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CCS/LM3463: Driving 6 LED strings using two LM3463

Ryan Delfino
Prodigy 150 points
Part Number: LM3463

Tool/software: Code Composer Studio

Hi there,


We have 6 LED strings (28 LED / string) in  our design and we plan to drive them with below circuit:

Here are some specs of these LED strings:

  • LM3463 [Slave]
    • Current per string = 300mA
    • Max.Vf per string = 93.8V
  • LM3463 [Master]
    • String #1 current = 300mA
    • Max.Vf string #1= 89.6V
    • String #2 current = 460mA
    • Max Vf string #2 = 47.6V

Our concern is that on the LM3463 [Master], we may have MOSFET power dissipation issues.

Can you help and confirm if this may be true, and help advice a workaround if needed ?

Thank you.

Regards

  • 0
    TI__Mastermind 28655 points

    Hello Ryan,

    Yes, you will have MOSFET power dissipation issues.  As an example, 93.8V - 47.6V = 46.2V, at 460 mA the MOSFET will have to dissipate 21.25W.  The MOSFET you have chosen is not intended to dissipate power so it will probably have issues with the other strings as well.  A 4V difference in the LED string voltage will cause 1.2W of dissipation, it will also exceed the SOA (safe operating area) of the MOSFET chosen (use the DC line since this is a linear).  The EVM uses FDD2572 at lower current, 200 mA.  The FDD2572 SOA allows it to be used with full voltage across it at 0.2A and it can have proper heat sinking.

    I would recommend either using a separate converter for Master #2 string or arranging the LEDs so it is closer to the other string voltages.  If you use a large enough MOSFET it will work however it will be dissipating 20W or so.

    You also do not have dynamic headroom control connected, see page 20, 21 in the datasheet.  Your strings are close to the maximum operating voltage of the LM3463, I would check circuit operation to make sure it does not have issues with maximum voltage such as at start-up.

    Best Regards,