Fast modulable Laser diode driver with high side current control

[chad]

Cool! Looks pretty good.

Nice Job!

chad

[flyowynd]

Hi again

I was looking at your design and maybe I got something wrong but I don't see any AOP on the board you design.
It seems to me that the board and the schematic are different.

Another question, do you think it's possible to do it with normal component instead of CMS. I will have four of these to build for my diodes so it could be easier for me.

Thx

[andythemechanic]

Hi again

I was looking at your design and maybe I got something wrong but I don't see any AOP on the board you design.
It seems to me that the board and the schematic are different.

Another question, do you think it's possible to do it with normal component instead of CMS. I will have four of these to build for my diodes so it could be easier for me.

Thx

Hi,

what did you mean with AOP, maybe the Opamp ? It's hardly visible on the circuit because it is a very tiny SOT23 package. There is one difference between the schematic and the board, on the board is a 10uF capacitor between opamp V+ and V- which is not in the schematic.
When you want to do it with normal components, you have to find an opamp in a DIL case with similar specs which could be difficult. All other components are available. Hope that helps!

[andythemechanic]

Hi Everbody,

Here an update to this circuit: I changed the current sense and limiting resistors to 0.1Ohms to be able to use the circuit for much higher currents. Now I have a damped oscillation on the current each time I switch it (see attached image). The oscillation is always the same regardless of current, frequency etc.
So here the $64 question to all the electrical engineers out there: Any Idea what this could be?

[krazer]

By reducing the current sense resistor, you increased the gain and it is now under damped, probably due to delays in the circuit. I am not really familiar with the components you are using, but it could be due to the gate capacitance of the mosfet, or possibly the opamp is just to slow for the slew rate you are asking of it. Also, if you are measuring the voltage at the output of the driver its very possible that the issue is the inductance of the leads (or if you have one, the capacitance of the esd protection capacitor), and the driver is in fact giving a constant current output but that requires varying the voltage a bit.

Good luck!

[andythemechanic]

By reducing the current sense resistor, you increased the gain and it is now under damped, probably due to delays in the circuit. I am not really familiar with the components you are using, but it could be due to the gate capacitance of the mosfet, or possibly the opamp is just to slow for the slew rate you are asking of it. Also, if you are measuring the voltage at the output of the driver its very possible that the issue is the inductance of the leads (or if you have one, the capacitance of the esd protection capacitor), and the driver is in fact giving a constant current output but that requires varying the voltage a bit.

Good luck!

Hm Thanks, but I don't think it's that easy. Slew rate of the Opamp is really high (130V/us) and the mosfet was the one with lowest gate capacitance for the wanted power rating.
Also it should be possible to compensate this effects, by changing C4,R9 and R6 and R4. Doing this had no effect at all on this oscillation it always stayed there and with the same frequency.
My nex thought was, that it could be parasitic capacitances on the inputs of the opamp. To test this I removed the copper of the ground plane under the opamp and the leads toward it, no effect.
Any other ideas what it could be?

[flyowynd]

Hi

I may be completly wrong but when I look at the trace, it seems that you have an instability like in a simple proportional driver. I've seen the same when driving motors and you want a precise control of it. That's why we use PID drivers.
But normally using a single proportional driver will cause the first oscillation to be above the targetted point.

I pretty sure it won't help but who knows.

[andythemechanic]

But normally using a single proportional driver will cause the first oscillation to be above the targetted point.

When you look at the rising edge of the pulse closely you see that the oscillation starts with going down a little bit.
Also it is not a pure proportional driver, the capcitor c4 integrates a bit. If it is something like this, shouldn't I be able to influence the oscillation with changing c4 ? Maybe not removing it but changing it's frequency? Nothing like this happened. When I make it much smaller then the circuit starts to oscillate at high frequency what is expected. When I make it bigger the rise and fall times get slower but the oscillation always remains like it is. I don't understand this.

[flyowynd]

That's really sound strange.
Just a question like this, in most of the current controlled source I've seen, they are using 2 Opamps one for current sensing and the other as a simple follower to drive the transistor.
Could the problem come from this? adding a second Opamps after the first one will isolate the two parts of the circuit and maybe get rid of the oscillation.
(I'm really not sure of this)

[krazer]

I still suspect that delays in your circuit may be contributing to the issue.

Looking at your mosfet (I can't quite read the part number, but I am guessing it is a IRLU9343) has a gate charge of about 5e-9 C Your opamp is supposed to be good for .1A, which can charge the capacitor in 5e-8s, or 50ns. The fet also has about 30ns of delay times on the turn on (10ns delay time, 20ns rise time), which all adds up to somewhere between .05-.1us of delay, which really isn't negligible in the circuit.

Although thinking about it more, the fact that the current takes a dive before it hits the set point is more worry some, and I have a hunch is being caused by stray inductance in the circuit somewhere. You did use a low inductance .1r resistor, right?

Also, what are you using for your dummy load? And what current are you running at for that matter.