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LM3409. 1.5 A less in LED string than expected

Gonzalo Garcia
Prodigy 150 points
Other Parts Discussed in Thread: LM3409

Hi all:

I made a design using LM3409, following one of the example design in the datasheet.

It has been designed with the following parameters and components:

Vin=44, Vout=29.2V, I=4.5 A, Enable pin : dimming at 15 Hz, Iadj pin totally Open. With those requeriment, I put  

Roff:33k

Coff:470 pF

Ruv1:6.98k

Ruv2:49.9k

Cin:56uF

Cf:1uf

Rsns:50mohm

L1:22uH

Cout:none

I have made my circuit calculating 4.5 A , But I only get about 3A. Leds string can reach 5 A pulsed

I have recalculate the components values, but it seems correct. Do everyone have the same problem, or the expected intensity value is aproximately the same tan the one you get in the real circuit? I am quite confusing about which componets influences these values or what is the problem in the desing or component selection  to get 1.5 volts less that expected.

I am thinking  in the PFEt, that has a ver high RDsON (280 mOhm) or L1, but I dont really know why.

Any help about what can I check could be very usefull

Best regards

  • 0
    TI__Expert 3495 points
    Gonzalo:
    Please measure the LED current without any output cap ( That is also the inductor current ). If everthing is correct, you should see a peak current of 5A and about 1A peak to peak ripple.
    The MOSFET RDSON is too high for 4.5A cuurent. It will get hot. You need a better MOSFET. I don't what inductor you are using. Please make sure it can handle at least 5A current.
    David Zhang
  • 0 in reply to David Zhang
    Prodigy 150 points
    Hi:

    Sorry, I do not understand what "output cap" means . Do you mean meake the measure without inductor?

    I have measure intensity putting an amperimeter in series with the led string and also i have measure the output voltage at the same time with osciloscope. As i am strobing, I can see eficaz intensity. So I can calculate maximun I. This value is the same than the one i obtained from checking the intensity in led datasheet from the voltage value i obtained. More than that, i do not know how i can see peak to peak intensity

    I do not know what I am doing wrong, but i have been making some changes in the components value in order to see how the intensity value changes. I was shocked when i changed rsns to 30 mohm and the intensity was exactly the same.

    Maybe this could help

    Best regards
  • 0 in reply to Gonzalo Garcia
    Prodigy 150 points

    Hi

    I just realized that you mean without any output capacitor.

    I just have measure without any output cap and the result values are the same. I just get about 3 A, the same tan before

    Also:

    Q PFET:MOSFET-P IRF9Z24SPBF

    Diode: b560c-13-f

    L1:Panasonic ELC18B220L

    Could  a high RDSON drops the intensity so much? Lets say my RDSON is 280 mOhm, compared with a PFEt with RDSON about 25 mohm?


    Any ideas?

    Thanks and best regards

  • 0 in reply to Gonzalo Garcia
    TI__Expert 3495 points
    Gonzalo:
    What I meant was to use the current probe to measure the LED current waveform. Do you have a current probe to use?
    David Zhang
  • 0 in reply to David Zhang
    Prodigy 150 points

    No. I dont  have any current probe for the osciloscope. I use amperimeter in series and extract the intensity from average intensity knowing duty cycle. I have obtained 3.5A .

    At the same time i use a normal voltage probe and see the voltage waveform and i see that voltaje values are the expected for 3.5 A for that leds.

  • 0 in reply to Gonzalo Garcia
    Prodigy 150 points
    Sorry i meant 3 amps, not 3.5