# help with: ZD850 1.5 Amp Constant Current Driver



## imageicon (Aug 23, 2007)

Hey guys i am looking at building some more bikelights and to cut down costs i am looking at cheap constant current regulation and i found this piece of tech.
http://www.cutter.com.au/proddetail.php?prod=cut755

as i understand it its just to hook up power leds and a resistor between RLIM and PGND ... it says in the datasheet that regulation is determined by an equation 0,2V/(1.15*iLED) where iLED being current wanted so say i want 1000 mah .. can any of you tell me wich resistor to put in?

what else needs to be done i see the typical application diagram and it seems to require some more work but is that necessary?

Cheers Jesper


----------



## frenzee (Aug 23, 2007)

For 1A, you'll need a .174 Ohm resistor. You could make your own from a 5 3/4" length of 24 ga. copper wire. btw, I think your link is for the chip only. You'' still need to put together the rest of the board or get the eval board.


----------



## imageicon (Aug 23, 2007)

ya it dosnt say if its the eval board or the chip.. id like the eval board i think... that was the resistance i calculated too it just seemed so small that i thought i was wrong!

think ill email them ask if its eval board or what it costs!


----------



## macforsale (Aug 23, 2007)

*


----------



## imageicon (Aug 24, 2007)

hey guys i wrote to mark " cutters ... the eval board is 10$ not to bad for a driver ... so do you guys recon thats a full functional led drive or do i still need to work on the circuit? im abit of a noob with elektronics and the whole led thing...


----------



## Luminescent (Aug 25, 2007)

imageicon said:


> hey guys i wrote to mark " cutters ... the eval board is 10$ not to bad for a driver ... so do you guys recon thats a full functional led drive or do i still need to work on the circuit? im abit of a noob with elektronics and the whole led thing...



Click on the link at cutters and download the eval board manual (here's the link):* 

 AN020_ZD850EVB_MANUAL_07 30 07.pdf

*Here is a quote from the beginning of the manual.
*
ZD850EVB Evaluation Board Factory Default Settings*
The ZD850EVB was configured to operate at a voltage supply
input of 12VAC/VDC to the VAC1/VAC2 of the device
driving 3 high power LEDs in series with ILED current of
300mA~350mA (details shown in Figure 4 under the Typical
Applications Section).

For 350 mA operation they show a 220uH inductor being required, and it looks like this MIGHT be what is supplied on the board based on the pictures ('Zywyn' didn't bother to SHOW the schematic and parts list of their eval board right up front like EVERY OTHER MANUFACTURE ON THE PLANET, so it's a little hard to tell). There is a 'Bill of Materials' at the end of the manual, but it suspiciously fails to list the inductor, so it's anyone's guess.

So 12volts supplying 3 White LED's at 350mA is pretty much guaranteed right out of the box, but higher current will require a different 'current sense' resistor, and possibly a new inductor.

The board seems to be mainly programmed by changing a single current sense resistor, but there are limitations based on the desired current, the number of LED's in series, AND the supply voltage.

For example for a 12-16 volt supply with 3 LED's and 1.5 amps of current, figure 5.3 shows that the current sense resistor will drop down to 0.06 ohms, but it also shows a different 68uH inductor.

The resistor value seems to be a little different from what the formula predicts so I guess there is some fudge factor based on actual testing needed.

So it's not quite as simple as you thought, but you should be able to deal with it if you can deal with fabricating a very low value resistor and bridging it into the PCB. It doesn't look like they provided terminals for this so you would have to bridge across an existing TINY surface mount resistor.

The inductor issue is a major pain in the *** because if you set the current too high for the inductor supplied, the core will saturate and the current will suddenly go very high and most likely destroy either the ZD850 chip, or your LED's (or BOTH).

Unfortunately, as I mentioned above, the inductor that is supplied doesn't seem to be spec'ed anywhere in the ZD850 Eval Board manual.

Without this info, and a little more information about just WHAT YOU PLAN TO DO it's a little hard to tell if this will work for you or not.

*Can't do much about the missing Zywyn info execpt contact the company, but perhaps we CAN nail down what you want to do a little more accuratly.*

- How many, and what type of LED's do you want to drive?

- What current were you planning to drive them at? (you mention 1 Amp).

- What battery voltage [or voltage range] are you planning to use?

If you are planning to use NiMH batteries to drive the LED's, they have a nice flat discharge curve, so you might be able to find a nice simple linear regulator constant current circuit and still get 80% efficiency or so.


----------



## macforsale (Aug 25, 2007)

*


----------



## Luminescent (Aug 26, 2007)

macforsale said:


> Luminescent,
> Why trash the manufacturer? Nothing out of line with their data sheet.
> 
> I ask that you edit your post. The forum's membership is not well represented by your comments.
> ...



Ok, in deference to your sensitivities, I have toned it down.

The data sheet is indeed reasonable, but the "evaluation board documentation", is just a BAD rehashing of the datasheet, the included schematics are at CHIP level NOT at BOARD level, so it is nearly USELESS for determining how to wire up the eval board.

Call me silly, but I come from the old engineering school when 'evaluation board documentation' at least told you SOMETHING specific about the eval board.

The closest Zywyn comes to actually explaining anything specific about the eval board, is to say that details can be found in Figure 4 (except figure 4 is really 4.3 and is just a general chip level schematic that doesn't give ANY specifics like the inductor part number or current ratings, or diode part number or voltage or current ratings, or pcb layout or wiring diagram)

True, I have seen a lot worse, but the fact that the eval board docs lacks even a simple wiring diagram or hookup information for the eval PCB alone makes it totally unacceptable.


----------



## WeLight (Aug 26, 2007)

Luminescent said:


> Ok, in deference to your sensitivities, I have toned it down.
> 
> The data sheet is indeed reasonable, but the "evaluation board documentation", is just a BAD rehashing of the datasheet, the included schematics are at CHIP level NOT at BOARD level, so it is nearly USELESS for determining how to wire up the eval board.
> 
> ...



These are not unreasonable comments, I will forward to Zywyn to try to get a little more 'meat' on the detail

Cheers
Mark


----------



## WeLight (Aug 26, 2007)

I have just uploaded a newer Eval board datasheet which addresses these questions
http://www.cutter.com.au/proddetail.php?prod=cut755

Cheers
Mark


----------



## macforsale (Aug 27, 2007)

*


----------



## Luminescent (Aug 27, 2007)

macforsale said:


> Gets one to the same document that 'Luminescent' used, but he made all the comments about missing information. :shrug:




Oh it's different all right. 

Actually, it IS a differently dated version but, sadly, actually seems to be WORSE in some respects :sigh:

For example the figure I thought was called out as 4 but which looked like 4.3 due to funky formatting, actually WAS "Figure 4. _ 3 LED something or other" but was jammed together by poor formatting which made it look instead like "Figure 4.3_LED something or other"

Now they still call out the 3 LED example at the beginning as "Figure 4" but the actual 3 LED example in the main body of the document has been change and is now "Figure 5"! (So it went from slightly confusing to flat wrong).

Other than the above NEW error in referencing the proper figure number, their doesn't seem to be any change at all. :shrug:

It was damn nice of Mark to make an effort to respond so quickly, but the document is now apparently worse than ever, and still seems to lack even a minimal "Hook-Up Diagram" at *BOARD* level, and full parts breakdown with the INDUCTOR specs listed for the actual part used on the eval board. Sure, there is tons of info about PICKING an inductor, but we shouldn't need to PICK anything, since there is an inductor already on the board, just tell us WHAT THE HELL IT'S RATINGS ARE, so we can know what current level the board can run with it's built in inductor.

Without this info, it's impossible to know if it's safe to reconfigure the board for 800mA. because if you drive the inductor into core saturation it can melt right off the board (after the high current destroys the ZD850 and your test LED's).

So again, thanks for the effort, Mark. This board does indeed look like it could be a real nice option for CPF type projects, because unlike some Eval Boards, it's not overpriced, or as big as a house, but a board-level hook-up diagram and spec info for the actual inductor on the board is still needed.


----------



## macforsale (Aug 27, 2007)

*


----------



## Luminescent (Aug 27, 2007)

macforsale said:


> Yes the formating is confusing.
> 
> Fig 4 is the 3 LED at 300-350 ma configuration and Fig 5 is the 3 LED at 1.5 amp. Values of R1 and L1 adjusted for current levels.
> 
> ...




Hi mike, I think you missed my earlyer point. Download the PDF doc again, all the figure numbers DID change.

The originally posted link was to - AN020_ZD850EVB_MANUAL_07 30 07.pdf

(This matches your figure numbers)

The currently posted link points to - AN020_ZD850EVB_MANUAL_08 24 07.pdf

(notice the changed date)

Maybe you need to clear your browser cache and update the page to see the new link?

The 3 LED 350 mA. example that WAS figure 4 is NOW figure 5, and figure 7 and figure 8 on page 11 are NOW figure 9 and figure 10.

None of the above changes anything substantive about your comments, but it could be a little confusing if someone new to the thread doesn't know that there are two versions.

The figures you referenced on page 11 are just poorly drawn board views, so one would have just about as much luck figuring it out for oneself by just checking out the board with a 10x loop (since it only appears to be a simple two layer board and everything is right there up front).

Of course the chip level diagrams are already available in the ZD850 chip documentation, and if I have to guess at component values and trace circuit pads, then what EXACTLY is the 'EVB MANUAL' doing for me?

Answer NOTHING, it's pretty much USELESS.

Dispite the bad docs for the eval board, I agree that it would be wrong to let the documentation issues spook you and prevent you from considering this chip, which could be a very good performer once you get the connections sorted out.

I also agree that it should not be too hard to sort out these connections on your own, with very little danger that you will goof something up and blow the driver.

This is not a boost converter so accidentally breaking a connection to the LED looks like it will also open circuit the inductor removing drive (I would still be careful to have everything hooked up correctly with good solid solder connections first because some outputs don't like to be open circuited, so it just makes sense to be cautious)

As far as looking at the Bourns site goes, that's not very practical, because they list several Bourns inductor series, but we have NO ASSURANCE THAT THEY ACTUALLY USED ANY OF THEM.

Everyone lists big name suppliers like Bourns, TDK etc for inductors in their datasheet recommendations, but inductors tend to be pricey, so if you can't work the kind of huge volume deal that impresses these companies, you usually end up subbing a cheap generic inductor from some no name vendor, and this could have quite easily happened on the eval board itself.

I am sure the factory can supply this info though.

Again, I don't want to sour anybody on the ZD850 eval board, because at only about 10 bucks it could be a great option, and bad documentation is a pretty common problem that all engineers have to deal with every day (no one will remember or care once you get things figured out and it's working).


----------



## imageicon (Aug 28, 2007)

hey guys ty for all the information! geesh no wonder a noob light maker like me cant figure it out 

my application is bike lights
i have produced some lights 2x cree XR-E emitters some taskled bFlex boards driven off 9.6v nimh batteries!

and some of the guys i bike with really likes them one of them work at a machine shop so he can make some aluminium housings for em, and we would try to cut down the cost on making them by using cheaper components than what i used for my prototypes. so really some differend design ideas are okay.. it was just when i stumbled across this chip it seemed all i would need from the product text but indeed it wasnt so easy when i started reading the manual 

cheers jesper


----------



## Luminescent (Aug 28, 2007)

imageicon said:


> hey guys ty for all the information! geesh no wonder a noob light maker like me cant figure it out
> 
> my application is bike lights
> i have produced some lights 2x cree XR-E emitters some taskled bFlex boards driven off 9.6v nimh batteries!
> ...



What do you think about a high performance linear constant current driver?

At high current your LED's will drop up to about 3.4 volts each, so two times 3.4volts = 6.8volts.

To calculate efficiency we devide this by the supply voltage 6.8volts / 9.6volts = 70.8% efficiency for the (fully charged pack).

In a linear regulator the efficiency actually goes up as the batteries discharge and their voltage decreases, so for a battery pack which has been discharged to 1volt/cell, we hit a much higher efficiency:

8volts/9.6volts = 83% efficiency

With the shape of the NiMH discharge curve, the overall average efficiency will probably be close to 74% which is just about as good as some switchers.

If you would like to try a linear driver, I have a fairly simple circuit with only about a dozen parts, which can drive any current level you want to well beyond 2 amps, but for CREE or Rebel, or SSC P4 LED's I would keep the drive down to about 700mA (and to even handle this current level you will need fairly good heatsinking). 

The circuit is designed to be extremely forgiving about substituting other low cost alternate parts, so you should be able to get all the components required for less than $5. 

It would require some soldering skills to build the circuit on a small piece of electronic breadboard, but if you know someone handy in such matters, it isn't a particularly complicated circuit.

Everything should fit on a tiny 1x1.5 inch PCB (and SHOULD BE, in order to keep the connections reasonably short). A small heatsink should be used on the MosFet Q1. If the same heatsink is used for Q1 as for the LED emitters, then the TO-220 metal tab on MosFet should be electrically isolated with thermal washer (anyone who has worked with this type of device will understand how to properly isolate the device from the heatsink).

Here are some additional notes:
All resistor 1/4 watt
Capacitors are low voltage monolithic, rated 25 volts or higher
Q1 is a T0-220 power mosfet. The IRF540 can be substituted with any 24Volt to 100Volt low RDSon N-Channel MosFet type.
U1 is not indicated but can be any low power opamp suitable for operation from 5 to 6 volts DC which is stable at a gain of 1 [internally compensated, unity gain stable] and that has a "Common Mode Input Range" which includes the negative supply rail [ground in this case]. Characteristics similar to MC33171 should be fine.

Here is the circuit -






Note: This example circuit is set up for a current drive level of about 730mA, which close to the maximum at which you can safely drive Rebel and CREE LED's, to drop this down to 350ma. cut the value of R3 and R5 in half to 5k.


----------



## imageicon (Aug 29, 2007)

would prefer an off the shelf product thou 
i will go support goerge at taskled i fancy his drives... and the guys who im making lights for will just have to pay thoose extra 20$, that is if you dont know of any linear controllers off the shelf thats cheaper... 

i really admire you elektronic guys my lvl is like i know how to solder it and i know how to follow a plan but i cant do the calculations or the setups my self


----------



## Luminescent (Aug 30, 2007)

imageicon said:


> would prefer an off the shelf product thou
> i will go support goerge at taskled i fancy his drives...



I thought that this might be the case, but I thought I would show you what the circuit would look like, so you could see what the level of complexity would be. With only about a dozen components, your average ham radio operator or other electronic hobbyist could tinker something like this together in about ten minutes, but if you have never tackled something that looks like this before, it would definitely be better to stay with a commercial product.

Sorry but it doesn’t look like the ZD850 would be a very good choice at 9.6 volts either. As your batteries discharge they drop down to about 1 volt/cell (or about 8 volts in this case), and with 2 LEDs in series the ZD850 seems to need 10VDC or more to drive it, so maybe one of the TaskLED boards would indeed be your best bet despite the higher price if you already know that one of their BUCK drivers would be ok with TWO LEDs at this 8 to 9.6 volt drive voltage.


----------



## imageicon (Aug 31, 2007)

well actually i have handled diagrams and making of boards some years ago but i dont really have components or tools and when you say local i think the nearest elektronik store is 50km away... 
and for my application wich will recive a pretty rough treatment on the mountainbike i like drives that is premade seems more durable 

but ty for your help... i threw a volt meter on my batts and they start out at 10.6 or something and your right after an hours or 2 use they are noticely dimmer than when i plug it on but still plenty of light for my use.

i think ill change thou to 3x cree and then serie up 2 batts to give it 19,2v should ensure max brightness for the full duration.:devil:


----------



## bikerjay (Sep 6, 2008)

I am now looking at this driver as well. Listed as $8.08 on the cutter.com.au site now. Thats hard to pass by. I am planning on driving 2 cree xp-e and want to use a 2cell Lion battery pack. The xp-e is rated at 3.5vf @700ma. The 7.4V nominal battery and forward voltage of 7V seems really awkward. Best solution I can come up with is to wire them in parallel, and hope they are well matched. Maybe I will just bump the drive current down a little to 1300ma. So I would be using this part with An input voltage of 6-8.4v and a current of 1300ma and a vf of ~3.5. I am guessing the zd850mr comes with the wrong inductor and the wrong resistor values, and I will have to change them. This could be difficult for me since I have never done surface mount before. Additionally I would have to calculate values based on that cryptic data sheet and source the parts which could be expensive. Can some one suggest some part values, or a different regulator all together. The data sheet seems to suggest a 22uH inductor. I am not sure about what resistor value to use for 1 led and 1300-1400mah current. I am set on 2 cree xp-e and a 2cell lion battery. Can a SOB do 1300ma off of 7.2V ?


----------



## Oznog (Sep 6, 2008)

You know I've been watching the ZD850. It's got some excellent properties. The internal transistor drivers are awesome in that they not only save you a difficult-to-select component but are typically driven at higher freq than you're try to do with an external. The app notes I saw on external drives recommended 100KHz as a good balance due to transistor switching losses and only go higher if the design's tuned real carefully. The ZD850 is 500KHz straight out and that shrinks the inductor size way down.

Still it doesn't seem to be available in any of the major outlets- Mouser, Digikey, even Future. So I'm kinda wondering what's up with that. Right now it's over $14 to have them ship any quantity to the USA which totally blows.

...do you actually HAVE a datasheet for the part itself? And if so could you provide a link? All I have is the "advance specs" sheet and the Eval Board spec sheet. The "datasheet" link on Cutter Electronics is broken.

As per the Eval Board AN020:
Vled=total LED output voltage, such as 3.4v * N

L=(Vled * 1.5uS)/(0.15 * Iled) 
R1=0.2V/(1.15*Iled)

inductor Ipeak=Iled*1.08 (choose an inductor with a DC current rating say 50% greater to keep the inductor temp down, and Isat significantly higher than Ipeak)


----------



## WeLight (Sep 6, 2008)

frenzee said:


> For 1A, you'll need a .174 Ohm resistor. You could make your own from a 5 3/4" length of 24 ga. copper wire. btw, I think your link is for the chip only. You'' still need to put together the rest of the board or get the eval board.




This is the Kit board
http://www.cutter.com.au/proddetail.php?prod=cut793
AUD9.99
Cheers
Mark


----------



## WeLight (Sep 6, 2008)

Oznog said:


> You know I've been watching the ZD850. It's got some excellent properties. The internal transistor drivers are awesome in that they not only save you a difficult-to-select component but are typically driven at higher freq than you're try to do with an external. The app notes I saw on external drives recommended 100KHz as a good balance due to transistor switching losses and only go higher if the design's tuned real carefully. The ZD850 is 500KHz straight out and that shrinks the inductor size way down.
> 
> Still it doesn't seem to be available in any of the major outlets- Mouser, Digikey, even Future. So I'm kinda wondering what's up with that. Right now it's over $14 to have them ship any quantity to the USA which totally blows.
> 
> ...



Will fix the link, here is the original
http://www.zywyn.com/files/zd850_ds.pdf


----------



## Oznog (Sep 6, 2008)

Huh. I see TRG Components is a US distributor for them, but their website is not exactly up to par with a lot of "coming soon" or "call us" endpoints. Anyhow:


> Pricing for the ZD850LEY is US $1.27 each in 1K quantities. Samples and evaluation boards (ZD850EVB) are available from stock and can be requested through TRG Components who distribute Zywyn's products througout N. America and Europe. Lead-time for production quantities is about 12 weeks ARO.


----------



## bikerjay (Sep 7, 2008)

Most of this is probably obvious to EE people but it took me a while to hunt down all the info. What ever Zywyn is paying there technical writers is simply too much...

The ZD850MR and the ZD850eval are two different boards. The one cutter stocks for 9.99ausd is the ZD850MR, which cutter conveniently calls zd850mr. After spending a long time studying the data sheet I have made a few determinations. The data sheet stinks and is incomplete 9pgs but the details are on the application notes 12pgs. The board does not totally live up to the specs on the 1page quick reference page which is listed as the specs on cutter, but it comes pretty darn close. The board comes with a .28-.3ohm current sense resistor, and a bourns inductor rated at 220uH/.49A rms max current/saturation current of .80A. There are two ways of wiring the board! One configuration gives you the option of 2-3 series wired leds at 300-350ma, the other gives you 1-5 series wired leds at 700-800ma. If I understand correctly there are no component changes necessary for either of these configurations. If you want only 1led at 300-350 you should change out the inductor and sense resistor. If you want more than 800ma up to the full rated 1.5A you MUST change out the inductor. In fact 800ma is really pushing the thing since its close to the saturation point. I plan on running it at 1350 ma with 2 parallel leds connected. I will be swapping out the inductor and sense resistor, which will be a pain since I will be trying to reflow with a regular iron. At least I have a B-flex that I fried that I can salvage a better inductor from. Its got a 68uH coil craft that saturates at 2.3A. May get better efficiency by having less resistance from the inductor. 

WeLight if your listening....
This thing would be the hot ticket if cutter sold different part numbers with different sense resistors and inductors and gave different specs for the versions. Would not change the cost much if at all, and I for one would gladly pay a couple dollars more for the option of different components.


----------



## Oznog (Sep 7, 2008)

The documentation does mention that it's a constant-Toff device and there are issues running Vin/Vout ratios greater than 2:1 (don't recall how major those issues are). You might want to check up on that if you use such a ratio.

That board is hardly "complicated", it's not difficult to build one.


----------



## WeLight (Sep 8, 2008)

bikerjay said:


> Most of this is probably obvious to EE people but it took me a while to hunt down all the info. What ever Zywyn is paying there technical writers is simply too much...
> 
> The ZD850MR and the ZD850eval are two different boards. The one cutter stocks for 9.99ausd is the ZD850MR, which cutter conveniently calls zd850mr. After spending a long time studying the data sheet I have made a few determinations. The data sheet stinks and is incomplete 9pgs but the details are on the application notes 12pgs. The board does not totally live up to the specs on the 1page quick reference page which is listed as the specs on cutter, but it comes pretty darn close. The board comes with a .28-.3ohm current sense resistor, and a bourns inductor rated at 220uH/.49A rms max current/saturation current of .80A. There are two ways of wiring the board! One configuration gives you the option of 2-3 series wired leds at 300-350ma, the other gives you 1-5 series wired leds at 700-800ma. If I understand correctly there are no component changes necessary for either of these configurations. If you want only 1led at 300-350 you should change out the inductor and sense resistor. If you want more than 800ma up to the full rated 1.5A you MUST change out the inductor. In fact 800ma is really pushing the thing since its close to the saturation point. I plan on running it at 1350 ma with 2 parallel leds connected. I will be swapping out the inductor and sense resistor, which will be a pain since I will be trying to reflow with a regular iron. At least I have a B-flex that I fried that I can salvage a better inductor from. Its got a 68uH coil craft that saturates at 2.3A. May get better efficiency by having less resistance from the inductor.
> 
> ...


Dont have a problem with that, but you need to advise me the list of preferred parts to hold


----------



## bikerjay (Sep 8, 2008)

This is the part where I am having trouble with the data sheet but I will give it the old college try as a versatile cheap buck driver will be useful for other DIYers. 

I would like a 22uh inductor and sense resistor at .15 ohm. This seems ideal for my desired 1350mah current into 2 parallel cree xp-e and supply voltage of 2cell Lion at 6-8.4v. .12ohm would give the full 1.5A and I could come down from that with analog dimming using standard resistors.

The configuration with the stock 220uH inductor and sense resistor at .3ohm would be good for 3 led at 350ma. 
A 180uH inductor and .3ohm resistor would handle 1-2 led at 350ma. 
Next up would be stock 220uH and .23ohm resistor for 1-5led at 700-800ma. 

The trouble is this, in the following documenting, http://www.zywyn.com/files/AN020_ZD850MR_Manual.pdf
On pages 5-7 the data sheet lists typical configurations and gives some values it say will work for a few configurations. It shows that 1-5 led(S) at 1.5A are valid and then dost list an inductor or sense resistor. Now this is not a huge problem because page 9 is a gold mine of information. Page 9 shows L1 R1 component values for 1-5 leds at 350-400, 800,1000, and 1.5A This would suggest a whole lot of different values, but the afore mentioned pages 5-7 make it clear that these values are ideals. 

Cheers


----------



## rschultz101 (Apr 12, 2011)

looks like we need a new pcb for the ZD850.
considering it, but would need a bunch of input.
once it's done, definitely need the parts.

- where to get the ZD850 in small qty, 1-10
- docu, app note etc.
- what parts to use for 1.5A 1.3A etc
- board size, shape, would think, a single sided one, be better for mounting
on a heatsink, say 20mm round , for example ? if it fits
- extra resistor solder spots, to change the current
- pwm input 
- module, maybe a version , with inline row of connector,

also consider other candidates, seen some from national, up to 50-75V, and higher current. but small and simple first. contact me
Rob mtbl.robs-x.com


----------

