Laser Diode driver - kHz modulation & 1% duty cycles - Page 1
Forum section: electronics / projects

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 23, 2019, 06:30:15 pm
content: Hello EverybodyI've been working with laser diodes most recently and have been able to come up with an almost-working design, albeit with some significant hurdles I have yet to overcome. Basically, I cannot get the transient response I am looking for when PWMing at a few kHz and going to very low duty cycles. Submitting here to get your expert advice. First, the schematic:The theory of operation: When a current flows through the LD, a reverse current proportional to the light output is generated through the PD. The LD and PD are in the same 3-pin package ( this being an N-type LD ) Rpd converts this reverse current into a feedback voltage. The OPA's inverting input is connected to this feedback point. The OPA non-inverting input is connected to a 10-bit DAC which I can digitally control through the MCU. Therefore, this DAC sets the peak LD current/optical power. The OPA drives enough current through Q1's base in order to achieve the requested peak voltage at the Rpd point.The LD I am using are all either IR or red LD's. Their monitor photodiode current can varying wildly: I've seen as low as 50uA all the way to 250uA for the same optical output power using LD's from the same box.Now in order to achieve varying optical powers, I utilize PWM and vary the duty cycle of a special feature of this OPA. This particular OPA can switch its output pin within 50ns between (a) actively driven and (b) high-Z When driving, the OPA servos current to Q1's base and ramps to/holds the peak laser power. When in high-Z, Rbe removes base charge and quickly puts Q1 into cutoff.The Vref, DAC, and OPA are all actually integrated onto a single die along with the MCU itself. For reference, it is a PIC16F17** series MCU. The OPA has a 3.5MHz GBWP, 40 degree phase margin, 3V/us slew rate, a common-mode range of 0-VDD and an open-loop gain of 90dB. I'm running this MCU and thus this OPA at 3.3V. For reasons I cannot get into here, I am more or less forced to use an integrated approach like this rather than a separate OPA, MCU, DAC, etc.About the feedback network: I had to really wing this part of the design and just solder and unsolder caps until I got a somewhat workable response. Cf1: anything below 3.3nF and the base drive oscillates wildly. I finally settled on Cf1 = 10nF and get somewhat stable results. Cf2 and Rf are currently just placeholders, to be used if I can figure out how to properly do Type II compensation. I am not sure how I would calculate this beforehand.Q1 is a 2SD1979GSL NPN. I have several other NPN's on hand I've been experimenting with. The best results I've found are with NPNs that have a high hfe and a lower ft range. I tried originally using several MOSFETs in this scheme, but all of them horribly oscillated and I could not get anywhere near a flat response with any NMOS I tried here. Probably had to do with the OPA's ability to drive a capacitive load.Now, some scope images to illustrate my problem:YELLOW trace is the feedback voltage at Rpd. GREEN trace is the voltage on Q1's base. PURPLE trace is the LD Anode ( the LD_SUPPLY ) RED trace is the LD cathode. LIGHT PINK trace is (LDA - LDK) for a differential measurement from LDA-LDK, giving me an idea of the LD forward voltage. Here, I have the PWM frequency set to 4kHz and illustrated is a 5% duty cycle. I start with the DAC set to something very small, like 100mV. Then I adjust the DAC output upwards until I can trigger on the edge of the Rpd feedback signal in YELLOW.I see a rather fast ramp up with significant overshoot on the Rpd signal. Not sure why that occurs here.Also, I see a rather fast ramp up to ~500mV on the base drive as soon as the PWM signal goes high and ... but then a slow and steady linearly increasing ramp of the base voltage until finally it flattens out. It is this slow and steady ramp up of Vbe that hinders low duty cycles. A zoomed in scope shot:Upon the PWM signal going high/OPA driving, the base voltage shoots up quickly to about 500mV -- below threshold -- and then slooooooowwllly ramps up as you can see in the above scope shot. This is a 5% duty cycle ON pulse shown. If I go to 1% duty cycle, the entirety of the ON PWM signal is spent in this slow ramp-up, which keeps Vbe below threshold the entire time, effectively giving me zero output on the LD. When this happens surprisingly, I still get a photocurrent feedback signal, which I don't understand at all. If there is no lasing action, there should not be any feedback signal ...I'm hoping to gather insight into the following points:(1) Is there anything in this circuit that strikes you as faulty or terribly incorrect? Are any of my assumptions flawed or incorrect?(2) Is there a better way to go about compensating this OPA for a better transient response and reduce the ringing on the rising edge? Can I do this empirically as before? I do not have access to a VNA to do fancy stuff like the loop gain/phase measurement ...(3) Why does the OPA seem to quickly slew to 500mV, but then exhibit that slow ramp-up to threshold?(4) Can this integrated OPA do what I am asking of it, given its GBWP and slew rate?(5) Any other ways to improve this circuit by adding or deleted external elements?(6) Would an NMOS work better here, if I were somehow able to get the circuit to not oscillate?(7) Eventually I would like to get this circuit to work up to 10kHz modulation rates with 1% min duty cycles. Would that be possible given this OPA?Thanks for reading!

author: janoc
author_rank: Super Contributor
timestamp: April 23, 2019, 07:25:11 pm
content: Well, this is a bit outside of my field of experience but I have a hunch that the opamp in the PIC is not designed to be abused like this. It is a very slow opamp - 3V/us slew rate is not stellar and this will directly affect the current ramp-up. If I see correctly, your scope captures are at 20us/division, so that ramp-up seems to pretty much match the declared slew rate of your opamp.The other thing is that as soon as you turn the laser on, the photodiode is doing negative feedback and is acting against the rise of the current.The fast rise/overshoot may be spurious and due to your probing (parasitic inductance of the wires).Also who knows how the opamp output behaves when you are turning the pin driver on and off (well, on and high-z). That's not normally the way PWM is done and the PIC pin drivers may not like that too much.I would probably prototype this first by using a discrete opamp and a signal generator, just to make sure that the rest of the design isn't doing something weird before messing with the PIC.

author: adamcord
author_rank: Contributor
timestamp: April 23, 2019, 07:40:13 pm
content: QuoteThe way you have the OPA configured, with a feedback capacitor, is like an integrator, which is my guess as to why it shows that ramping behavior. Can you probe the output of the OPA before the 330 Ohm resistor? I would expect that to look like a ramp.

author: StillTrying
author_rank: Super Contributor
timestamp: April 23, 2019, 07:49:51 pm
content: A nice long post, but too much for me to digest in one meal.I don't think the first bit of your yellow PD signal is the PD, I think it's the op amp's output rising when enabled, and 'the ramp' is CF1 discharging. then the op amp 'works'.The PWM plus DAC control seems complicated, especially as it's the opamp's output which is switched on and off.

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 23, 2019, 08:18:45 pm
content: I think you're right about the photodiode ...To make sure my LD/PD is still good, I took the LD/PD device out of its mount and into a LD controller to measure the photocurrent. Then I forgot to put it back in circuit, and powered up everything with probes in place. Surprisingly the yellow trace remains the same ... in the total absence of the photodiode current! How does this make sense? There is no DC path from OPA OUT to OPA -IN ... I don't understand how this could happen.

author: jmelson
author_rank: Super Contributor
timestamp: April 23, 2019, 08:21:56 pm
content: Rpd is too high, by a couple orders of magnitude. Instead of using a voltage amp, why not a transconductance amp. There, the effective impedance is very low.You will need a feedback resistor across the op-amp, value sets the conversion from current to voltage.The rest of this circuit should work. I built something pretty close to this some time ago, for writing images on photographic film with a laser diode.Jon

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 23, 2019, 08:39:34 pm
content: I'll try building up this circuit with a discreet single-supply OPA, with a GBWP somewhat higher than 20MHz. I'll also use a potentiometer rather than a DAC. Try to test out the concept with higher bandwidth components and work from there. Maybe it is something weird to do with the PIC's integrated OPA.Regarding the toggling between driving and high-Z mode: This is actually a feature of the chip built into the registers and hardware. This PIC has the OPA "output override" function which can be internally routed to toggle based on the internally generated PWM signal. Also there is at least one app note out there from Microchips website suggesting to use this high-z toggling functionality, albeit in an LED dimmer, DC/DC conversion sense.Using discreets, I can't toggle the OPA between driving and high-z because thus far I haven't found any OPA that advertises this. If I can't toggle between high-z and driving mode, how else to achieve this variable duty cycle modulation? I can't turn the OPA on and off fast enough with an EN pin ( OPA startup/shutdown time ). I can't toggle an external DAC between 0V and some setpoint fast enough ( DAC settling time + cost of a fast settling DAC + size of an external DAC )

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 23, 2019, 08:43:09 pm
content: Quote from: jmelson on April 23, 2019, 08:21:56 pmThe nominal photocurrent at peak power output, measured by me, is 150uA. But this can vary between 50uA to 250uA in the same part number laser diodes...150uA * 2.49k = 0.374V0.374V is about 1/3 of the headroom of the DAC's possible output ( Vref feeding the DAC is 1.024V ) So I think this resistor is more or less in the correct range, no? Some room for adjustment down, more room for adjustment up.Also: you mean a transimpedance amplifier ( not transconductance ) ?If you meant transimpedance amplifier, there is a single reason I cannot use one: This is a single supply system, and as such, I cannot use a TIA because using a TIA with an N-type laser diode would generate a negative output voltage in proportion to Ipd * Rf Since I don't have a negative supply, I cannot use a TIA in this fashion.

author: janoc
author_rank: Super Contributor
timestamp: April 23, 2019, 09:07:32 pm
content: Quote from: smoothVTer on April 23, 2019, 08:39:34 pmGBWP is not what matters so much, do make sure your opamp has sufficient slew rate.Quote from: smoothVTer on April 23, 2019, 08:39:34 pmWhat's wrong with using an extra transistor there?

author: StillTrying
author_rank: Super Contributor
timestamp: April 23, 2019, 09:11:48 pm
content: "If you meant transimpedance amplifier, there is a single reason I cannot use one: This is a single supply system, and as such, I cannot use a TIA because using a TIA with an N-type laser diode would generate a negative output voltage in proportion."All you've got to do is leave out RPD, and provide a resistance path opamp output to -IN, the PD's current is already in the right phase for overall negative feedback.

author: mikerj
author_rank: Super Contributor
timestamp: April 23, 2019, 10:07:51 pm
content: Just to make sure I understand your intentions, you want to provide a constant average power output independent of PWM duty cycle, so low duty cycles require much higher peak currents than high duty cycles? I don't see any current limiting in the circuit, does the 2.5v laser supply guarantee you can't blow it up if the drive transistor is saturated?

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 23, 2019, 10:27:09 pm
content: Quote from: janoc on April 23, 2019, 09:07:32 pmYou're right ... I could have back-to-back NFETs at the OPA output pin to force the high-z state on any OPA I choose ... however ... at least in my preliminary experiments with this, I find large negative and positive going spikes upon the switch on/off state. This might have to do with charge injection of the series mosfets, in which case I'd be worried about momentarily overdriving Q1's base and blowing the LD. For example:Here's switching the gate of the back-to-back NFETs and the signal transmission of a 1MHz sine wave:Zoomed into the rising switching edge, I get some nasty 1V spikes positive on the ON edge and -1V spikes on the OFF edge:This may or may not work ... I could choose much higher Ron NFETs to minimize the gate capacitance and thus the charge injection.

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 23, 2019, 10:34:58 pm
content: Quote from: mikerj on April 23, 2019, 10:07:51 pmThe idea is to be able to set the peak LD current to a constant, known level, independent of the LD's own Ipd characteristic. Once the LD is "peaked", so to speak, the idea is to modulate at several kHz and vary the duty cycle to scale the output power. The complication arises because the same part number laser diode ( red or IR, in this case ) can have an Ipd anywhere from 50uA to 250uA for an output power Po = 10mW The LD is most efficient at this "peaked" output level, so to achieve highest efficiency, you want to always reach that peak power during the PWM "on" time.I guess another way to word it is that I want to achieve a constantpeakpower, that I can then PWM/vary the duty cycle to scale the perceived average power. To achieve this constant peak power, I run this system as a fast linear regulator, using the DAC as my peak power control signal.

author: ajb
author_rank: Super Contributor
timestamp: April 24, 2019, 12:37:27 am
content: There's a fundamental problem with switching the output of the opamp as you're doing: you're breaking the feedback loop. An op amp without feedback is a comparator, and not a very good one, often exhibiting problematic behavior when changing state. The fast rising edge, resultant ringing, and slow ramp up to the set point is entirely consistent with an op amp coming out of saturation and getting a hold of its feedback again. You will almost certainly achieve better results by switching the setpoint on and off, and allowing the op amp to stay closed loop at all times.

author: duak
author_rank: Super Contributor
timestamp: April 24, 2019, 03:55:57 am
content: I last worked with laser diodes in the early 90's. I don't think things have changed much though:https://www.newport.com/medias/sys_master/images/images/he9/hd7/8797049520158/AN05-Laser-Diode-Characteristics-Overview.pdfIt explains some aspects but for what it's worth, here's my spin on it too:- it's normal to have a variation in monitor current. The photodiode sits at the other end of the laser diode chip and measures how much light leaks out of the cavity back facet or mirror. I can't remember the exact values for the leakage but you can see that a difference between 99.9% and 99.99% reflectivity is an order of magnitude variation in monitor current.- a laser emits non-coherent radiation as an LED below lasing threshold. The amount and wavelength is not usually specified.- the LD_supply voltage seems a little low. It should work but Q1 is running close to saturation ie., VBE > VCE- you've got to be really careful not to exceed either the forward current limit or the power limit spec or you will blow the facet off the die and it will no longer lase. This may have changed over the years, but back then the manufacturer wasn't kidding about exceeding limits. We joked that you could kill a $100 device in 100 ns for a burn rate of $1B per second.- back when I worked with these things, if the laser was modulated, the monitor was sampled and used to determine the drive current. This was then switched or steered so as to not cause any current spikes that could damage the laser diode.

author: mikerj
author_rank: Super Contributor
timestamp: April 24, 2019, 11:00:33 am
content: Quote from: smoothVTer on April 23, 2019, 10:34:58 pmSounds to me like you want a constant current current drive. Using a continuous feedback with an integrator to control laser current will result in the loop trying to maintain a constant average power, i.e. laser current will rapidly increase at low duty cycles.If you want closed loop power control only whilst the laser is operating you will need some kind of sample and hold on the feedback signal gated by the PWM. You may be able to do the entire power control loop by triggering an ADC conversion from the PWM output and using this to adjust the DAC which controls a current source.

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 24, 2019, 04:50:05 pm
content: Quote from: StillTrying on April 23, 2019, 09:11:48 pmOK, I tried this approach but I am not sure how to get it working correctly. As a starting point, here is a simulation of the steady state system in its current form ( ignoring any PWMing of the LD )The laser diode is modeled more or less with D1/D2/D3B1 is a simplistic model of the photodiode: below lasing threshold, the photocurrent is very low. As it approaches lasing threshold ( ~20mA ) the photocurrent increases drastically, and thereafter, approximates a linear curve for increasing laser diode forward current. C2/R4 are there for OPA stability. With this simulation, I can finely control the forward current through D1/2/3 using the Vref_drive ( which would come from my internal DAC )When I apply your suggestion to use the PD feedback current directly in a TIA approach, I cannot get a usable current through D1/2/3 anymore, no matter what value R4 I choose. It seems to regulate, albeit to a very low current:Does the 2nd schematic/simulation I have shown here reflect the general idea of your suggestion?

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 24, 2019, 05:04:56 pm
content: Quote from: duak on April 24, 2019, 03:55:57 amThanks for the resources and your inputs.The devices I am working on use very cheap laser diodes ( 635nm reds and 850nm IR's ) that cost about $3/ea. so even if I blow several it is not a big deal ( phew! ) The system itself is tiny and runs off a small, weak battery. Therefore the smallest PCB I can get away with steers me towards a fully integrated solution, if I can get away with it. Also having these weak batteries makes it critical to drive these LD's at peak efficiency to get the longest battery life.

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 24, 2019, 05:16:27 pm
content: Quote from: mikerj on April 24, 2019, 11:00:33 amThanks mikerj,I found this app note from Maxim about doing pretty much what you describe in the first half of your reply, I think. Yet I do not understand how this circuit could possibly work ( regulating average power ) because then there is no limit to how much the current can increase at low duty cycles ... this circuit below would push the current very high and blow the laser diode at low duty cycles.Taken fromhttps://www.maximintegrated.com/en/app-notes/index.mvp/id/1811

author: pwlps
author_rank: Frequent Contributor
timestamp: April 24, 2019, 06:42:36 pm
content: Quote from: smoothVTer on April 23, 2019, 06:30:15 pmI don't understand the principle of operation as you explained, but I admit I didn't make much effort to analyze it in details.I don't get: why should you need any digital stages if your pwm duty cycle is only controlled by an analog input ?You might be interested to compare it with my approach here:https://www.eevblog.com/forum/projects/simple-lifi-transmitting-audio-signal-with-a-led/msg2282771/#msg2282771A slight modification would be needed if you want to have the feedback go through a measuring photodiode rather than to be taken directly from the LED output voltage, otherwise this sigma-delta converter circuit can give a high-fidelity PWM without any digital components.

author: jmelson
author_rank: Super Contributor
timestamp: April 24, 2019, 07:16:09 pm
content: Quote from: smoothVTer on April 23, 2019, 08:43:09 pmYes, in a strictly DC sense, but your problem is poor AC bandwidth. High impedances wreck your bandwidth.QuoteYes, right, incorrect term, I did mean transimpedance. Well, if the output is at the wrong DC resting point, then you have to shift it. Since you don't have a negative rail to bias the input, you need to shift the non-inverting input more positive. When set right, that will cause the output to idle at a more positive value, and then move in the minus direction with increasing laser output. But, this presents a low impedance to the photodiode, and will therefore improve the AC response.Jon

author: chris_leyson
author_rank: Super Contributor
timestamp: April 24, 2019, 08:48:01 pm
content: Hi smoothVTer, it's been a few years since I last worked with laser diodes and back then we were driving them CW.I probably had a mini rant about fixing some of the legacy laser drivers that we used because 50% current overshoot was not uncommon, all down to putting too many 100n feedback caps around op-amps and then not testing the transient responseAny new laser drivers, ~50mW power, just used a single op-amp driving a mosfet but needed to have a select on test resistor to set the correct PD feedback voltage for each laser diode. I used just enough capacitance to roll off the high frequency gain and managed to get a good step response with 5% overshhot. The only problem with a single op-amp solution was that if you have a set point of zero then the laser diode is always driven at it's threshold current.I was looking at fast(ish) laser drivers for a personal project but the company I worked went belly up so that never got finished, I had more important things to think about.Anyway, something you might find useful is a little circuit to mimic or emulate a small laser diode with an N style pinout.The laser diode drive transistor or mosfet is represented by the current source I1 and diodes D1 and D2 together with Q1 Vbe mimic the LD forward voltage somewhat. R1 sets the threshold current for the emulator, 30 ohms gives about 20mA, Q1 and R2 drive an LED at about 10mA for a max LD drive current of 50mA in this example and Q2 and R3 mimic the PD feedback current set to 330uA at 50mA drive. It's a reasonable approximation to a small laser diode and a lot cheaper to fix if you let the magic smoke out.

author: StillTrying
author_rank: Super Contributor
timestamp: April 25, 2019, 06:43:27 am
content: I don't think any of these solution work very well, so here's yet another.

author: mikerj
author_rank: Super Contributor
timestamp: April 25, 2019, 09:24:42 am
content: Quote from: smoothVTer on April 24, 2019, 05:16:27 pmWhen using a laser for transmitting data the data signal is always DC balanced, i.e. the average duty cycle (over some number of bits) is maintained at 50% i.e. an equal number of ones and zeros for an NRZ signal. Not only does this maintain a fixed average power output, it makes the receivers job far easier as the slicing level can be set based on average power.That app note does raise another issue that you might run into; biasing the laser. For the fastest response you don't want to turn the laser fully off as this slows the response considerably. Typically the laser will be driven so that it always remains above it's threshold current, though at kHz speeds this may not be an issue.

author: awallin
author_rank: Frequent Contributor
timestamp: April 25, 2019, 06:30:07 pm
content: not sure if it's been said/linked already but the usual constant-current circuit used for sciency-stuff is:Libbrecth-Hallhttp://www.submm.caltech.edu/kids_html/DesignLog/DesignLog179/MillerMUSICReadoutDocs/HEMT%20Power%20Supply/Libbrecht%20and%20Hall,%20A%20Low%20Noise%20High%20Speed%20Diode%20Laser%20Current%20Controller.pdfEricksonhttps://arxiv.org/abs/0805.0015Seckhttps://arxiv.org/abs/1604.00374

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2019, 01:01:49 am
content: Quote from: smoothVTer on April 23, 2019, 06:30:15 pmWhen the output is off, the external capacitor used for frequency compensation is charging up. This is called integrator windup and it results in the slow recovery.QuoteThe configuration of Q1 is a major problem; it is adding uncontrolled voltage gain within the feedback loop. Reduce the value of the series base resistor so that it only suppresses parasitic oscillation and add a low value resistor in series with the emitter to stabilize Q1's transconductance.With Q1 fixed, the external feedback capacitor should not even be required removing the slow ramp up.QuoteI suspect this comes from the operational amplifier recovering from saturation of its internal circuits.QuoteThe topology is the problem and not the speed of the operational amplifier. A faster amplifier will not recover from integrator windup any more quickly.QuoteI might use a different circuit configuration. Replacing the operation amplifier with an operational transconductance amplifier would be one option but they are not common. (1) This provides a direct current output so Q1 is no longer needed and frequency compensation is considerably simplified.There is a trick I would at least evaluate though. Ground the operational amplifier's output through a low value resistor to a low impedance source, and then use the positive supply pin to drive the laser diode. This has the effect of replacing Q1 with the high side transistor of the operational amplifier's output stage and the output resistance now controls the transconductance.U111 in the first example shown below gives some idea of what this looks like. The power supply pins become current outputs, the normal inputs stay high impedance inputs, and the output becomes a low impedance input.The second example below shows the same idea but the output of the operational amplifier is being used for local feedback. Essentially this circuit configuration is an operational amplifier with a current feedback amplifier added to the output.QuoteA bipolar junction transistor is actually a better choice here because of low and predictable threshold voltage.QuoteAbsolutely, the operational amplifier is more than fast enough.(1) You could use a 723 voltage regulator; it has one.

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 26, 2019, 02:28:25 pm
content: Quote from: awallin on April 25, 2019, 06:30:07 pmI have not seen this paper despite googleresearching for a couple of months now. A lot to digest in this paper! Thank you, I hope to gleam some insight from this.

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 26, 2019, 07:53:17 pm
content: Quote from: David Hess on April 26, 2019, 01:01:49 amI will implement some of your suggestions here, David. Already have had some success with the base resistor swap out ... this and some other changes from replies above have been helpful. Once I figure this out, I will report back here with an updated status.I've been keeping scope shots along the entire process for posterity and will post back some of my findings.Thanks everyone! Couldn't have gotten this far without all ya'll.

author: StillTrying
author_rank: Super Contributor
timestamp: April 26, 2019, 09:43:31 pm
content: Quote from: smoothVTer on April 23, 2019, 06:30:15 pmWith a 12pF photo diode and a 3.5MHz op amp 1us wide pulses will just be a little rounded hump. I use just a PD and 1 or 2 fast transistors to view square-ish 1us pulses.

author: LaserSteve
author_rank: Super Contributor
timestamp: April 29, 2019, 11:54:02 am
content: I received your PM, fast drive is not my cup of tea, unfortunately.Having had my share of blown diodes due to poorly designed drivers in the laser show industry, I can suggested the following general additions. You haven't lived until 16 single mode red laser diodes die in a brief flash and go LED. This happened due to defects in a brand new commercial driver board, and at 35$ a diode... Some of my diodes cost 200$ each, so I tend to carefully obtain commercial drivers. When I'm not in the lab, my laser diodes are used in arrays for combination into one beam, so I tend to drive 4 diodes in series at a time. I only need 30 Khz at up to 1.5 amps, and no light feedback. I buy the drivers from a vendor known to not have issues.So I suggest the following, as my concerns are more practical, and not about the driver bandwidth:Add a Lasorb, make sure you have a fast, low Vf, reverse protection diode in series with the device, An independent upper limit current clamp circuit is a must, and insure you are ramping the power to the output stage after you know the op-amps in your circuit have stabilized. Thus some form of soft start.A warning about working with LDs on the bench, the little 10 to 100 uf capacitors across the current limited output stages of bench power supplies tend to store charge and blow up diodes, as does the surge that many PSUs have during startup. In other words, commercial constant current bench supplies often have only the steady state current well defined.If I could, I'd have a normally on FET (Depletion?) of some kind across the diode, until the circuit is stable. Normally closed mechanical shunt relays have traditionally been used for this, shorting the diode till the laser head cable is connected, or the PSU is stable, and they just are not fast enough.The Lasorb is a static discharge protection device that has a structure like a Mosfet/SCR hybrid and is triggered by a fast DV/DT. I know the inventor personally, they do work, and I strongly suggest having one within 6 cm of any valuable LD.www.lasorb.comI would have also sent you to Libbrecht and Hall as a start. There is a paper that was written in response to L&H that is worth a read:An Ultrahigh Stability, Low-noise Laser Current Driver with digital controlChristopher J. Erickson, Marshall Van Zijll, Greg Doermann, and Dallin S. Durfee Department of Physics and Astronomy, Brigham Young University.REVIEW OF SCIENTIFIC INSTRUMENTS 79, 2008Good, it is on line:https://www.physics.byu.edu/faculty/durfee/Publications/Erickson08au.pdfWhich was a follow on to this:https://tf.nist.gov/general/pdf/739.pdfIf you don't have academic access, emailing the professor will generally result in your receiving a preprint, if they are allowed.If I had to design a very low current diode driver, I'd drive the diode off a high side current source and shunt it with a FET or Bipolar for the modulation, being careful to ensure there is a always a below threshold "leak" of current through the LD to protect it from surges.Where you really should just go is here:I'd suggest taking a look at APC style LD driver chips with PD feedback by ic_Haus:https://www.ichaus.de/keyword/Laser%20Diode%20and%20LED%20DriversTake a look here for addressing some additional concerns, his writeup is pretty good..http://hololaser.kwaoo.me/laser/red_diodelasers.html#LDdrvhttp://hololaser.kwaoo.me/electronics/myLDdriver.htmlOne last note, the traditional current source used for testing LDs by the laser show industry and laser hobbyists is a LM317 configured as a current source with just a non-inductive current sensing resistor. If the leads are kept short and the input power is filtered, this combination tends to lead to long diode life when used by amateurs. Evidently National's LM317 has the right startup characteristics, but others work as well. Hence my note on shunting the diode for regulation.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: April 29, 2019, 02:17:40 pm
content: I finally found something EE worthyDesign and Stability Analysis of a CMOS Feedback Laser DriverP. Zivojinovic, M. Lescure, and H. Tap-BéteilleIEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 53, NO. 1, FEBRUARY 2004IEEE is anal on uplinks, or I'd post it for you.. However you may be amazed about what shows up in Google Images for a preview..While the pictures show an array of FETs, the circuit is modeled on traditional components.I'm pretty sure from reading it that they modeled it on the bench before making the IC... >> then 1 Mhz from my reading.Steve

author: smoothVTer
author_rank: Regular Contributor
timestamp: June 03, 2019, 02:58:07 pm
content: Sorry its been a long time. I mostly figured out this laser driver, and this post is to update my findings and trial-and-error scope shots.The topology I settled on came about after taking suggestions from prior posters:At first pass, the circuit didn't have the schottky on the OPA output; there was no Re; there was no Rpe; Rbase was a few kohms;and Cf was about 1nF. Theory of operation is during the laser "on" time, Q2 is off and all feedback current is generated by the monitor photodiode. OPA1 pushes enough base current to bring the inverting and noninverting inputs to the same potential. During the laser "off" period, Q2 injects a small current into the inverting node, raising the voltage there, and then OPA1 turns current off to the base of Q1. Consequently, this puts OPA1 into saturation because the + and - inputs are rather far apart in potential.The yellow trace is the base voltage; the green trace is the voltage at the inverting node; the red voltage is a pseudo-differential voltage across the laser diode by taking 2 scope probes across the diode relative to GND and doing some math in the scope. As you can see, (1) The output oscillates (2) There's a large negative going excursion of the OPA output as soon as Q2 turns off; I think this has to do with the Cf capacitor's stored charge suddenly being reversed, as well as the "integrator windup" effect. (3) There is a long slow ramp of of the base voltage, not desirable for generating a sharp, fast laser pulse.The first thing I changed was to put a schottky on the OPA1 output, to prevent that large negative going excursion:This seemed to help the recovery time from saturation.Next, I empirically determined a Cf value in my circuit that would just quell the oscillations on the rising edge of the base voltage. This improved my waveforms considerably:Still wasn't happy about that long, rising edge. Another poster suggested to (a) lower the base resistor just enough to prevent parasitic oscillation and (b) add an emitter resistor to stabilize Q1's gain. Well, all this sort of reminded me of BJT design class and made a lot of sense.That rising edge was a little strange looking so I had to go back and forth and trying out different values for Rb/Re.After playing around with values for Rb and Re, and lowering Cf even further, I finally got something approaching a good waveform with good turn on/turn off characteristics:The laser diode voltage falling slope is determined by a large part to Rpe ... Rpe is basically there for that purpose, plus a larger purpose of always allowing a tiny bit of current through the laser diode so it is faster to turn on from the off state, with less of a transient excursion. Of course, power consumption was an issue in my design so it had to be a balance between response and power consumption.I am sure there are many, many things I can improve on this circuit. For example: I used an AD8029 for OPA1, for the only reason that (a) it was available in my lab and (b) it is a tiny package size and space was a consideration. But perhaps there is another OPA better suited for this role?Also: Q1 I found the NPN's that work best with this circuit are high-beta, low Vce NPN's. Why is this so? This one in particular is a 2SD1979GSL NPN. I basically soldered and de-soldered NPN's here until I found something that seemed to work best. What combination of characteristics are best for Q1 in this application? Some NPN's I tried oscillated no matter what ... some had such a slow turn-on that the 2us pulses above never reached steady-state.Also: Re adds to the power dissipation of the entire driver; it stabilizes Q1's Vbe and thus its gain, but also is in series with the laser drive current, raising the voltage dropped by it, and this necessitates a higher laser supply voltage. I settled on a value of 15 ohms. I would like to lower this resistor as much as possible but it seems the lower value I choose, the worse that rising edge of the base voltage appears. How can I choose Q1 so that I minimize Re as much as possible?Thanks everyone for contributing, I hope these screenshots sort of show what was happening during the design/tuning process of this circuit.