I am trying to create a current source for laser direct imaging and I have a problem in the circuit I cannot solve.

The laser is a 120 mW 405nm diode and I use it in a laser module from a laser printer. The circuit for the BLDC motor with the hexagon mirror works fine but the circuit for the laser is designed for a 100mA infrared laser diode. This is too much for my UV diode, I have destroyed one diode already. Also the 405nm diode has a common cathode where the infrared types have a different connections.

I am going to modulate the laser with a STM32 microcontroller with the SPI MOSI signal at 18 MHz SPI clock. I have tested that already and this work fine too. This may be a much to high frequency but I have to start somewhere. Don't want to scale down the specs at the beginning of the project.

The problem I encounter is a spike at the beginning of the pulses. See the attached pictures. I have already tried to reduce this effect but I am an embedded software programmer, not an electronics engineer. I hope to find someone here who can tell me what I need to add to reduce the current spike during the start of a pulse.

The bias current is required to calibrate the optics for a sharp image. The images below show a LTSpice simulation with the minimum and maximum currents set.

With a resistor I try simulate the internal photo diode (PD) in 10 steps between 10k and 100k

enter image description here

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I can provide the LTSpice simulation file when someone wants to give it a try in the simulator.

@Andy aka, Is this what you meant with using a high side driver? I cannot connect the cathode of the PD to ground completely as the base of Q2 will not receive enough voltage to start. Now I had to add a resistor (R1) to make Q2 work. I am pleased with the shape of the wave although it is not a square. enter image description here

  • \$\begingroup\$ "This may be a much to high frequency but I have to start somewhere." I would start by calculating the actual analog bandwidth supported by your lenses and scanner so that you can spec the frequency. 3dB bandwidth in a laser scanner is 0.31*spot_velocity/spot_width. This is usually a much smaller number than people expect. How many RPM for your polygon? What is your laser beam diameter and lens focal length? \$\endgroup\$ – user1850479 Oct 12 '20 at 14:08
  • \$\begingroup\$ So far I have tested creating a pattern with a module from a HP laserjet 3. With that module I can control the motor speed in software using a PWM signal but the projected pattern vibrates a little caused by a too low motor speed. That module came from a used printer. I have obtained a newer module in which the motor speed is much faster but this speed is fixed. With the HPL3 module I was able to make a projection with a square mirror and a buffer of 2kb (16384 pixels) in about 20us for a line of pixels. \$\endgroup\$ – hennep Oct 12 '20 at 14:31
  • \$\begingroup\$ The new model has a hexagon mirror and a very high speed. It sounds like a dremel tool :-) I cannot calculate the exact speed. The formula in the datasheet requires the value of two resistors and two caps. Unfortunately SMD caps have no markings. That is why I started with the highest frequency. A lower speed does not solve the problem with the spike, also a lower frequency square wave starts with that same spike. \$\endgroup\$ – hennep Oct 12 '20 at 14:31
  • \$\begingroup\$ You want to sweep carriers from the LED during turn-off to kill the illumination quickly. That is optical and something you won't see from an electrical simulation. \$\endgroup\$ – DKNguyen Oct 12 '20 at 14:46
  • \$\begingroup\$ 20 microseconds per scan would imply 0.5 million RPM. That seems too fast to be correct. \$\endgroup\$ – user1850479 Oct 12 '20 at 15:01

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