• State
  • Locked Locked
  • Replies 5 replies
  • Subscribers 64 subscribers
  • Views 445 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.

LM3429 current limit without R(lim) - how?

bowerymarc
Expert 1935 points
Other Parts Discussed in Thread: LM3429, LM3421

In the LM3429 datra sheet page 10, section "CURRENT SENSE/CURRENT LIMIT" it is mentioned that there are two methods to sense transistor current.  It goes on to detail method two, using a resistor in series with the main MOSFET and connecting the IS pin to the source.  But method 1 is not detailed, it's only mentioned that the Rds(on) is used as the current sense resistance. 

So, what's the formula?  Would it be:

I(lim) = 0.245/Rds(on) ?

(also is there any way to get web-bench to use this topology)

looks like the same question would apply to the LM3421 etc.

  • 0
    TI__Expert 7250 points

    Hello Marc,

    The current sense/limit is further detailed on page 14 in the LM3429 datasheet or page 16 in the LM3421 datasheet. Specifically the paragraph following figure 7 which notes:

    Alternatively, a sense resistor located in the source of the MosFET may be used for current sensing; however a low inductance (ESL) type is suggested. The cycle-by-cycle current limit (ILIM) can be calculated using either method as the limiting resistance (RLIM):

     

    Also please note in figure 7 the connection change required between RDS-ON and RLIM sensing methods. I will check with the webench team to see if there is a way to model this in webench. If you have any other questions in the meantime please let me know.

  • 0 in reply to PC
    Expert 1935 points

    Patrick: please carefully reread my post.  I explicitly asked how to calculate I(lim) without the resistor, using only the MOSFET Rds(on). 

    Thus, my original question remains unanswered.  Please try again.

    Thanks,
    Marc 

  • 0 in reply to bowerymarc
    TI__Expert 7250 points

    Hello Marc,

    First, you cannot calculate Ilim without a resistor. There is an internal resistance to the Mosfet  and this should be used. Please see below the step by step proofs of how to calculate the Ilim using the Rdson of the Mosfet.

    Please note that if you choose to use the Rdson of the Mosfet you will need to size the FET based on the Rdson. The Rdson of the FET will change across temperature as well so this will need to be taken into account when calculating your current limit.

  • 0 in reply to PC
    Expert 1935 points

    Hi Patrick,

    So you have verified that for my first question, I had the correct answer.  I suggest a section be added to the data sheets to clarify this.

    My second question is still unanswered - how to get WebBench to use the topology where you're using Rds(on) rather than a separate resistor? 

    If so, WebBench should be able to chart I(lim) vs MOSTFET temperature, since it knows the latter as part of it's simulation.  It could also use a user input desired I(lim) to select the MOSFET.  Please investigate and let me know.

    In fact, it might be advantageous to use Rds(on), as you get a kind of foldback as the temperature goes up.  I hope National/TI engineers can investigate this and write something up for the users. 

    Thanks,

    Marc

  • 0 in reply to bowerymarc
    TI__Intellectual 2930 points

    Hi Marc,

    Im sorry, but this setup for the schematic is not currently enabled in WEBENCH. 
    If this is something you're trying to do I'd be happy to provide support while you design though. 

    Best regards,
    Tommy Jewell
    WEBENCH Apps