• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 995 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.

LM3409: PWM Calculated vs Real

Dominic Leone
Prodigy 120 points
Part Number: LM3409

So i designed per the document and get a calculated value which is fine. of course the t_of and operating frequency is based on input voltage so if i am designing for a swing of voltage from 10-28VDC @ 250kHz, is there anyway to calculate the value at test voltage for 12V DC? Also i thought i read previously that this particular chip does a frequency shift during operation, so even though i calculate for 250kHz, it will frequency hop some range. did i miss understand this when i was reading it? the reason i ask is because i dont have usually frequency shifting i am used to seeing which looks like a vibration movement left and right on the Oscope, but rather i am getting a solid 2 frequencies during operation. It appears to switch between 297kHz and 350kHz. Is this a normal operation for the chip? Or what would cause the output to do this. I am measuring the frequency at the recirculating diode cathode. where the PMOS diode and inductor all intersect. I designed for 10-28VDC input, 3.1V output and 470pF and 10.5kOhm resistor for Coff and Roff. and use a 47uH inductor. 

  • 0
    TI__Mastermind 25500 points

    Hello Dominic,

    We can investigate further if needed because I'm not 100% clear on this entire post. But I think at least what you might be seeing based on the "frequency shifting" comment that it might just be the chopping action of the current sense comparator. Are you referring to the on/off times changing from cycle to cycle? That could be due to the chopping to eliminate input offsets and increase accuracy.

    If it is something else could you provide some scope shots of what you are seeing and clarify some questions? Just as a side note hysteretic control has many benefits but it generally also means that you rarely are exact with design equations since component tolerance comes into play. Usually a little bench time to tweak component values is required to tune it for the components you are using.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 120 points

    Yes i am referring to the on/off times/ for period 1 it operates at 297kHz. for period 2 it operates at 364kHz. for period 3 it goes back to 297kHz. From what you said i just want to confirm this is an expected result because my post was a little unclear. If this does not clear up what i am seeing i will go geta picture of the Oscope which shows 2 frequencies pulses overlaid on each other. you see a 297kHz and a 364kHz signal overlaid

    I complexity of the circuit operation within the chip and tolerances does make it tough to get absolute and i did find some source of errors in my calculations and results. 1 of which i calculated for a swing voltage, but since frequency is based on Vin, my frequency is going to shift as voltage changes. so i got closer to what i was seeing within 7% which is good for me.

  • 0 in reply to Dominic Leone
    TI__Mastermind 25500 points

    Hello Dominic,

    Yes, that is expected. You can look at section 8.3.3 of the LM3409 datasheet for an explanation of what you are seeing.

    Regards,

    Clint