Rethinking our approach to the DDL?

[rhd]

I just stumbled on these regulators today.

http://www.advanced-monolithic.com/pdf/ds1085.pdf

I know you guys were looking for a lower dropout regulator than the lm317, and these seem much improved in that regard, with a higher current capacity as well.

We've been using the 1085 for a while now. I'm not sure many people are still doing serious 445 builds with the LM317. In fact, jib77 is already selling a 1085-based driver that does 1.8A beautifully, and a few newer members have created driver runs with their own PCB layout. There are a number of good threads on the board re the 1085. It's a good IC. I've used it fairly exclusively for all of my builds these last few months.

 

[qumefox]

Grr. Blame the search for that. I searched for ams1085 before I posted and got zero hits. heh.

Forget I said anything.

 

[chefla]

I just finished my first 445 build using a micrel driver IC. This thing got me 1100mA@4.5V from 2xCR2 3.0V batteries. With the 1085 and the same config I only got 900mA@4.2V. I think, I have a new favorite regulator IC....
I will post a review once I got my pictures sorted out...

 

[Jufran88]

Would that happen to be a Micrel MIC29312? Rhd and I did some testing with those ICs and they have the lowest drop out I have seen so far, for linear regulators, 250mV at 1A and 600mV at 3A (full load).

But I'm a little bit confused. 1100mA@4.5V and your using 2 - 3V CR2? Shouldn't it be 1100mA@6V?

 

[rhd]

Would that happen to be a Micrel MIC29312? Rhd and I did some testing with those ICs and they have the lowest drop out I have seen so far, for linear regulators, 250mV at 1A and 600mV at 3A (full load).

But I'm a little bit confused. 1100mA@4.5V and your using 2 - 3V CR2? Shouldn't it be 1100mA@6V?

I think he means that the diodes is consuming 1100mA and 4.5V, not the driver... ?

 

[chefla]

Sorry for the confusion. The diode is consuming the 1100mA at 4.5 Volts, not the driver. It was late at night and maybe I should have used a clearer wording.:tired:

EDIT: I used the MIC29152. Maybe I should have ordered a few samples of the MIC29312 as well and compare them. The datasheet looks promising. It has got worse regulation but the dropout seems to be lower at the same current.

Jufran88, did you and rhd use it as a voltage regulator? Or wire it for current? If the latter, than I doubt that you got the 250mV voltage drop total. You would still need the 1.24V for the sensing resistor on top of the Vdrop. Or not?

 

[Benm]

Today, we regulate voltage and current external to the power source (battery) so that we must consume energy (watts) to perform the regulation: hence the drop-out problem. I'm thinking that one would need to fully abstract the battery using a DC-DC converter with variable efficiency feedback - thus giving the ability to use both voltage and current feedback simultaneously and in real-time. What we would be doing is essentially building a miniature lab power source with both voltage and current regulation. It would consume power, but essentially from the view of the LD, the current and voltage both would be in lock step and regulated. This would obviously require the removal of the capacitor that we almost universally see across the LD.

Of course, maybe the coffee is doing the talking...

I suppose this is what switchmode drivers do already. The good ones (for laser use) are constant current, and will only output a limited voltage.

Perhaps it would be best to seperate threads for design of linear and switchmode drivers, since the approach is quite different, and the choice of either would be mostly based on what you need to work with.

Linear drivers are only useful when the battery voltage is slightly higher than the diode voltage, but this IS a typical scenario when running a bluray or 445 from 2 lithium cells in series. It applies to running an 808 pump diode from a single lithium cell too.

 

[Jufran88]

Ah, ok I get it now.

From the looks of both datasheets it seems to have about the same dropout voltage at the same current (MIC29312 vs. MIC29152).

It looks like the MIC2950X series has a lower drop out (~170mV @ 1A). However, the datasheet it says its drop out in Volts which is probably a mistake. It can't drop out 170V @1A.. :wtf:
I might order some for testing, but its about $5.60 USD for one at digikey... Yikes!

We wired it up as a current source, but we found it does not have continuous ground. I'm not too sure if that would affect its use or not. I'm planning my LPC-826 soon using the MIC29312 and I'll see if it works well or not.

And yes you would have to add the reference voltage along with the drop out voltage to get the total drop out needed. It's still pretty good compared to a 1V drop out plus the reference voltage.

Sorry for the confusion. The diode is consuming the 1100mA at 4.5 Volts, not the driver. It was late at night and maybe I should have used a clearer wording.:tired:

EDIT: I used the MIC29152. Maybe I should have ordered a few samples of the MIC29312 as well and compare them. The datasheet looks promising. It has got worse regulation but the dropout seems to be lower at the same current.

Jufran88, did you and rhd use it as a voltage regulator? Or wire it for current? If the latter, than I doubt that you got the 250mV voltage drop total. You would still need the 1.24V for the sensing resistor on top of the Vdrop. Or not?

 

[chefla]

I just posted my MIC29152 story here.
And Jufran88, try to sample the MIC295xx from Micrel. Thats what I did ;-)

 

[Jufran88]

Here's the one I was looking at: *Digi-Key - 576-2241-ND (Manufacturer - MIC29502WU)*

And here is the work done on the MIC29312: *http://laserpointerforums.com/f67/mic29312-based-linear-driver-62028.html*

 

[chefla]

You are right, this MIC29502 is the 5Amp version and has an even lower Vdrop than the 1.5Amp version I used. Bummer
Higher current by less internal resistance -> lower Vdrop at the same current? Sounds right.