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Weird issue with STM's LED2001 and PWM

iceman1979

Hi All,

 

 

I am working on a laser diode driver design using the LED2001 ic from STM for use with these little laser engraver kits we are seeing coming from places like Banggood.com and Gearbest.com. The reason for the design was to get better linear control of the laser diode, particularly for use with engraving images. So far the testing has gone fairly well with the exception of the first time the driver has power applied to it and the PWM signal is sent to it.

 

For those interested in seeing the design, the Kicad source can be found here https://dl.dropboxusercontent.com/u/5570668/Laser%20Drivers/PWM_CC_LaserDriver.zip

 

The laser engraver machines come with a controller board that uses an Arduino Nano to driver the laser, stepper motors, and communicate with the PC. The Nano outputs a PWM signal of about 1khz which drives a mosfet that drives the laser module. My driver design is meant to replace the laser module's internal driver and to be driven directly from Nano's PWM pin. My design takes the 12v from the controller as power for the driver and the 5v PWM signal from the Nano to the dimming pin on the LED2001.

 

The issue I'm having is that when the laser driver gets power for the first time, it requires a pulse width of about 25% or more before the output comes on. Once it comes on through, the pulse width can go down to 1% and still control the laser. For testing i just fire at 100% for a second and then turn it off and as long as I don't remove power from the board it will give me linear control from 0-100% just fine. If I forget to pules the laser after power up and I go to engrave a picture, the picture will be messed up until the color of the pixels are dark enough to turn it on.

 

Any idea what might be going on here?

Thanks

John

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  • 0

    Do you still have those input capacitors for the LED2001 in the circuit?

     

    If so you may be waiting on them to charge up.

     

    What about adding an enable switch to the output to the laser diode?

     

    You could turn on the driver at full power for a second, drop it down to your start power, then engage the laser.

     

    Al

  • 0 in reply to ajens23

    Yeah, they are still there. I'll test that by powering it up and leaving it for a while. This will give the fan time to spin up to full speed and charge the caps. After say, 10 minutes or so I'll try turning it on with the lowest "ON" setting (the GCODE command would be "M03 S1", max power would be M03 S255).

     

    Al Jensen wrote:

     

    Do you still have those input capacitors for the LED2001 in the circuit?

     

    If so you may be waiting on them to charge up.

     

    What about adding an enable switch to the output to the laser diode?

     

    You could turn on the driver at full power for a second, drop it down to your start power, then engage the laser.

     

    Al

    I'm afraid I'll probably have to do something like that. I had not thought about doing that but it is a good idea. I don't want to add an MCU to this so I'll have to think about how to implement some circuitry to hold the dim pin high while disabling the output until the caps are charged or a set time has passed. I think this design just got a bit more complicated lol.

     

    BTW, I know this chip was designed for LEDs but its output looks really clean and there is zero overshoot with this. Other then this little issue, it has been a really great driver for my 2w laser diode and the linear control it gives is really nice. Now, to go noodle on some circuits lol.

     

    Thanks for the great suggestions.

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  • 0 in reply to ajens23

    Yeah, they are still there. I'll test that by powering it up and leaving it for a while. This will give the fan time to spin up to full speed and charge the caps. After say, 10 minutes or so I'll try turning it on with the lowest "ON" setting (the GCODE command would be "M03 S1", max power would be M03 S255).

     

    Al Jensen wrote:

     

    Do you still have those input capacitors for the LED2001 in the circuit?

     

    If so you may be waiting on them to charge up.

     

    What about adding an enable switch to the output to the laser diode?

     

    You could turn on the driver at full power for a second, drop it down to your start power, then engage the laser.

     

    Al

    I'm afraid I'll probably have to do something like that. I had not thought about doing that but it is a good idea. I don't want to add an MCU to this so I'll have to think about how to implement some circuitry to hold the dim pin high while disabling the output until the caps are charged or a set time has passed. I think this design just got a bit more complicated lol.

     

    BTW, I know this chip was designed for LEDs but its output looks really clean and there is zero overshoot with this. Other then this little issue, it has been a really great driver for my 2w laser diode and the linear control it gives is really nice. Now, to go noodle on some circuits lol.

     

    Thanks for the great suggestions.

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