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