LED Driver Voltage Issue

Hey All,

I'm having an issue setting up the LTM8042 current driver from linear technology.

My issue is the supply voltage after PWM is to high at 11volts to the LED. Current is good at 1 amp.

I've attached the schematic for LTspice, you just need to change the .txt to .asc for it to work.

I've also attached the data sheet for the led.

Any help would be greatly appreciated. Thank you.

View attachment XLampXML_Color.pdfView attachment Schematic.txt

its working fine, I am not sure what you are worried about. I haven't read that datasheet ltm8042, BUT YOU HAVENT ANY OVERCURRENT IN THE LED (sorry about caps).

Current is good and working like it should. The issue is it's supplying like 11 volts to the led, max volt rating is around 3.7 volts.

Also here are some photos (of simulation and schematic) for people who do not have LTspice program.

Thank you!

**broken link removed**
**broken link removed**

The LED voltage is 3.4V. I guess you have issues to measure it correctly.

Hint: It's the difference between two node voltages.

Hi,

Did you measure voltage across the LED? V_anode - V_ cathode?

If yes, then i assume the model for the LED is not correct.

Klaus

Added:
... just some minutes too late..

Thanks for the help guys.

Voltage drop across the LED is correct at around 3.5 volts. But voltage at the anode is 11.8, and volts at the cathode is 8.4 volts.

But voltage at the anode is 11.8, and volts at the cathode is 8.4 volts.

Click to expand...

Why "but"? That's how the circuit works.

The LED doesn't "see" the absolute node voltage, only the difference.

Thanks for you help very much appreciated.

Trying to visualize how that doesn't damage the LED due to having to much potential going through the LED, current flow is regulated with the transistor on the IC, but trying to shove to many free electrons through than the LED material consists of going to burn it up?

Can you source a good read or video explaining it for me? Still learning all this stuff.

Hi,

As said before: your LED only sees the difference between anode and cathode, and this is what counts.

It doesn't know what "you decided to be GND".

******

Imagine two 9V batteries. Connect them in series.

* Now decide the free "-" to be GND. Now the center point is +9V, and the free "+" is +18V.
* Now decide the center point to be GND. The free "-" now is -9V, and the free "+" is +9V.
* Now decide the free "+" to be GND. Now the center point is at -9V, and the free "-" is at -18V.

... This are just three different decisions, without changing a single connection.

Obviously every single battery is 9V all the time..

Klaus

I believe, the confusion is already in the terms. A potential or a voltage goes never "through a component". I suggest basic literature about DC voltage and current, e.g. the first chapters of the "Lessons in electric circuits".

https://www.ibiblio.org/kuphaldt/electricCircuits/DC/index.html

Klaus,

The way you worded the fact that it doesn't know what ground is allows to to better visualize your point. The diode needs the difference in voltage to close and conduct, doesn't matter what levels they are. Thank you for that! Very helpful.

FvM,

Thanks for the link loads of free information there! And different analogies I've not seen before. Thank you!

My concern wasn't how the diode conducts through differences in voltage levels on Anode and Cathode side, but you did help me discover something I didn't know in that the difference doesn't matter if its referencing 0v or 8v as long as it's proportional. My concern is that the voltage rating of insulation on the diode and the insulating characteristics in the diode will be exceeded at 11 volts. I can't seem to find the absolute maximum voltage rating, only forward voltage.

Thanks again guys, helping me learn means a lot!