Driver for the new 505nm and 480nm diodes

Thanks!

Finally received the parts and built the driver, but it isn't working well.

Schematic:


Datasheet:
https://cds.linear.com/docs/en/datasheet/3083fa.pdf

I built it without a PCB, with point-to-point soldering, to keep the footprint small.

It was supposed to regulate between 1 and 278mA, but instead goes from 1 to ~330mA.

Up to about 100mA it works like expected, but on the higher limit it behaves weirdly. It doesn't regulate properly and the current goes a lot lower (180-250mA) while I touch the potentiometer (even through a plastic knob). Even hovering my finger neat the pot or the pot's wires makes the current go lower. Seems like some sort of capacitive effect but I have no idea what's causing it or how to fix it.

It was tested with a dummy load set for around 7V, scope shots below are across the dummy load's 1 ohm resistor.

Scope shot at max current without my finger near the pot:


Scope shots at max current while touching the pot's knob:



I could really use some help from the electronics experts.

Hi Atomicrox, quoting from the datasheet:

"Since the SET pin is a high impedance node,
unwanted signals may couple into the SET pin and cause erratic behavior.
This will be most noticeable when operating with minimum output capacitors at full load current.
The easiest way to remedy this is to bypass the SET pin with a small amount of capacitance from SET to ground, 10pF to 20pF is sufficient."


Adding more (ceramic) output capacitor might helps as well.

About the output current mismatch, I believe it's because of the tolerance of the resistor you used



EDIT:

I just measured my body's floating voltage using oscilloscope, it's whopping 32Vpp with the frequency of 50Hz (which is line frequency in here) :can:
That of course could affect a sensitive high impedance node if you ever got your body/finger/hand near the sensitive feedback node of some regulator

Thanks a lot, I shouldn't have missed that! I'd rep you but the forum won't let me.

The resistor ir supposed to be +-1%, maybe it's the pot. Didn't actually measure either before assembling. Either way if it stabilizes at 330mA it's not much of a problem.

Will attempt to fix it sunday.

You are welcome :beer:

FYI, those potentiometer (every pot out there) usually have about ±20% tolerance.
Even those multiturn does have ±20% tolerance.


Anyway, let us know if it works!

Here's what I added:
-20pF ceramic cap between SET and GND
-Two 100nF ceramic caps as decouplers on input and output
-100k resistor in parallel with the pot to lower equivalent resistance

The capacitive coupling is gone. Now the crazy swinging only happens if I touch the metal case of the pot directly, but that won't happen on the final build. Nothing happens when touching the plastic knob.

Current range is closer to what I wanted (1mA to 305mA on the DMM), but the maximum current seems to lower to about 280mA after a while. I'm assuming that's due to the sense resistor warming up, even though I used a 2W part.

There's still a considerable amount of noise.

Scope of the total voltage across the dummy load, at max current:

Zoomed in to see the noise better:


Scope of current across dummy load 1ohm resistor
Just after starting up:

After the components are warm:

It also seems to have some sort of lower frequency oscillation when you look at the waveform on a longer timespan:



I should note that my test setup isn't optimal - long wires, alligator clips, components touching a wooden table.

What do you think? Is it safe to use with a laser diode or would you suggest any improvements?

That's a great improvement.
BTW could you capture the startup response?

About the noise, 60mA for a linear regulator seems big but that is no problem for laser diode as many of them are meant to be driven by fast and high current pulse. But if you are concerned, you can add more ceramic cap on the output (should be 1uF or 2.2uF). General rule of thumb are, on similar capacitance, ceramic capacitor (MLCC) gives you the most ripple/noise reduction due to very low ESR compared to other type of capacitor.

I looked at the startup and shutdown behavior, seemed clean but I didn't take a screenshot. Will have to figure out an easy way to do that since the graph changes quickly and the screenshot button is laggy.

It'll probably have to wait till next weekend, I don't think I'll have time to do any electronics during work days.

Thanks again for your help. Unfortunately the forum still won't let me rep you.

Oscilloscope should have a single trigger mode, you could use it to capture the startup and shutdown, set the trigger to rising, at 100mV, and click start capture (or something like that, might be different on each brand)

Hi Ast,
Thats great to see you helping we all need new drivers , You have come a long way refining your work its greatly appreciated here at the forum.

Rich

This is Paul's daughter. He is still in the hospital and will be there until next month. He wanted me to let you all know that he hasn't forgotten anyone.

He'll be back ASAP

paul1598419 said:

This is Paul's daughter. He is still in the hospital and will be there until next month. He wanted me to let you all know that he hasn't forgotten anyone.

He'll be back ASAP

Click to expand...

Thank you for the update.
Please let him know that we all miss him and we all wish him a speedy recovery.
Regards
RB

:beer:

RB astro said:

Thank you for the update.
Please let him know that we all miss him and we all wish him a speedy recovery.
Regards
RB

:beer:

Click to expand...

Ditto!

-Alex

Give him our best regards!

Thank you for letting us know.
Wish him well from me and the rest of us. :beer:
Also I appreciate him thinking about us as well when he needn't of.

paul1598419 said:

This is Paul's daughter. He is still in the hospital and will be there until next month. He wanted me to let you all know that he hasn't forgotten anyone.

He'll be back ASAP

Click to expand...

Hi,
and thank you for letting us know...
It takes some pressure off us not knowing
how the surgery went.
Give your Dad all our best and we all hope his
recovery is swift without complications.:yh:

Jerry