• State TI Thinks Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 2 answers
  • Subscribers 62 subscribers
  • Views 552 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.

LM3488: Rsl Value & Vsl

hideyuki sakai
Mastermind 6250 points
Part Number: LM3488
Other Parts Discussed in Thread: LM51551, LM5155

Dear, Sir.

My customer is designing Flyback using LM3488.

I would like to make sure following. Please give your advice.

1. Figure 24 shows the relation between Rsl value & Vsl voltage.

    The other side Vsl volatge is defined 52 to 132mV on the datasheet.

    iF Rsl = 0 ohm, 52 to 132mV will be maintained?

    Or any protection will be activated?

2. I wonder Vsl setting will affect to Vsc(short-circuit CL) voltage?

Best Regards,

H. Sakai

  • 0
    TI__Genius 17590 points

    Hi Hideyuki-san,

    The 52mV to 132mV internal compensation ramp voltage is internal to the IC itself and not dependent on Rsl. If Rsl is involved, then the current limit will be changed since you are adding a ramp to the inductor current across Rsen. 

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Mastermind 6250 points

    Dear, Richard-san. 

    Thank you so much for your teaching. 

    Please teach me about Current Llimit, again. 

    Page 22, formula (15) is referred Vsense. 

    1. I wonder Vsense means the spec defined on Page 6? 

    2. If yes, there are 2 type of Current Limit, normal & short-circuit? 

    Best Regards, 

    H. Sakai

  • 0 in reply to hideyuki sakai
    TI__Genius 17590 points

    Hi Hideyuki-san,

    I've formatted my answers to your questions below.

    1. I wonder Vsense means the spec defined on Page 6? 

    ANS: Vsense is defined as the voltage that will trigger the cycle by cycle current limit. This is different than Vsc, which is the short-circuit current limit sense threshold voltage. 

    2. If yes, there are 2 type of Current Limit, normal & short-circuit? 

    ANS: Yes, you are correct- there are 2 types of current limit. Normally you will trigger the normal cycle by cycle current limit which is set by Vsense, but you can hit the short-circuit current limit if you exceed the threshold set by Vsc during the blank-out time of the IC. Further detail can be seen in section 7.1 of the datasheet here: http://www.ti.com/lit/ds/symlink/lm3488.pdf

    Additionally, has your customer considered using the LM5155? It is our newest part and has lower quiescent current and higher efficiency in designs than the LM3488. It also has a variant called the LM51551, which contains hiccup mode, which is used for short circuit conditions. You can see it here: http://www.ti.com/product/LM5155-Q1

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Mastermind 6250 points

    Dear, Richard-san. 

    Thank you so much for your advice and teachings. 

    I would like to make sure the relation between Vsl & Vsense. 

    According the definition on the datasheet, Vsense min. = 125mV

    and Vsl max. = 132mV. So, if this combination was existing, 

    cycle-by-cycle CL would be always activated. 

    Is there a any relation or tendency between Vsl & Vsense? 

    Please give your advice. 

    Best Regards, 

    H. Sakai

  • 0 in reply to hideyuki sakai
    TI__Genius 17590 points

    Hi Hideyuki-san,

    Vsense is not a constant value and can change as a function of the internal compensation ramp. You can see this in section 8.2.1.2.4

    Vsl represents the maximum amplitude of the slope. The addition that you are seeing is dependent on the duty cycle so your case may not actually be feasible.

    However, if you are dealing with extremes, it is advised to not use the maximum and minimum limits of Vsense and Vsl. Instead, the electrical characteristics table has a relation between Vsl & Vsense called Vsl ratio.

    You can use that and eq.18 in tandem to approximate a reasonable Isense limit or Rsen range. 

    Best Regards,

    Richard

  • 0 in reply to Richard Fu22
    TI__Genius 17590 points

    Hi Hideyuki-san,

    I will now be closing this thread since there has been no response. Thank you for asking these questions.

    Best Regards,

    Richard