What is the best way for 3 LEDs on ADD5211s?

Dear Sir/Madam,

Our customer will use 3 LEDs/200mA each LEDs on ADD5211.

So what is the best design for 3 LEDs?

eg;

case-A) FB1===LED1, FB2===LED2, FB3===LED3, FB4===open

case-B) ganged FB1+FB2+FB3===LED1+LED2+LED3, FB4===open

etc,,,,,

Please advice us what is the best way.

Thanks Kaos

  • Please advice us customer will start to design PWB tomorrow by your ADD5211.

    thanks Kaos

  • Hi Jon,

    I already informed like as following to our customer.

    ganged FB1+FB2+FB3+FB4===LED1+LED2+LED3+LED4 by serial connection

    Additional comment also thanks.

    Kaos

    •  Analog Employees 
    on Jul 6, 2016 10:40 PM

    Hi Kaos,

    The best way to run the ADD5211 is to have all four current sinks operating.  This reduces the current per sink and also reduces the headroom voltage required for the sinks.  So that saves a lot of thermal power dissipation on the ADD5211 package.  So I would recommend having the 3 LEDs connected in series and then connect all 4 FBx pins together.  And then set each sink to 50mA.  So that will give 200mA through the LEDs. 

    Also, please keep in mind, that the ADD5211 is a boost regulator.  That means that its input voltage must always be less than the output voltage.  With three LEDs in series, the output voltage will be:

    Vout=3*Vf(200mA) + Vhr(50mA)

    Where Vf(200mA) is the forward voltage of 1 LED at 200mA of current.  And Vhr(50mA) is the headroom voltage of one current sink at 50mA.  Which will be about 0.44V (typ).  You can find that equation on p. 11 of the datasheet. 

    Best Regards,

    - Jon