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Old 10-01-2009, 05:41 AM #1
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Default Soft starts and stops

As others have done, I'm building my own LD driver based on the LM3410. However, I'm thinking of designing the drive to provide soft starts and stops by controlling the slew of the output. My rational for this is that slow rises to full output current (say 0.2 sec for arguements sake) and slow reductions to zero current would reduce thermal stresses within the LD die - perhaps providing longer LD life. Does this line of thinking make any sense to anyone else?

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Old 10-01-2009, 08:58 AM #2
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Default Re: Soft starts and stops

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Originally Posted by Giface View Post
As others have done, I'm building my own LD driver based on the LM3410. However, I'm thinking of designing the drive to provide soft starts and stops by controlling the slew of the output. My rational for this is that slow rises to full output current (say 0.2 sec for arguements sake) and slow reductions to zero current would reduce thermal stresses within the LD die - perhaps providing longer LD life. Does this line of thinking make any sense to anyone else?

Dave
Not really. Look at data sheets, laser diodes have no problems running in pulsed operation in the high kHz range ( or even MHz range; many datasheets list sub-microsecond plus widths, and if the whole pulse is a microsecond, the turn-on has to be shorter than that). If a diode is capable of being turned on and off a million times a second, is it going to make a huge difference to take it from whatever start up it is at now up to .2 second? Not really. Do you think your driver is going to turn on in the nanosecond range?


The only other problem could be if somehow a driver was dumping more current or voltage into the diode in its "fast start up" mode than in its "slow start up" mode. If the driver, for some reason, performs better with a soft start, then go for it. But if the driver is clean on fast start up (which it seems it should be), then it's not going to hurt the diode.

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Old 10-01-2009, 09:13 AM #3
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Default Re: Soft starts and stops

Uhm, about slow-start, or softstart circuits, it can help a bit in decreasing initial spikes of boost drivers, so diodes lives longer and happy

About slow stop, i don't know, but for logic, it must not help so much, nor in switch cycle (LDs are planned for be used in 2 MHz modulation, so built on purpose for extremely start-stop cycles), nor for the thermal cycle, cause the mass of the diode body itself is so big, compared with the mass of the bare chip, that a soft stop don't change in any appreciable way the thermal derive of all the assembly, not for short times, at least.
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Old 10-01-2009, 03:53 PM #4
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Default Re: Soft starts and stops

Thanks, I think the ouput capacitor should sufficiently keep the driver from overloading the LD.

Dave
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Old 10-01-2009, 06:53 PM #5
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Default Re: Soft starts and stops

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Thanks, I think the ouput capacitor should sufficiently keep the driver from overloading the LD.

Dave
Overloading the LD in which way....

Jerry
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Old 10-02-2009, 12:36 AM #6
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Default Re: Soft starts and stops

Super Caps-The Electronic Goldmine

throw one of these tiny super caps in parallel with the diode... it with charge at startup, but it's resistance will grow about about 2-3 seconds to where your diode is near full power. It depends on the value of the cap you get, and the current you're providing.
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Old 10-02-2009, 03:57 AM #7
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Default Re: Soft starts and stops

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Overloading the LD in which way....

Jerry
Yeah, not well worded. I should have just said thanks for the advice, I'll be scraping the soft-start idea. I was just referring to the output cap of the boost driver. Without that element the LD would be subject to current spikes...I think.

Supercaps, they're neat, but I haven't used any as of yet.

Dave

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Old 10-02-2009, 04:47 AM #8
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Default Re: Soft starts and stops

They're perfect for this application since they are rated for low voltages, and they stay in a compact size.
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Old 10-02-2009, 05:25 AM #9
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Default Re: Soft starts and stops

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They're perfect for this application since they are rated for low voltages, and they stay in a compact size.
Just make sure they don't come disconnected, and then reconnected to your lasr diode while charged. Although they may provide a soft start, they will certainly dump a large amount of power into your diode if reconnected.
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Old 10-02-2009, 10:33 AM #10
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Default Re: Soft starts and stops

I've built some drivers with soft start. One thing i noticed back in the day of the 16x burner diodes, is that they always died on startup, never when having been on for a while. There is no technical explanation for this, just an observation.

Using soft start does help that problem, but only shifts it to a point where the diodes die in mid operation anyway.

The more recent diodes like the LPC's and the PHR and later blurays seem a fair bit sturdier though, i have yet to kill one on reasonable current without further abuse.

As for soft-stop: I've never tried as that would require a big buffer cap, or smart wiring where the switch doesnt actually interrupt battery supply power.
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