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LM3489 Current limit operation

Martin Novotny
Prodigy 50 points
Other Parts Discussed in Thread: LM3489

Hi,

lm3489 buck controller has an internal 5.5uA current source that vary from 3uA to 7uA (lm3489 datasheet). What function is current source of (ie. what causes the current change 3uA-7uA)? If I measure for example 5uA current can I assume this current is relatively stable and vary only with temperature approximately in range 4,8uA to 5,2uA(datasheet) or even this measured source(5uA) can vary from 3uA to 7uA?

Thanks.

  • 0
    TI__Genius 11490 points

    What current are you talking about Martin? Can you write the spec name? Usually internal currents vary becasue of multiple things including process and temperature.

  • 0 in reply to Vijay86929
    TI__Prodigy 310 points

    Hi Martin,

    I guess you were referring to the current limit ADJ current. There is a typical performance curve of the ADJ current vs. temperature on the page 5 of the datasheet. That can give you some idea of how ADJ current varies with temperature and VIN.

    Regards,

    Min

  • 0 in reply to Vijay86929
    Prodigy 50 points

    I am talking about the current limit ADJ current (ICL_ADJ) as mentioned Min Chen. In the graph (page 5 of the datasheet) the current vary approximately within interval 5,2uA - 5,7uA but the ICL_ADJ is characterized by 3uA - 7uA (page 3 min and max value). I need to set the current limit to 2A - 2,5A, but it seems if the ICL_ADJ vary 3uA - 7uA the current limit vary 2A - 4,6A. I need higher accuracy. It is possible with LM3489? Is the min and max value (3-7uA) given only by production and then the ICL_ADJ dont vary so much and vary only as shown in the graf (page 5, Current Limit ADJ Current vs Temperature) ?

  • 0 in reply to Martin Novotny
    TI__Prodigy 310 points

    Hi Martin,

    The min, max limits in the EC table have considered lot variations, test guard bands, statistical correlation etc.

    Below are some trip-temp characterization data. Please note that these are just for reference only and won't be guaranteed by the production.

    Test Name

    Units

    Temp (deg C),      TA=TJ

    Avg

    Min

    Max

    Current Llimit ADJ current source

    uA

    25

    5.63868

    5.03

    6.13

    125

    5.39723

    4.98

    5.76

    -40

    5.56205

    4.69

    6.150
  • 0 in reply to Min Chen
    Prodigy 50 points

    Hi Min Chen,

    my english is poor and probably I cannot explain the issue well. I will ask you a different way... I produce DC/DC with LM3489 and I set the current limit circuit (figure 5 on page 10) to limit 2A using ideal resistor Radj and Rcs. When I test the DC/DC first time I just measure limitation of 2A. Then I provide the DC/DC to customer that will use DC/DC in stable temperature 25°C, what min, max value of limited current can I guarantee?

  • 0 in reply to Martin Novotny
    TI__Prodigy 310 points

    Hi Martin,

    If you are making mass production, it is important to include sufficient margin in your design to accommodate parameter variations.  We cannot guarantee the spec limits different from the datasheet.  Sometimes, additional screening can be applied in the production to guarantee tighter spec limits. However that is per customer's request, requires a special procedure and extra cost.

    Hope this helps.

    Min

  • 0 in reply to Min Chen
    Prodigy 50 points

    Hi Min,

    I am not making mass production, I am making only one prototyp needing set 2A-2.5A limitation but with current source 3-7uA is not possible. I only wanted to know if I set this one prototyp to 2A limitation and I measure the current source has for example 5uA whether the current source can change over the whole specified range 3-7uA or change aproximatley only with temerature.

  • 0 in reply to Martin Novotny
    TI__Prodigy 310 points

    Hi Martin,

    The ADJ current limit of an individual unit  varies mainly with the temperature. If you have tested the current limit of your prototype under the required operating conditions, customer should expect the similar results under the same operating conditions.

    Min

  • 0 in reply to Min Chen
    Prodigy 50 points

    Hi Min,

    that is what I need "Current limit varies mainly with temperature". Thank you very much!

    Martin