I have a learning opportunity that I need some help with :)
I have current regulator design I put together using the LED2001 IC for driving laser diodes. This is mounted on top of a heatsink module which is mounted to an open frame CNC machine.
Here are the Kicad source files. Kicad Source
The issue that I'm having is that when power is first applied to the board, the dim/enable pin needs to be held high for a brief period before the output of the LED2001 will operate on pulse widths of 1-99% (0% being 0vdc & 100% being 5vdc).
If I don't apply this technique when I first power it on, the output will not turn on until I go above 25% on the pulse width. When cutting stuff this is not a problem as the power is usually set to 100% for the cut opperation but when I'm engraving an image and the start of the engraving happens to be in a light area of the image it will not turn on the laser until it reaches a dark part of the image.
At first I thought it might be an issue with my design but I have since found references to similar designs that seem to indicate that this has something to do with the LED2001 IC.
A suggestion was made to me to disable the output while holding the dim pin high for a brief period at start up (to let the caps charge) and then bring the dim pin low and then enable the output, allowing the PWM signal to control the output after that.
I could slap a mcu on the board to do this but I thought that this would be a good opportunity to learn how to accomplish this without using a micro. I have been searching google for ideas but I don't think I'm using the right key words.
If this is trivial and you want to post a schematic, that is fine. I'll take it and try to learn how it works. OR, if you don't mind pointing me in the right direction or helping me learn the parts that would make this work, I can certainly do the research.
Edit: Here are some screenshots from my O'scope.
- 16% PWM after applying power to the board.
- 97.3% PWM, not quite fully on yet..
- 97.7% PWM, the driver is on and full PWM range is available.
- back to 16% PWM.
Edit: since LTSpice didn't have a NOR gates built in, I found an example using some inexpensive BC817 BJTs. Here is what I came up with that seems to work ok based on the feedback provided in the comments.
I ended up with two NOR gates and one NOT gate which seems to do what I need it to do. When the device is powered on, it should hold the DIM pin high for about 1.5ms and then return back to 0V and stay there until the PWM signal is present, at which time it will control the DIM pin.
The output of the first NOR gate is also used to offset the feedback pin voltage at startup which should limit the output current and basically shutting off the output without adding a mosfet as a switch inline with the output.
Here are some screenshots from LTSpice.