analog controlled 5A laser diode driver. pwm solution wanted - Page 1
Forum section: electronics / projects

author: bob808
author_rank: Frequent Contributor
timestamp: December 09, 2019, 09:08:37 am
content: 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.

author: bob808
author_rank: Frequent Contributor
timestamp: December 09, 2019, 07:23:48 pm
content: 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?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 09, 2019, 09:28:16 pm
content: 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.

author: bob808
author_rank: Frequent Contributor
timestamp: December 10, 2019, 02:43:16 am
content: 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.

author: bob808
author_rank: Frequent Contributor
timestamp: December 10, 2019, 05:11:57 am
content: 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.

author: bob808
author_rank: Frequent Contributor
timestamp: December 10, 2019, 06:12:58 am
content: Quote from: Kleinstein on December 09, 2019, 09:28:16 pmI chose this current sense chip for my design:https://datasheets.maximintegrated.com/en/ds/MAX4376-MAX4378.pdfIt is a high side current sense amplifier. I don't think I can use it at ground side. I presume this chip gives better performance than just using an opamp to sense the current.I managed to trick Simetrix to let me simulate the circuit, by transforming the current sense into the topology suggested by you, and it seems to be working fine this way.I attached the final schematic showing the current going through the diode string. Also my supply for them is messed since using an opamp it needs some voltage drop to pick up a sense of what's happening.So anyway, I'll use a chip for sensing, just that I need to figure out if I'll do it on the high side or low side of the mosfet.This is kinda great as I use the PWM line for blanking, and I can use the DAC lines to set the operating conditions.For example, I might try to overdrive the diode in pulsed operation. This can be easily set up this way. Also I do care about overall efficiency.I've used TI's Webench Power Designer for the TPS54531 chip for parts spec with 5mVpp output ripple at 5V/5A output. That device is current controlled I think. The 2x47uF caps on the output shouldn't be a problem for a 3.5A diode. I'll have a small resistor in series anyway.Also the PWM frequency in the attached photo is at 10KHz.Also not being able to adjust for the diode I use, makes for losses of power on the pcb, that I do not want. I'll need to switch between 450nm/405nm diodes, without much fuss. There's like 0.5V difference in operating points. As in 450nm has lower operating voltage, and that means that I have to dump the extra 0.5V at anywhere between 3-5A. Depending on the 445-455nm diode I use. That's extra 2.5W, that means another heatsink etc etc. It's an exercise in efficiency so to say.And with 24V input, you could control, just the same, two laser diodes in series. There's extra losses on pcb in the dc/dc converter but theoretically it should be doable. Just set double the output voltage, current control is the same.