# 10A Buck Switching Regulator



## KillingTime (Sep 17, 2009)

Hello all,

[PLEASE NOTE: I'm no longer suggesting people use this voltage regulator on its own, without some sort of external circuit to overcome the negative voltage drop on the SST-50 led as it heats up) . The negative temperature coefficient of the LED on extended runtimes causes increasing amounts of current to be drawn - resulting in LED damage. My advice - stick to current regulation. This regulator may be of interest to other projects, so I'm leaving the data up.]

[EDIT: (again!). See pepko's post further down, a solution to overcomming the negative temperature coefficient on the LED may have been found.]

Just thought I’d share my experiences with a regulator that I purchased for an SST-50 project.

For those that don’t know, a company called Phatlight have brought out two emitters that can be run at 5A and 10A (@3.6v). The SST-50 and SST-90.

Driving these emitters is not a problem. You can do direct drive from a single Li-Ion, or run 4 x Shark Buck regulators from the shoppe. The problem with direct drive, is that you don’t get full output from the emitter over the battery output range – but the flashlight build is simplified due to the lack of regulator. Using 4 x Shark Bucks @ 2.5A each is possible, but much more expensive, and there’s the space consideration. So, I decided to look for an ‘off the shelf’ regulator that could be bought cheaply, and would do the job of running an SST-90 at full power (10A).

I’ve found what I thought might do the job, and my experiences are detailed here.
The regulator I used is made by Artesyn Technologies, and is called the LD010C. Basic specs below (please note, I'm not selling these):

Cost: 11.50GBP (16.1USD) Single Quantity
DC-DC Buck Switching Voltage regulator. 50W max handling capacity.
Input Voltage: 3-18.8V (2 or 3 Li-Ion battery)
Output Voltage: 0.59 – 5.1V
Output current max: 10A
No input or output capacitors needed.
Small: 11 x 21mm (including pins).
Convection cooled – will need custom heat sink if no fan is used (see below).







You’ve probably noticed by now, that this is a voltage regulator (and not a current regulator like the Shark Buck). That’s the one drawback. Current regulators are better for this application (high power LED) because the power delivered to the LED is not affected as much by LED temperature. The SST-50 and 90 have a negative voltage temp coefficient, which means as they heat up, the Vf goes down. You can overcome the worst effects of the neg temp coeff by good LED heatsinking, and starting with a slightly lower regulator voltage. This is what I did, and it worked for me.

As this regulator was unknown (to me), I did some testing on the output regulation. The results can be found below. If you’re designing a LED flashlight around this regulator, then the data could come in handy because it details the output drift over input voltage and temperature. This regulator has a nasty habit of raising its output voltage as the input voltage drops (see graphs). If you’re driving the 50 or 90 at high power levels to start with, and the output rises, even a small rise can lead to huge increases in current. The trick to achieving good (spec. sheet) regulation (0.042 dV) over the input range, was to use short leads between the regulator and load (<9cm). I used 30cm lead initially, and the regulation was poor (0.27 dV rise).

The V/A curve for the SST-50 is shown below:






You can see at 3.5V, the current drawn is around 4.5A. If the voltage were to shift up by 0.27v, the current would disappear off the graph (8 – 9A ?). Clearly unacceptable. A shift of 0.042v from 3.5v takes us to 5A. While the dVout was acceptable for me, it might not be for you if your application is different. You will need to download the spec sheet for your LED.

As for the regulator, it only needs one external component to set the output voltage. I used a 22 turn miniature preset (10k). The standard one turn presets will not offer enough fine control. As the resistance drops, the output increases.











Some tips:

1.	The regulator is designed to be cooled by air convection. I made an aluminium heat sink (pictured above) that did the job in the absence of air convection (using 2mm thick aluminium strip). The copper braid pictured is de-soldering wick. All stuck together with Arctic Alumina Epoxy. Both chips on top and the one chip underneath are cooled. You still have to attach this heat sink to something metal. For testing, I used a TO220 heatsink. The regulator pictured managed a 2 hour burn in test. 1 Hr at 5A and 1 Hr at 7A (see results).

2.	I’ve noted dVout on the results. This is the maximum rise in output voltage over the whole input voltage range (worst case). If you’re using protected Li Ions, then they’ll give up way before you hit the minimum input voltage (4.5v). For example, 3 x Li Ions would give you (3 x 4.2v) 12.6v, but they’d cut out at (3 x 2.25) 6.75v. Likewise, 2 x Li Ions would start at 8.4v and end at 4.5V. This is why I’ve listed the dVout over 13v – 7v and 9v – 4.5v ranges.

3.	I did manage 10A through a 0.35 Ohm load (3.5v). Only tried it for a few seconds though as my load heat sink could not remove all the heat generated.

4.	The regulator seemed to function with greater accuracy when no input or output capacitors were used.

5.	Even though I was using a Pentium 3 fan cooled heatsink (pictured), the heatsink was too hot to touch at 7A after 1 hour. People running the SST-90 for extended periods at 10A are going to need to think about thermal considerations....






Listed in order are:
1.	5A load test, using short load leads and no I/P or O/P capacitors.
2.	5A load test, using short load leads and 220uF I/P and O/P capacitors.
3.	5A load test, using long load leads and no I/P or O/P capacitors.
4.	1Hr run time test at 5A.
5.	1Hr run time test at 7A.


























Some other pictures:


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## HarryN (Sep 18, 2009)

*Re: 10A Buck Switching Regulator (SST-50 & SST-90 application)*

Hi - Thanks for posting this.

You are exactly right - it is really hard to find a current controlled buck regulator in anything above 2 amps or so. It is not hard to see just why these are so difficult to produce - fundamentally, a current regulator is just a voltage regulator where all of the current is measured by dropping it through a resistor - at least that is my understanding. That much current takes a heck of a resistor, and its related losses.

I think that TI also markets some similar voltage regulators (using their parts of course).

Its actually fairly impressive that the Vout maintained all the way down to about Vin - 1 volt. The efficiency drop trend is similar to what other controllers seem to do as this converges - not entirely sure why. Its amazing just how hard it is to get a regulator design above the 90% point, so this isn't all that bad. I am not a big fan of open inductors like that, but they are common (and cheap)

Similar to what you have found commercially, I have been working on a 6 amp regulator with some other features, but similar in concept. I don't see a short term way around using voltage regulators with some finessing to deal with these high current draw LEDs - but who knows. :shrug:

It is curious that the output cap didn't help. Did you happen to take any ripple measurements ?- not that this is a big deal in this application of course

Thanks for pointing this and take care.

HarryN


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## KillingTime (Sep 18, 2009)

*Re: 10A Buck Switching Regulator (SST-50 & SST-90 application)*

Hello,

I've not done any ripple tests.

Further to my post above, I've been doing some runtime tests on the SST-50 I have.

Not good news I'm afraid.

Even though the regulator was able to maintain a steady voltage to the LED, the negative temperature coefficient of the LED is just too high for extended runtimes - even with a copper heatsink on the emitter (no star).

I did a 30 min runtime test this evening and by the time I'd finished, the regulator had gone in to hickup mode to protect itself (>10A).

It's a shame. This regulator looked promising. Best I point this out now, rather than other people try to emulate the same and fry their emitter.

Back to the Shark Bucks then .


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## 2xTrinity (Sep 18, 2009)

This could probably be reconfigured into a current regulator by adding an additional opamp. Use the op-amp to compare the current through a sense resistor, to a fixed reference voltage. This reference voltage can be made using a zener diode and a voltage divider -- or even using a voltage divider across the LED itself (I have done this before -- use the LED itself as a ~3.3V zener diode voltage reference for a current regulator which operates as I have described) 

Then, if the driver has an anolog input control pin (eg higher voltage applied causes higher voltage output) it would be possible to connect the output of the opamp, to the control pin on the voltage regulator. In the datasheets you have linked, it appears that the external resistor you have applied is part of a voltage divider network. Depending on what resistance you choose, the voltage to the analog control pin will vary accordingly. You could likely elect to not include the sense resistor, and place the output terminal of your opamp there to actively manipulate that voltage. Although I can't say for sure unless I get a chance to play with this driver, there may be a way to rig it to operate as a current regulated driver using external feedback with an opamp.


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## KillingTime (Sep 18, 2009)

Hello 2xTrinity,

I was wondering when someone would suggest that...

My original idea was to turn this into a current regulator. I tried the circuit below (with some different values to those pictured), and it did work as a current regulator up to 5A, but latched up beyond this, and even below 5A, stability was an issue. If the load was changed too quickly, the regulator would latch up (5.1V).






What I found was this:

The regulator will try to adjust its output so the voltage on the feedback pin remains at 0.59V, BUT, the trim resistor that you're supposed to add draws current to ground (measured around a few mA). I originally omitted the trim resistor, adn connected the op-amp output to the trim pin. The oputput went low.

After some thought, I realised, the converter was using the op-amp feedback network as a trim resistor to draw current to ground.

So, I added a trim resistor (R3 - 1K) to allow the regulator to send current to ground and jacked up the value of the feedback resistors by 100x, so they wouldn't interfere with the current drawn by R3 from the regulator. This worked - as I say - up to around 5A - with some stability issues.

The diodes keep the ouput from rising above 1.2V. The feedback pin on the reg isn't meant to shift too far from 0.59v and having the op-amp applying 5v (supply) could damage it - just a development precautionary measure.

With hindsight, I think the problem was the length of the leads going to the prototype board holding the op-amp. You can see from the test results that the regulator does a poor job with long leads (~30cm). If the voltage reg isn't doing it's job properly, then the current reg addition will function poorly too. I gave up on the current reg addition because I didn't know what was causing the instability. It was only after I played with the reg in pure voltage mode that I noticed (by accident) that the length of the load leads was causing regulation issues.

I've run out of time with this now. Perhaps someone else could continue if they had the time. The longform datasheet does warn you not to place circuits in the feedback network of the regulator - because it could give rise to stability problems - but I had to try.

Nice idea though.

EDIT:

A 'poor mans' solution to creating a 10A current regulator using off the shelf parts, might be to use the LDO10C to drop the voltage down to 0.1 - 0.2V above that needed to run the emitter at your chosen current,.......and then place a load of AMC7135 boards from KD or DX between the voltage regulator output and the LED. The AMC7135 only has an overhead of 0.1V, so you'd get the best of both worlds - efficient voltage dropping with current regulation. 10A would require 7 x 1.4A AMC7135 boards though (9.8A output). Would take up some space.


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## mpf (Sep 19, 2009)

KillingTime said:


> Hello 2xTrinity,
> ...
> A 'poor mans' solution to creating a 10A current regulator using off the shelf parts, might be to use the LDO10C to drop the voltage down to 0.1 - 0.2V above that needed to run the emitter at your chosen current


 I am going to try this is a different scenario.
I design voltage chargers into my torches and my latest one is using a buck/boost voltage supply to both charge the batteries and to run the boost current supply that drives the leds.

The problem is the batteries NiMh want to be charged at approximately constant current but the battery voltage rise with charge also changes with temperature. 

The solution I am going to try is to use a uC controlled variable resistor to the control the output of the buck/boost voltage supply and measure the current going into the battery with an Attiny uC and adjust the resistor to adjust the voltage and hence the current. I expect to be able to adjust the voltage within 70mV over 10V to 22V need to charge the batteries. I intend to use MAX5453 variable resistor and an LTC3780 for the boost/buck voltage supply.

If you are only trying to adjust for changes in Led temperature then a similar approach to this might work together with the http://www.forward.com.au/uCLedDriver/build_basic_uC_led_driver.html to drop the last little bit of voltage to control the current


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## HarryN (Sep 19, 2009)

So I have the dumb question of the day here. A current driver can be more or less simulated using a Li cell and a resistor in series with the LED. I might be wrong, but I think of a cell as a voltage source with current limitations. (depending on design).

This would seem to imply that you could make this voltage driver work by simply putting some resistance in series with the voltage regulator and the LED. (Say .2 - 1 ohm) Is this really just a matter of not driving the LED up to the edge - in other words - set it up to drive at 4 amps instead of that steep part of the curve at 5?

What am I missing ?


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## LED Boatguy (Sep 20, 2009)

I've had good experiences with the National's 5A Simple Switcher LM22678-ADJ. I've only used it for voltage regulation, but always wondered if you could tweak it to run a SST-50. The LM315X series seems to be newer and (more importantly) has Ilim and Soft Start pins. Below is the LM3150.






First, you set max amps with the Ilim resistor. The chip will "skip a beat" if amps are over the current limit set here.

Next, you set voltage by the voltage divider Rfb1/Rfb2. That gives you a fixed voltage. Well, how about throwing in a thermistor here to throttle the voltage when things heat up? Also, the divider lends itself to multi-stage clickies for intensity control.

Just a thought.

The LM22678 has fewer components and could easily be made to fit a D-sized board.






PS IMO, dedicated current limiting is the way to go, but we may have to kludge it as the current regulation chip mfgrs aren't keeping up with the LED makers.


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## KillingTime (Sep 20, 2009)

Hello HarryN,

The 'resistor in series' approach would waste too much power. You'd need to heatsink the resistor against the flashlight body....which would already be very hot. At low drive currents this approach is an option. At the currents wer're trying to use it's an no go. Simulate it if you like on LTspice.

I think the way forward for the time being, is to go with the option I suggested above, Use the regulator in voltage mode to drop to 0.1 - 0.2 above that needed, and then use 5 to 9 AMC7135 boards. This has the advantage that all parts come off the self. No custom stripboard circuit construction (which would be a barrier to most people). I would imagine that quite a few on here are comfortable with soldering wires together, but whole circuits are another step up.

Someone should try this though before it's confirmed as a working option. There could be probs with that many 7135 boards in a confined space.


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## jtr1962 (Sep 20, 2009)

KillingTime said:


> My original idea was to turn this into a current regulator. I tried the circuit below (with some different values to those pictured), and it did work as a current regulator up to 5A, but latched up beyond this, and even below 5A, stability was an issue. If the load was changed too quickly, the regulator would latch up (5.1V).


Try a 1000 pF cap between the output and op-amp inverting input. I've found I needed to do that whenever I'm using opamps as part of a current regulator.


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## georges80 (Sep 20, 2009)

Putting an opamp to sense current (voltage drop across a sense resistor) and drive the feedback pin of a switching regulator "sometimes" works. 

The issue is that voltage regulating switching regulators assume a passive voltage divider as part of the whole control loop. By putting an active device (op-amp) into the loop you are changing the inherent control loop and often this will 'confuse' things enough that you'll get all sorts of strange operating (or not...) modes. Voltage regulators are designed to have very fast transient responses to varying loads (something that isn't needed for a LED load) and this makes them even more 'twitchy' to having an active device in the control loop.

Some switchers have external pin(s) to connect compensation networks (to set the appropriate poles/zeroes of the control loop) and some don't. With external compensation pins it is sometimes possible to get the control loop stable.

Finding a switcher that is designed ground up for current regulation (modern LED driver chips) simplifies things a lot. For the current levels you're looking at you would likely need a 'controller' and use an external power FET.

cheers,
george.


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## Linger (Sep 20, 2009)

*Re: 10A Buck Switching Regulator (SST-50 & SST-90 application)*



HarryN said:


> it is really hard to find a current controlled buck regulator in anything above 2 amps or so. It is not hard to see just why these are so difficult to produce


 
Actually you're right actually, I found a person who's made them for years, just not on CPF. A 9amp driver has been produced, it has been received for assembly and testing. It works. A person who has a lot of experience with these emiters is assembling a demo to test the driver's capability. Results pending very soon...

Best,
Linger


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## acgriff (Sep 20, 2009)

I may be in the wrong thread but it sound like you guys are the pros. I haven't built a circuit in 25 years, but I'm dying to try the new SST-50 in a mag. I was looking around the web for ideas and saw this:
LM338 - 5 amp adjustable regulator.

http://www.national.com/mpf/LM/LM338.html

Am I starting out way off base here?
Andrew


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## gSPIN (Sep 21, 2009)

LM338 is a linear volt reg, not switching.


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## BillyNoMates (Sep 21, 2009)

Sometimes a hall-effect current sensor is a better option for sensing the current than a series resistor - especially when you are talking about currents of >5A.
When converting a voltage reg to a current reg, just bear in mind the point raised by georges80 - changing the gain in the control loop can make cause it to go unstable - especially if one is regulating for different load/supply conditions. Most of the time it is not possible to tell if the loop is unstable without the proper test equipment as the LEDs will still come on - it just may not be regulating in the way that one intended and can cause the driver or LED (or both) to fail prematurely.


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## HarryN (Sep 26, 2009)

Thank you for the respones - it is interesting to see just how complicated life can get building these current regulating drivers.

One aspect of the original poster's project has me thinking about something we used to do - "burn in" the LED to drop the Vf. It is not so important anymore, as modern LEDs have pretty low Vfs, but a number of people (including me) used to run the LED at spec for 24 hours from an AC supply. This would commonly drop the Vf .2 - .4 Volts, and which then kept them pretty stable.

I wonder if some of the Vf shifting observed here was not so much from the dV/dT (change in Vf from temp) but instead from this initial burn in effect?

_____________

Killing Time - thank you for the comment about resistors and losses, but please check my math on something:

Suppose you have 1 LED junction so approx 3.5 volts Vf (after burn in). Just for fun, if I set a voltage regulator for 3.75 volts, and drop the last .25 volts x 5 amps with a resistor, then:

V = IR so 0.25 = 5 x R R = .05 ohms, which isn't that much resistance.

Power lost in the resistor = VI = .25 volt drop x 5 = 1.25 watts

Power to the LED = 3.5 volts x 5 = 17.5 watts

So the loss in the resistor is really not much in the big picture.

For me, the more challenging question is - at what point is the resistance too small for the system to work stabily?

I realize that I cannot safely come close to the max drive of an LED with this kind of setup without taking on risk, but for somewhat under driven approaches, I am still not really clear on the difference between a battery + resistor approach vs a constant voltage supply + resistor approach.

___________________

BillynoMates - Thanks for the post about hall effect sensors vs sense resistors - make sense.

___________-

George - always good to hear from you. :wave:


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## pepko (Oct 16, 2009)

LDO10C is very good regulator for SST-90 ...


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## Techjunkie (Oct 16, 2009)

*Re: 10A Buck Switching Regulator (SST-50 & SST-90 application)*



Linger said:


> Actually you're right actually, I found a person who's made them for years, just not on CPF. A 9amp driver has been produced, it has been received for assembly and testing. It works. A person who has a lot of experience with these emiters is assembling a demo to test the driver's capability. Results pending very soon...
> 
> Best,
> Linger


 
Any results yet?

Also... I came up with the following Linear Technology part numbers after searching for DC-DC regulators. I suppose one or two of these might work for an SST-90 driver that would fit in a Mag neck. (I wish I knew how to do it):
LTM4605
LTM4608A
LTM4600HV
LTM4614
Maybe someone here could piece together a prototype.


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## pepko (Oct 19, 2009)

I use very simple design - LDO10C as a voltage regulator with decreasing voltage with increasing temperature.

It contains one thermistor (1kohm) parallel with 510ohm resistor and serial with 100ohm trimmer for exact setup trim pin of LDO10C driver. 

You can see values on this picture:


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## KillingTime (Oct 19, 2009)

pepko said:


> I use very simple design - LDO10C as a voltage regulator with decreasing voltage with increasing temperature.
> 
> It contains one thermistor (1kohm) parallel with 510ohm resistor and serial with 100ohm trimmer for exact setup trim pin of LDO10C driver.
> 
> You can see values on this picture:



I'm glad someone has found a solution that works.

Have you done any extended runtime tests (w\current measurements)?

Sometimes the simple solutions are the best.

Well done.


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## luckybucket (Oct 20, 2009)

georges80 said:


> Finding a switcher that is designed ground up for current regulation (modern LED driver chips) simplifies things a lot. For the current levels you're looking at you would likely need a 'controller' and use an external power FET.
> 
> cheers,
> george.





I like the idea of a simple solution. With a sharkcage or even individual shark sinks from the shoppe the fet's could be sinked to the side of the light, even multiple ones if necessary. None of my lights are overdriven and its time to step it up and make a real light. Im thinking sst-90 at 12 amps on high, but a conservative 3 amps for normal use. This would require a very versatile regulator and pushing 4 sharks to their limit kinda worries me. IMO things run better when their is some headroom.


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## stinky (Oct 21, 2009)

pepko said:


> I use very simple design - LDO10C as a voltage regulator with decreasing voltage with increasing temperature.
> 
> It contains one thermistor (1kohm) parallel with 510ohm resistor and serial with 100ohm trimmer for exact setup trim pin of LDO10C driver.



Would you mind sharing some more details on your design for us newbs? I'm thinking that this *LDO06C-005W05-SJ* might be a good item for the SST-50. 6A should be sufficient for that emitter, and avnet has them in stock for around $11, with min order of 1. Paired with either the KTY83/110 or the KTY83/122 (specs are very close). http://www.mouser.com has those in stock for under $1 also with min order of 1.

In your picture I see the emitter clearly, but not the driver.

Thanks!


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## pepko (Oct 21, 2009)

stinky said:


> Would you mind sharing some more details on your design for us newbs? I'm thinking that this *LDO06C-005W05-SJ* might be a good item for the SST-50. 6A should be sufficient for that emitter, and avnet has them in stock for around $11, with min order of 1. Paired with either the KTY83/110 or the KTY83/122 (specs are very close). http://www.mouser.com has those in stock for under $1 also with min order of 1.
> 
> In your picture I see the emitter clearly, but not the driver.
> 
> Thanks!



This is my design, but it's only first version ...


Police (maglite) 2D with KD 52mm aspherical lens ...





http://www.mypicturehost.com/show.php/60336_DSC7302.jpg.html


DX 2mode switch - low mode with 82ohm resistor (about 0.8A from batteries), off with 10kohm resistor (stand-by = 0.050A consumption)




http://www.mypicturehost.com/show.php/60337_DSC7309.jpg.html

LDO10C with 100ohm trimmer and 510ohm resistor parallel to thermistor KTY83-110




http://www.mypicturehost.com/show.php/60338_DSC7310.jpg.html

LDO10C pin 'enable' is not used ...




http://www.mypicturehost.com/show.php/60339_DSC7311.jpg.html


Solarforce L900M modded with 6x Cree XRE Q5




http://www.mypicturehost.com/show.php/60340_DSC7295.jpg.html


Police Maglite SST90 current 9A - wide focus




http://www.mypicturehost.com/show.php/60341_DSC7297.jpg.html


Police Maglite SST90 current 9A - narrow focus




http://www.mypicturehost.com/show.php/60342_DSC7298.jpg.html


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## stinky (Oct 21, 2009)

pepko said:


> This is my design, but it's only first version ...
> 
> LDO10C with 100ohm trimmer and 510ohm resistor parallel to thermistor KTY83-110
> 
> ...



Thanks pepko! I also found this design (diagrams and equations) in the docs for the chip here, for those who are interested (see p.7-8):

http://www.powerconversion.com/assets/an_ldo03c_06c_10c_1199502565_appnote.pdf - Powered by Google Docs

the specs for chip are very nice. seems like you could just solder or embed the whole thermistor right into the heatsink without damage to ensure good thermal transfer. using different resistor combinations, you could have very wide or extremely narrow Vout ranges at differing temperatures. does anyone know the formula for how the Vout drops on SST chips as the temp rises? with that info, in theory at least, it shouldn't be too hard to figure out the components for a nice driver for these chips


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## LED Boatguy (Oct 21, 2009)

Very nice. Now we just need the SST90 in warm or neutral white...


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## Mike cz (Oct 23, 2009)

Beautiful work. SST-90 is with the aspherical lens great ...:twothumbs

_Appendix:
Idea of the thermistor was mine, but I did not know about this forum ... During production (2.9.2009) [email protected] P7 (published 17.9.2009 here http://forum.fotonmag.cz/index.php?showtopic=436&st=40 and now here https://www.candlepowerforums.com/posts/3131681#post3131681) I used 30Amps voltage DC-DC schwith and I needed something to compensate for temperature, because it is a voltage DC-DC schwith. So I added a thermistor to control DC-DC switch. Then was created this thread... Then Pepko used a similar DC-DC switch__ with termistor for SST-90.:thumbsup: I and pepko are friends from the Czech forum: http://forum.fotonmag.cz/index.php?
_


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## pepko (Oct 24, 2009)

Mike cz said:


> Beautiful work. SST-90 is with the aspherical lens great ...:twothumbs
> 
> _Appendix:
> Idea of the thermistor was mine, but I did not know about this forum ... During production (2.9.2009) [email protected] P7 (published 17.9.2009 here http://forum.fotonmag.cz/index.php?showtopic=436&st=40 and now here https://www.candlepowerforums.com/posts/3131681#post3131681) I used 30Amps voltage DC-DC schwith and I needed something to compensate for temperature, because it is a voltage DC-DC schwith. So I added a thermistor to control DC-DC switch. Then was created this thread... Then Pepko used a similar DC-DC switch__ with termistor for SST-90.:thumbsup: I and pepko are friends from the Czech forum: http://forum.fotonmag.cz/index.php?
> _




That's right, this was your theory and your experiences were very helpful for me. Thanks for your advice.

Now I'm trying to use SMD thermistor KTY82 (2kohm) paralel to 470ohm resistor. I think I don't have to use trimmer with that. And I want to place it directly under PCB of LED and minimize thermal transfer between thermistor and LED chip.


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## Alan B (Nov 22, 2009)

Good thread here. Some suggestions and thoughts:

1) the initial posting indicates 18.8V max in the text, this is a typo. Clearly 13.8V elsewhere. Might be worth fixing that.

2) I found a similar part at Mouser for $12. Murata LSS-T/10-W12. 50W 6V 10A output. They also had another similar part for a higher price.

3) There is a 2V Vin to Vout minimum on the Murata part so Vbatt minus losses needs to be at least 5.6V. Still likely ok for 2 Li-Ions if other various voltage drops are kept to a low enough value.

4) I'm considering driving one of these regulators with a microprocessor. Have it close the current loop. Since this is effectively a thermal compensation loop it does not need high bandwidth. The LED current can be read by a hall effect current sensing chip so no shunt resistance is required, or a low value shunt can be used. Many of the microprocessors have built in high gain differential channels on their ADCs for reading small value shunts.

The interface between the micro and these regulators would then be the issue. The regulators do not appear to take a voltage control input directly but an FET can be biased with a pwm derived voltage to replace the trim resistor. This setup can be used to get dimming control as well as current regulation.


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## Justin Case (Nov 22, 2009)

pepko said:


> DX 2mode switch - low mode with 82ohm resistor (about 0.8A from batteries), off with 10kohm resistor (stand-by = 0.050A consumption)



A 50mA parasitic draw seems rather high to be practical, depending on what batteries you are using. In one day, you'll burn 1200mAh. Even for a 10000mAh capacity D cell, the shelf life seems somewhat short.


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## Alan B (Nov 22, 2009)

Not sure what others were planning, but I'm planning to remove power from the regulator when the light is off.


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## Justin Case (Nov 22, 2009)

A flashlight without an immediate source of power is like a 1911 that's not in Condition 1.


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## Alan B (Nov 23, 2009)

Justin Case said:


> A flashlight without an immediate source of power is like a 1911 that's not in Condition 1.



I will use an FET controlled by the micro. No delay.


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## OldNick (Nov 30, 2009)

stinky said:


> Thanks pepko! I also found this design (diagrams and equations) in the docs for the chip here, for those who are interested (see p.7-8):
> 
> http://www.powerconversion.com/assets/an_ldo03c_06c_10c_1199502565_appnote.pdf - Powered by Google Docs
> 
> the specs for chip are very nice. seems like you could just solder or embed the whole thermistor right into the heatsink without damage to ensure good thermal transfer. using different resistor combinations, you could have very wide or extremely narrow Vout ranges at differing temperatures. does anyone know the formula for how the Vout drops on SST chips as the temp rises? with that info, in theory at least, it shouldn't be too hard to figure out the components for a nice driver for these chips



Interesting thread indeed.

It's the regulator you need to match, and let it take care of the LED (??)

How about using a trimpot in parallel with the thermistor, as well as the one in series? Then you could set for as little voltage drop or as much as required (???), within reason. Use the parallel trimpot to set it up, testing on a simple resistive load, controlling the thermistor with controlled temps.


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## ahx66 (Mar 3, 2010)

I was wondering if anyone has put a potentiometer in series with the trim resistor? You could choose your POT to be the max setting around the 9 A, and dial down the brightness.


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## lolzertank (Jan 31, 2012)

This is an old thread, but I believe I have a way to make the LDO10C current regulated well enough for LEDs.

The regulator voltage can be controlled by having an additional resistor and *a voltage source* connected to the trim pin. It's not too tricky to see how this works with the resistor divider, and it doesn't mess with the feedback loop so long as the voltage source is reasonably stable. The voltage source will be supplied by a microcontroller through PWM and a low-pass filter, which should allow current and temperature sensing and multiple modes among other things. Basically it's simulating a _really slow_ op-amp to avoid any instability, with some extra intelligence behind it.


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## mpf (Jan 31, 2012)

having tried both constant current (voltage boost) and constant voltage (buck/boost) switching regulators with out success. Could not get them stable. I have resorted to designing my own buck regulator uC controlled which does both constant voltage and constant current. Not very well regulated but good enough for feeding my torch as a table lamp and charging the batteries.

Will post the details later (much later at the rate I am working on it). Or contact me at www.forward.com.au if you want the present circuit.

p.s. mine is only a 2 to 3A buck so not actually in this league.


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