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Need help with the LM3466 multi Series/Parallel Configuration

Other Parts Discussed in Thread: LM3466

Let's say I am using the CREE CXB1816 LEDs which require 35V @ 450mA:

Using the LM3466 this would allow me to create a Series string of two deep and then based on my constant current limits I could do several Parallels.

For illustration purposes using your example I will have two deep and five parallels, so this configuration will require 5 LM3466:

(Each 35V 450mA is equal to one LED)

35V 450m  35V 450m  35V 450m  35V 450m  35V 450m

35V 450m  35V 450m  35V 450m  35V 450m  35V 450m

So in this example I need a power supply that provides 2*35 = 70V and 5* 450mA = 2,250mA

Question: How do I determine the Cin, Cled, Rsen, Req,Ceq for my application.

I apologize if this is a BASIC knowledge question but I would appreciate any help you can offer.

Thank you, Denis

 

 

  • Hi Denis,

    The power supply needs to be a current source not a voltage source so it needs to provide 2.25 amps of current with voltage capability above 70 volts for headroom and LED variance.  This needs a current regulator not a voltage regulator as the source.  Also since 70 volts is near the ABSmax of the LM3466 Vin I would not directly connect the current source to the LM3466 Vin.  There will be voltage overshoot during transients and other disturbances.  There is a section in the datasheet that describes high voltage applications.  The additional MOSFET may or may not be needed depending on the range of the LED Vf.

    Cin for the LM3466 is really the Cout of the current source (besides the local bypass capacitor).  It depends on what the current source is, single stage PFC, two stage, etc or this could be from a DC source?

    Cled is also, from a current ripple standpoint, Cout of the current source and the topology of the current source.  The Cled local to the LM3466 is for designs with long wires to the LEDs, the datasheet mentions this.  This is to reduce noise back to the Iled.

    Rsen I would set to around 0.65 ohms.  It can go lower if power dissipation is a concern but string to string accuracy will degrade.  This really isn't a current setpoint, the LM3466 just tries to maintain equal current through each device.

    Req and Ceq are recommended to be 51.1 ohms and 1  uF.  It will also depend on how the current source loop response behaves as a complete system.

    Also to take into account are fault conditions.  If an LED string opens the current source will voltage overshoot.  If an LED string shorts that voltage will be applied to the Iled pin.

  • Hello Irwin,
    I appreciate your prompt response. I have put in considerable effort in finding a power supply for this application, at the moment I am looking at the Genesys Gen 80-30 which will go up to 80V and up to 30 amps of constant current, both of these are adjustable on this unit. My challenge is protecting each Series should one or more LED fail. It concerns me a bit that you feel I am at the limits of the LM3466: Does TI have any solutions with higher Vin and balancing potential? I already felt that this unit was limiting me because I would prefer to have more than 2 LEDs per Series string, but the 35V requirement of the CREE LED CXB1816 puts me at the 70V limit. Any suggestions you have would be appreciated. The Genesys Power supplies look promising with some Very high voltage/amp offerings that will allow me to dial in the constant current and voltage. This would allow me to create much larger arrays of LEDs. I need to power and protect about 1,300 of these LEDs. Irwin, thank you again for any help. Also, you can not explain it in too much detail, I am very much a newbie at this. Denis
  • Hi Denis,


    I would be a bit concerned about the lab supply route.  They are intended to work as a voltage source though they can go constant current.  I would really check to see how it functions as a current source with load steps.  I would try and search for LED ballasts, current sources basically, that you can add the LM3466 circuit to the output.

    The datasheet shows how to go up in string voltage if desired.  The fault conditions may need to be looked at to make sure the circuit can handle it.