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I'm trying to make a current source using the following schematic for this laser-diode using an OP284 opamp and a TIP120 without heatsink:

enter image description here

Since the laser-diode have no SPICE model I used two 1N4001 in series (D2 and D3) in simulation to mimic it. Left circuitry to R4 is a slow start for the input. And R8 and C6 is for preventing transient peaks.

I'm going to use the laser-diode in continuous mode meaning no modulation. And I set the Rp as a 4.7 kΩ potentiometer where max current is 440 mA in simulation:

enter image description here

And here is the specs for the laser-diode:

enter image description here

And the voltage current plot:

enter image description here

My questions are as follows:

1-) When I look at forward voltage versus current plot of the laser-diode it shows 2 V forward voltage drop happens at 900 mA. But in the same datasheet table it shows at 400 mA operating current the forward voltage drop is 2 V. Is there something wrong here? I'm trying to know this because I wonder if my two in series 1N4001 roughly fine to mimic the laser diode.

2-) My second question is about R7 the 0.6 Ω sense resistor. When I set the current to 440 mA as in simulation, the opamp inputs becomes around 245 mV: enter image description here

And in fig 26 of OP284 there is this plot:

enter image description here

where it says Vin = 0.5-4.5 V. So does that mean in my case Vin is violating the spec rules? But even I increase R7 in my case Vin remains always around 250 mV. How to remedy this?

Any input about this or if you see any other fundamental problem in schematic I would appreciate.

Edit:

I tried to take points from plot and made an LTSpice model using this tool here.

enter image description here

And then set Rp to 5.6k to achieve max 500mA:

enter image description here

And here shows the voltage drop at 500mA:

enter image description here

Is this headroom enough?

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1 Answer 1

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  1. Each line in the datasheet is independent. Recommended continuous current is 400 mA. The conditions of the voltage drop line are not specified. Design your current source (headroom) to handle this much drop and you will be fine. If a 3V drop is difficult to achieve, you will need to estimate the max drop at your current.

  2. The voltage range in Figure 26 are the conditions for this test. You can ignore this test for your purposes. The specification that is important for you is the input voltage range. This circuit needs an opamp that has an input range that goes down to zero. The OP284 does.

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  • \$\begingroup\$ I made a diode SPICE model using the voltage-current plot points here i.sstatic.net/KnAf00IG.png Can I use such model instead? \$\endgroup\$ Commented Mar 5 at 17:07
  • \$\begingroup\$ I mean can I just follow the LD plot? It shows 1.4V voltage drop at 400mA current. \$\endgroup\$ Commented Mar 5 at 17:12
  • \$\begingroup\$ Please also see my edit. \$\endgroup\$ Commented Mar 5 at 17:18
  • \$\begingroup\$ The voltage-current plot shows typical values. This is fine for most purposes. If you are building a lot of these, you should analyze using worst case values. Sometimes worst case values are not easy to come by, you need to be creative. \$\endgroup\$ Commented Mar 5 at 17:21
  • \$\begingroup\$ The base resistor (R6) seems small to me. If the laser diode is always present, it should be OK. If the laser diode is missing, the opamp output will be stressed (max 10 mA recommended). Consider changing R6 to 470 ohms. \$\endgroup\$ Commented Mar 5 at 17:43

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