• State Resolved
  • Locked Locked
  • Replies 10 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 226 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.

LMR51450-Q1: Frequency foldback

DARSHAN M V
Prodigy 30 points
Part Number: LMR51450-Q1

Tool/software:

Hello team ,

I need to understand what is frequency foldback? which is mentioned in datasheet

  • 0
    TI__Genius 14456 points

    Hello Darshan

    The frequency foldback feature is helpful under Min-Ton and Min-Toff operating scenarios. Once device hits Min-Ton limit, it loses control over output and out ripple increase. While under Min-Toff scenario, device fails to regulate. This feature allows device to maintain regualtion under Min -Ton/ Min-Toff scenarios. I would highly recommend you to watch following video to get clear understanding

    Introduction to Buck Converters: Minimum On-time and Minimum Off-time Operation - YouTube

    Thank you

    Regards

    Onkar Bhakare

  • 0 in reply to Onkar Bhakare
    TI__Genius 14456 points

    Hello Darshan

    I hope the video was helpful. Let me know if you need further help

    Thank you

    Regards

    Onkar Bhakare

  • 0 in reply to Onkar Bhakare
    Prodigy 30 points

    Hello Onkar ,

    Thanks for providing me the technical support to understand the concept ,I have few more questions before I decide to use the IC

    • When does the IC shift from PWM mode to PFM mode ?- what is the value of current considered as low load current 
    • How will the IC sense the load current when it decreased or increased - to change its mode?

    During the PFM mode my understanding is frequency remains same .Let's assume if the buck is operating at 2.1MHz PWM mode during the full load and when there is a light load buck will shift to PFM mode from PWM mode by decreasing the ton time and increasing the toff time but the total time T remains the same - is my understanding is correct ?

  • +1 in reply to DARSHAN M V
    TI__Genius 14456 points

    Hello Darshan

    DARSHAN M V said:
    • When does the IC shift from PWM mode to PFM mode ?- what is the value of current considered as low load current 

    Device will enter PFM if inductor peak current falls below I_PEAK-MIN. Value of load current at the transition point will be determined by Vin, Vout, Inductance and Ton

     

    DARSHAN M V said:
    • How will the IC sense the load current when it decreased or increased - to change its mode?

    IC has inbuilt current sensor to sense FET current. This information is used to change the mode.

    DARSHAN M V said:
    During the PFM mode my understanding is frequency remains same .Let's assume if the buck is operating at 2.1MHz PWM mode during the full load and when there is a light load buck will shift to PFM mode from PWM mode by decreasing the ton time and increasing the toff time but the total time T remains the same - is my understanding is correct ?

    No, during PFM IC will reduce frequency. Relationship will approximately linear.

    Hope this clarifies the doubts

    Thank you

    Regards

    Onkar Bhakare

  • 0 in reply to Onkar Bhakare
    Prodigy 30 points

    Hello Onkar,

    Referring to the above question on frequency reducing in PFM mode - what is frequency it is going to reduce when it enters to PFM mode?

  • +1 in reply to DARSHAN M V
    TI__Genius 14456 points

    Hello Darshan

    DARSHAN M V said:
    what is frequency it is going to reduce when it enters to PFM mode?

    Frequency will be load current dependent. Additionally, choice of L, Vin, Vout would also affect switching frequency.

    Thank you

    Regards

    Onkar Bhakare 

  • 0 in reply to Onkar Bhakare
    Prodigy 30 points

    Hello Onkar, 

    I have one general query w.r.t Buck IC, Please refer to the below statement

    Let's assume if the buck is operating at 2.1MHz PWM mode during the full load and when there is a light load buck will decrease the ton time and increasing the toff time, but the total time T remains the same so that Frequency remains same, so that  efficiency will increase - is this feature can be seen in any buck IC , If yes what is the mode name ?

  • 0 in reply to DARSHAN M V
    TI__Genius 14456 points

    Hello Darshan

    If frequency remains same, you won't get any benefits under light load. Switching losses dominates over conduction losses at lighter load and these losses are function of frequency.  Thus, PFM helps here as this mode gradually reduces frequency with load once converter enters into it.

    Thank you

    Regards

    Onkar Bhakare

  • 0 in reply to Onkar Bhakare
    Prodigy 30 points

    Hello Onkar,

    I have few questions w.r.t the above part number.

    What is the FPWM mode what are advantages?

  • +1 in reply to DARSHAN M V
    TI__Genius 14456 points

    Hi Darshan

    Following are the advantages of FPWM over PFM

    1. Fixed switching frequency is maintained throughout the line, load conditions result in predictable EMI performance and thus easier to filter and comply with EMI standards 
    2. FPWM offers lower and more consistent output voltage ripple, especially under light load conditions
    3. It gives better transient response

    While FPWM has mentioned advantages, PFM is superior in for maximizing light load efficiency.

    Hope this helps

    Thank you

    Regards

    Onkar Bhakare