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Thread: Analog laser diode drivers?

  1. #41
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    Quote Originally Posted by lucerne View Post
    Hi all




    That would be interesting, WHY...
    No reason.... Just thought it might be a good idea given that at least some of their shortcomings appear to be overcome by using shunt mod.

    Thanks Robin, those posts are the best quick guide to what points really matter, that I've seen. Either I should read the LaserFAQ more closely, or there's stuff here that should be in there.

    Are there any special points to specify op-amp choice with? I'm hoping some general purpose FET input type like the LF411 or LF412 will do ok, because that is incentive to make sure I have more of those around. Anything that can help narrow a good selection for a general part for many tasks is a good thing, it helps prevent that foul obsolescence problem that is occuring far too often now, as well as making it easy to get bulk discounts for stuff. Would that those LF411 op-amps were made in that tiny 5 pin shape you're using, those are nice.

    One other question: could there be anything to gain from using high speed CMOS logic IC's like the hex schmitt triggers? The idea of building those into PWM oscillators and buffers for fast PWM proportional control is appealing, they're small and cheap, but I don't know yet what sort of risks they might have of overshoots and other nasties. I'll get one and try it but my findings are unlikely to be the best information to post here so I had to ask...
    Last edited by The_Doctor; 08-10-2007 at 04:56.

  2. #42
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    Would these op-amps be much use?
    http://cgi.ebay.co.uk/300-LOT-TI-TLV...QQcmdZViewItem

    They're very cheap, given there are 300 in each of 57 remaining lots. If the seller was selling fakes I don't think they'd have reached 7600+ feedback at 100%, nor risk damaging that record with a few dollars worth of bad sales. The unity gain bandwidth at 5V is better than a 741, the DC offset is better, they operate with low power on a single rail, the input resistance is high, the output drive strong, and they look like a good general purpose thing to have a lot of. Any catch to these IC's?

  3. #43
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    I don't think they would be much use.
    In 'high-bias' mode they have a small signal bandwidth of 700kHz & a slew rate of 3V per microsecond.
    Also looking at their o/p voltage vs Io they are only good for a few mA out.
    Not great for rapid charging & discharging a MOSFET gate.
    In conclusion - right price but a little slow.
    If you increased the feedback capacitor (470pF) by a lot it might be made stable but 30kHz modulation would be out.

    I have used LMH6645MF (55MHz, 40mAo/p) and LMV116MF (45MHz, 30mAo/p) both with great success. Any fairly quick (25MHz+) op-amp with a R-R o/p and an input whose common mode range includes the 0V supply and has a sensible o/p current would do.

  4. #44
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    Ok. I bought some anyway, as you say, the price IS right. When I need a general purpose cheap SMT op-amp for single supply use for any non-critical task I can grab one of those first, it will save me agonising over what else to try, if it works.

    Any thoughts on this question?
    One other question: could there be anything to gain from using high speed CMOS logic IC's like the hex schmitt triggers? The idea of building those into PWM oscillators and buffers for fast PWM proportional control is appealing, they're small and cheap, but I don't know yet what sort of risks they might have of overshoots and other nasties. I'll get one and try it but my findings are unlikely to be the best information to post here so I had to ask...
    It's an interesting idea because it could be very cheap and easy to do. Maybe with 74HC to directly drive a laser diode if you have enough buffers in parallel? I was told it might be viable. The idea of using hex schmitt trigger inverters like that was familiar to me, the idea that they could be safe for laser drive was not.
    Last edited by The_Doctor; 08-14-2007 at 00:12.

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