This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3409HV Noise

Other Parts Discussed in Thread: LM3409HV, LM3409

HI:

I have designed a circuit using LM3409HV in order to get a driver PCB, to pulse a string of two leds at 25 fps (unsing enable pin), with 500us of exposure time. The input voltaje is 48V, the output I is 3 A, and in that condition the expected output voltaje in the led string is 26.8V

Others parameters, (some parameters are just a guessing, following manual and datasheet examples) are:

Nominal Input 48
Nominal Output 26,8
Led String R 2
Switching frecuency 800000
Average Led Current 3
Inductor Ripple Current 0,6
Led current ripple 0,05
Input voltage ripple 0,2
UVLO
Expected Efficiency 0,9
Coff

4,7E-10

 

 

The final values for this calculations are

Roff 20442,81887
Toff 4,74537E-07
Fsw 800000
L1 2,1196E-05
Rsns 0,075151515
Ton 7,75463E-07
Cin 1,16319E-05

SO I choose the following values that you can see in my circuit.

My problema is that, despite my circuit "Works", is it not working as expected. If I run the circuit with no loads (no leds string connected), I obtained a clean signal at the output (between SAL and GND). Maybe is ok (I am not sure if output voltaje need toi be equal to input voltaje with no load)

But when I conected the load, The signal is tooooo noisy. And this is not suitable to make a reliable driver.

There are more than 3 V peak to peak in that signal. I dont really know what I have to check. I can see that also the output voltaje expected at 3 A it is not what I found (37 V at least in output signal), but this is also a question for the led manufacturer   .

Maybe I'm wrong with calculation, or I missed something that I didnt realized. Maybe It is needed some circuitery to prevent the return current of the L or the leds. Or maybe is the layout. I am not sure, but I appreciatte if you can give me some advise about how can I find this problem, or what can I try

I can give more details if needed.

Thanks

  • I am not 100% sure I understand what you mean by "noisy". Is there any way you could provide some scope shots? It would be good to see the waveforms at the switch node (where Q, D, and L intersect), the output voltage you mention, and the LED current if possible. Having said that are you using an output capacitor? If you have a full 600mA inductor current ripple through the LEDs a 3V resulting ripple isn't out of the question. Also, depending on the LEDs you are using and how they are specified you can get a significantly different output voltage. For example, if they are specified to be 26.8V at 1A and you drive 3A through them 37V might not be out of the question. You should check the LED Vf vs I curves to be certain.
  • Hi there:

    im sorry I have attached the scopes, but nothing appears. Let me check if I can attached some image in my next post.

    Regarding your question, 26.8V is the voltaje expected for 2 leds at 3 A in the I-V curve.

    Looking at the pictures it could be clarified.

    This is the output voltaje before load

    This is the output with load

    And this is my circuit:

    Best regards

  • I see the scope shots but I do not see the circuit. That would be good to see because I am not sure how you could get such a high output voltage. Do you have extremely long, high resistance traces to the LEDs? It would be really nice to see the LED current as well, that information is much more telling than the output voltage for an LED driver. In any case the LM3409 is inherently stable, so if there are noise issues it is more likely due to layout. If you could provide the circuit and the layout for it I would be happy to look over them for problems. You can do a private message if you do not want the information on a public forum.
  • Hi Clinton:

    I will be glad to provide you more information. Just let me know how can I send a private message

    thanks
  • You can now go to friends and click "connect" under my name to send private messages.