• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 62 subscribers
  • Views 410 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

LM3409HV-Q1_Advice for Total System Delay Calculation

hideyuki sakai
Mastermind 6250 points
Other Parts Discussed in Thread: LM3409HV

Dear, Sir. 

Delay from detection by LM3409HV to external PMOS ON/OFF(Total System)

would be related to Peak-current control, which in turn would be related to

averaging current accuracy. 

My customer would like to estimate that to calculate several portion which to be

creating the delay. 

PS) LM3409HV delay means the device detection to PMOS Gate On or Off

1. PMOS Turn-On. 

LM3409HV delay + external PMOS On delay(td-on + tf)

2. PMOS Turn-Off. 

LM3409HV delay + external PMOS Off delay(td-off + tr)

I wonder above understanding about delay creating portion

would be sufficient considering Total System delay(external PMOS)?

I would like to get your advice. 

Best Regards, 

H. Sakai

  • 0
    TI__Expert 3495 points
    Sakai:
    I understand the customer has concerns about the turn on/off dealys affect the LED current accuracy. I don't think it should be a big concern unless the customer operates the buck converter with high inductor ripple current and high switching frequency.
    If the peak to peak inductor ripple current is 10% of the average current and the total turn on delay is 10% of the on time, the peak inductor current is only 1% higher due to the turn on delay. The affects on the LED current accuracy will be less than 1% because the higher peak current caused by the turn on delay will be offsetted by the lower valley current caused by the turn off delay.
    The total turn on/off deay = IC internal delay + MOSFET rise/fall time. The IC internal delays are fixed. The MOSFET rise/fall time depends on the MOSFET gate charge.
    My suggestion is to keep the inductor ripple small and not switch at very high frequency.
    David Zhang
  • 0 in reply to David Zhang
    Mastermind 6250 points

    Dear, David-san. 

    Thank you for your valuable advice. 

     

    The IC internal delay are defined by  tDEL & tD-OFF. 

     

    Regarding PMOS delay(turn-on & turn-off), no enough explanation how

    to calculate those precise as well(referring Qgs, Qgd, etc). 

    Could you advice the calculation formula for PMOS delay(turn-on & turn-off)

    or introduce a written document? 

     

    Sorry, again and again. 

     

    Best Regards, 

    H. Sakai