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analog controlled 5A laser diode driver. pwm solution wanted
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bob808:
Hi all!
I'm trying to make a 5A laser driver to control my laser diode (3.5A one).
It works at about 4.8V/3.5A max. I'm trying to control it with the two DAC lines from the Arduino DUE.
One line sets the output voltage for a TPS54531 dc/dc buck regulator. The other DAC line sets the output current.
I've attached the schematic I use in the simulator.
X3 sets the correct DAC output range. DUE has 1/6-5/6 Vref output on the DAC lines. I'm using a 3V ref so that's 0.5V-2.5V output. X3 gets that range to 0-5V.
X1 is actually a current monitor chip that is implemented exactly the same as in the schematic. X2 sets the laser diode current (R1).
The simulation is multi-step for 0.5-2.5V output from the DAC. The graph shows the current through the R1 resistor.
What I'm missing from the whole deal is a way of blanking/PWM the laser. I tried using a mosfet to bring X3 output to ground but that upsets the whole system.
I'd love to be able to PWM cleanly at 1KHz-10KHz max. Any ideas?
I don't know if directly pulsing the DAC output is a good idea. I tried to simulate it and works pretty good at 1KHz-10KHz. Just that I would have preferred to use a separate hardware PWM line for the blanking job, and leave the DAC outputs to set the operating point of the diode.
I also attached the simetrix sim file.
edit: TPS54531 does have an enable line but it's integrated with the undervoltage lockout system, and the chip also has a slow start capacitor, and min value is 1ms so PWM on enable pin is out of the question. And not that great to be honest.
bob808:
Any way of getting the 3.3V PWM signal to mix it with the DAC output voltage for a PWM signal with a voltage amplitude that follows the DAC output voltage?
Kleinstein:
For a laser diode of similar load, there is no need to control voltage and current. One usually starts with a clean supply (e.g. 5 V) and used current regulation or alternatively power regulation with feedback from a photodiode.

Laser-diodes can be quite sensitive to even short events of over-current. So even the large unit can be ESD sensitive and react bad to overshoot. So for testing I would use a dummy made from LEDs, diodes or similar. To take out some uncertainties, I would start with a linear 5 V regulator to have a clean supply.

Current regulation is much easier with the shunt at the ground side, just as in the standard current sink circuit.

A 3 A laser diode is likely powerful enough to produce dangerous intensity. So be careful and protect your eyes.
bob808:
I want to control the voltage and current to have the least losses in the current regulating transistor. I have 24V main power supply, the DC/DC gets that to 5.5-6V or so. Having control over the voltage could let me use two laser diodes in series for example.
For testing I'll use some normal diodes in series. I also have protection glasses for the involved wavelengths.
I'll play with the location of the current resistor.
bob808:
Managed to find a way to mix PWM and DAC signal together. This should work but I can't simulate it all together as Simetrix is limiting me in using 3x LM358. And the LTSpice model I found for LM358 is crap, I get lots of errors and sims take long time.
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