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Old 05-16-2014, 06:15 PM #1
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Wink Driver for DVD diode base on LP2985

I got a few diodes from DVD.
I want to make a simple driver with TTL modulation.

I know that the red diode need 2.5 volts, but the store I found lp2985 3 volts.
I know that the diode current is important, but lp2985 may issue a maximum of 150ma. What should save diode from burn?
lp2985 has input on / off http://www.ti.com/lit/ds/symlink/lp2985-n.pdf
Whether it is suitable as a modulation ?

3 volts converted to 2.5 probably can be delivered to a resistor in parallel?

Thanx.


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Old 05-16-2014, 11:20 PM #2
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Default

First of all, you don't have to worry about the voltage of the driver as long as it is above the diode's forward voltage.
A diode is a current limiting device, so yes you only need to worry about regulating the current.
150mah should be fine, as long as the drive the diode came from is at least 16X or faster. Don't quote me on that, not all diodes are the same so it may kill it, but it will probably be OK.
Making your own TTL driver is, as I understand, is somewhat complex. So if you're not well versed in circuit theory it may be tough.
Just buy a TTL driver on fleabay for a few dollars.

EDIT: Are you aware a SOT-23 is an SMD (Surface mount device) package? You would need to make your own PCB's in order to use it.
Also why are you using that chip in particular? This sounds like it'll be your first build, so why not just make an LM317 Driver?
It won't have TTL input, but then again the chip you linked doesn't either. You could make an LM317 based driver and just use a switching transistor to create TTL modulation. Do a quick search on here I've seen a few TTL schematics floating around here.

Last edited by oahu99; 05-16-2014 at 11:38 PM.
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Old 05-17-2014, 08:35 PM #3
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Default Re: Driver for DVD diode base on LP2985

Quote:
Originally Posted by oahu99 View Post
You could make an LM317 based driver and just use a switching transistor to create TTL modulation. Do a quick search on here I've seen a few TTL schematics floating around here.
Yes, I have seen many drivers on the LM317. But I was so naive, I thought that input ON/OFF (enable) some LDO can be used as TTL.

And thus simplify and reduce the circuitry.
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Old 05-18-2014, 01:22 AM #4
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Default Re: Driver for DVD diode base on LP2985

The LM317 really isn't a very good choice for practical builds though.

I know its universal, well documented, commonly available and such, but it does require a dropout voltage of 3 volts between input and output, or 4.25 volts between power supply and load to function within stated parameters.

It often works with a lot less dropout voltage, but this operation is not guaranteed in any way, so you could be in for an unpleasant surprise using the lm 317 as a current limiter.
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