Using a DC/DC converter to power the laser

IgorT · Feb 11, 2008

drlava said:
Yes, that's what I'm saying. You could use a pot as Rout of the zxct1009 then to vary the current (pot in series with a fixed resistor to limit the max. current would be best). Run the Vout to the feedback of the max757. The only thing you'd have to test for is the startup transients of this pair, to make sure there isn't a surge.

But it says maximum continuous current 25mA.. What if i want more?

For a Blue Ray i would rather have 35-39mA..

drlava · Feb 11, 2008

IgorT said:
But it says maximum continuous current 25mA.. What if i want more?

For a Blue Ray i would rather have 35-39mA..

That's the max output for the power sensing lead. You use a shunt resistor for the load. A shunt resistor down to .1 ohms should be fine, which will only drop 35-39mV.

Have a look at how this part is supposed to be used, maybe that will clear things up for you:

IgorT · Feb 11, 2008

phenol said:
igorT, i wouldnt mess with inductors connecting more than one at a time. you never know what would happen if their fluxes intersect in some way.

Yeah, that's what i was afraid of.. Thanks!

phenol said:
why 47uH? it should be ok to also use a larger one if available at the expense of higher resistive losses, though.
you could also use toroids from a pc power supply and experiment with wdg number. ive tried 3 different inductors with MC34063 and all worked ok.

The MAX1674 recommends 22uH for most applications and 47uH for higher currents.

So you're using higher values than that?

I have several ferrite cores, with known AL values, so i tried winding these exact inductances. But i could try an even higher inductance, if you think it would work better...

Your project sounds interesting. I need to check out the datasheet of the IC you're using..

IgorT · Feb 11, 2008

drlava said:
That's the max output for the power sensing lead. You use a shunt resistor for the load. A shunt resistor down to .1 ohms should be fine, which will only drop 35-39mV.

I checked that schematic out, as soon as you explained to me what this thing does.

In that circuit higher currents are mentioned, that's why i didn't understand why it only says 25mA in the datasheet.

Can the zxct do up to 300mA as well? That would solve all my problems, even for the burners.

Again, thanks for bringing it up to my attention!

EDIT: From that circuit it would seem it can do 400mA or more. This is great! I'm ordering it immediatelly.

phenol · Feb 11, 2008

well, i calculate what Minimum inductance my booster needs and i use higher than it. for instance, i figured that i'd need 22uH, but i put 100, as thats what i had in hand. i also tried with 36uH and found no difference. we'll see if its gonna be that easy when i put an external NPN switch with Ipk>4A...
MC34063 has been around for more than 2 decades, it is a universal regulator. unfortunately, its ref voltage is rather high and its min operating voltage is 3V.

drlava · Feb 11, 2008

IgorT said:
EDIT: From that circuit it would seem it can do 400mA or more. This is great! I'm ordering it immediatelly.

The part itself doesn't 'do' any current, it merely measures the voltage across the current shunt resistor and produces a relative small current for measurement. If you had the right shunt resistor, it could measure multiple Amps.

IgorT · Feb 11, 2008

drlava said:
The part itself doesn't 'do' any current, it merely measures the voltage across the current shunt resistor and produces a relative small current for measurement. If you had the right shunt resistor, it could measure multiple Amps.

Yes, i noticed the current doesn't even flow through it..

Ok, so it measures the current through a resistor and feeds this back to the step up IC to adjust it's output voltage and keep the current constant... That still means it would work with the two of my ICs, that are adjustable. Need to order some more, since i need four circuits.

Sorry it took me that long to get it..

Again, thanks for your explanation (and your patience

)!

woop · Feb 12, 2008

if you want some inductors, coilcraft has free samples
I am going to order a few NCP3065's and see how they go, i would prefer not to have an extra op-amp for measuring current.
although does anyone here know how to wire up a lm311 for that use? I have a few lying around

phenol · Feb 12, 2008

LM311 is a comparator. Try LM358 instead, or at best - one with input and output common-mode range including supply rails.

IgorT · Feb 12, 2008

woop said:
if you want some inductors, coilcraft has free samples

Thanks woop!

I just ordered 6 different inductor samples 4x each to my company's R&D department. Now i'm set for pretty much any boost or buck IC i might experiment with.

If i were to order these from farnell, it would have cost me around 40-50 pounds...

Thanks again!

IgorT · Feb 12, 2008

drlava said:
As for current control, have a look at the arcls grey market board on the previous page. The ZXCT1009 is nice, but it needs at least 2.5V Vi/Vo differential to operate, making it unsuitable for use with the max757 for red lasers. It would work for blu-ray however (2.5V+1.25V= 3.75V < Vd (4.8-5V)).

Ok, now that i finally understand how it works, i have another question. You said it wouldn't work for burners, due to a too small voltage difference if i used the MAX757..

What if i use the circuit exactly as it is in the arcls board? Would it then work with a burner, since the ZXSC420's sense voltage is lower?

My LD needs 2.92V for 270mA.. Since the ZXSC has a feedback voltage of 300mV it would mean 2.5V + 0.3V = 2.8V < 2.9V, so it should just work, right?

I can actually get both the ZXSC and the ZXCT for less, than one LM1086..

BTW, would i need the schottky diode and the transistor also from ZETEX or could i use substitutes for that circuit?

phenol · Feb 12, 2008

IgorT, where are you ordering those zetex chips from? I can find almost any member of their E-line transistors locally, but nothing else....
BTW, coilcraft do indeed offer a broad range of inductors plus a handy tool for calculating the buck/boost inductor value. I ordered a toroidal coil and a bunch of RF broadband xformers and elliptical low pass filters.
On semiconductor also offers free samples but i think one has to cover handling/shipping expenses
IgorT, since you have some old dvd burners, you could look for low-drop regulators in them

drlava · Feb 12, 2008

IgorT said:
My LD needs 2.92V for 270mA.. Since the ZXSC has a feedback voltage of 300mV it would mean 2.5V + 0.3V = 2.8V < 2.9V, so it should just work, right?

I can actually get both the ZXSC and the ZXCT for less, than one LM1086..

BTW, would i need the schottky diode and the transistor also from ZETEX or could i use substitutes for that circuit?

Your observations are correct, and you could substitute the transistor and diode with similar performing ones. If there were a boost reg that had a 300mV or lower Vsense and internal xsistor/diode that would be ideal.

IgorT · Feb 12, 2008

phenol said:
IgorT, where are you ordering those zetex chips from? I can find almost any member of their E-line transistors locally, but nothing else....

I'm ordering through a Slovenian store, but they order from Farnell: http://uk.farnell.com/jsp/home/homepage.jsp

This is the UK branch, but they also have one in the US and retailers all over the world. They have all the ZETEX components needed, and you can get almost anything there.

phenol said:
On semiconductor also offers free samples but i think one has to cover handling/shipping expenses
IgorT, since you have some old dvd burners, you could look for low-drop regulators in them

Yes, On Semiconductor requires US$20 shipping and handling for samples.

But i managed to order samples of LM3402, a current regulated buck converter from National and MAX1570 & MAX1698 current regulated boost converters from Maxim for free, once i gave them my company's e-mail address.

I didn't think about looking for regulators in old DVD burners. I should check them out.. Thanks!

IgorT · Feb 12, 2008

drlava said:
Your observations are correct, and you could substitute the transistor and diode with similar performing ones. If there were a boost reg that had a 300mV or lower Vsense and internal xsistor/diode that would be ideal.

Yeah, that would be great.

Again, thanks for your explanations and all the help!

I think i'll order some of these ZETEX components.

It would seem Farnell is runing out of ZXSC420s and they don't intend to order more...

BTW: Is the sense resistor critical in this circuit? The datasheet recommends two specific ones.. Does it need exactly 0.3 Ohm?
Guess i should add this to my shopping list.. Farnell should have it.

IgorT · Feb 12, 2008

Oh, and does anyone know what the value of R1 in that original arcls circuit is supposed to be?

Using a DC/DC converter to power the laser

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