output to limit. When Iin/Iout hit current limit, controller will discharge SS capacitor. When voltage on SS capacitor is lower than 0.8V, controller will decrease voltage on Vout to limit current. And we could sense current any time.

In conclusion, I have two questions on it,

  • Why we need to apply two ways for current limit? Or what situation will apply two current limit way in same application?
  • If we want to apply two ways in one application, how can we set current limit on it? For example, 6V~24V->12V/5A, OCP=5.5A.

Thank you!

Best regards,

Ann Lien

  • 0
    TI__Guru* 78200 points

    They serve different purposes. The peak current limit is to protect the inductor from saturation, and the peak limit should always be set below the Isat of the inductor. Average current limit is to limit the dc current from the input or to the output, depending on where you place the sense resistor. In a switch mode power supply the switch peak current and the input or output dc current are related but not at a fixed ratio. The same peak current will support different dc current when input or output voltage changes. Most dc-dc controllers only have peak current limit. However, the LM5175's applications include the USB or battery charger, and both applications require well controlled dc current limit.  So, it is included in the LM5175.

    To your 2nd question: set the peak current limit to always support your 5.5A load within your entire operating range, as given in the datasheet, and set your average current limit to 5.5A.

  • 0 in reply to Youhao Xi
    TI__Genius 14430 points
    Hi Youhao,

    Thanks for your help!

    Bets regards,
    Ann Lien