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LM3914: LED current difference over four times

Yoshwa
Expert 4050 points
Part Number: LM3914

Hi team,

Our customer inquired large LED current difference between designed and measured value.

The attached schematic is a customer circuit aiming 3mA per channel.

The measure LED current was 136mA total.

What kinds of problems are considered?

We appreciate your help.LM3914.pdf

  • 0
    TI__Expert 6330 points
    Hi Yoshwa,

    I have notified our expert regarding this topic. Please expect a response by 12/13/18.

    Thanks,
    Aaron
  • 0 in reply to Aaron Goodson
    TI__Mastermind 18550 points
    Hello Yoshwa,
    I reviewed the schematics and since there is a 3.9K resistor (R17) across which the reference voltage of 1.25V is developed, constant current flowing through this resistor should be 0.32mA. Current through each LED should be ~0.32mAx10=3.2mA and since there are 10 LEDs, total LED current should be ~32mA. I do not see a problem with this schematics and unfortunately it is difficult to say what is causing the higher current.
    Kind Regards,
    Liaqat
  • 0 in reply to Liaqat Khan
    TI__Expert 4050 points

    Hello Liaqat,

    Very thanks for your response.

     However, at page 9 of the data sheet,

    “Current drawn by the internal 10-resistor divider, as well as by the external current and voltage-setting divider should be included in calculating LED drive current.”

     

    This means that the internal current passing through internal ten 1kΩ resisters should be added to ILED current calculation equation on page 2?

     Is this consideration correct?

    If so, should the ILED current be estimated from the following equation, assuming VOUT= 9V and R1=3.9kΩ?

    ILED = 12.5/R1 + 10×VOUT/10k＝12.5/3.9k　+ 10×9/10k = 12.2mA

    Which is correct?

    Customer measured value was 13.6mA per channel.

  • 0 in reply to Yoshwa
    TI__Mastermind 18550 points
    Hello,

    Yes, I am aware of the statement on page 9 of the data sheet but what threw me off was that you mentioned 136mA total LED current in your original e2e post. With that high a total LED current, I assumed that there is something grossly wrong with the application and I provided my 1st approximation explanation that excluded the impact of current flowing through the internal resistor divider. Now that you mention that the current in customer application is 13.6mA/channel, it makes more sense.
    So the calculation based on customer circuit will be:

    Vout=Vref * (1+R2/R1) + Iadj*R2 ------------ from page 9 of the datasheet
    Vout=1.25*(1+24.2K/3.9K) + 120uA * 24.2K
    Vout= 11.904V

    Please note that datasheet specifies +/- 30% tolerance of the internal resistor divider, so it can be total of 7K or 13K. With this, current through the internal resistor divider will be anywhere between 11.904/7K = 1.7mA to 11.904/13K = 0.91mA

    So total current out of the Refout (pin 7) can be between 1.25V/3.9K + 1.7mA = 2.02mA and 1.25V/3.9K + 0.91mA = 1.23mA

    So per channel LED current can be between 1.7mA x 10 = 17mA and 1.23mA x10 = 12.3mA

    You mentioned that customer measured value is 13.6mA so it is well within this range.
    Please note that I have not included other tolerances in this calculation above such tolerance of Iadj which is mentioned as typ.=75uA, max=120uA.
    I hope this explanation is helpful.

    Kind Regards,
    Liaqat