LED Automotive indicator selection and construction help

[dannyf]

3 x Richtek RT8472

Most simple buck converters can be configured as a constant current drive (like 33063 for example, or even a 555). Or other dedicated led drivers are a fair bit more common than this particular one.

[Corporate666]

SO... Just so I get things straight...

That RT8472 could run 2 x 2.5v 350mA Cree XP-C LEDs from a 6v source without issues?

My Current BOM (Feel free to criticize it)

6 x Cree Amber XP-C LEDs (XPCAMB-L1-R250-00401)
3 x Richtek RT8472

Total estimated lumen output at 350mA/LED is 330lm
(Passive components to suit)

Also, how would I calculate total current draw? I realize it'll be bugger all but it would be useful for correct fusing for the circuit

The RT8472 is a great LED driver that I've used many tens of thousands of with excellent success in automotive applications, and it is incredibly cheap!  But I don't think it would be well suited to your particular application - it is a buck (step down) driver, with a 5V to 30V input range, so you would be running really close to it's cut-out voltage and it won't be all that efficient, and you would only be able to drive one, maybe two LED's with it.

Instead, I would probably go with the Texas Intruments TPS61165.  With just 3 capacitors, an inductor, a diode and a resistor, you're good to go - it will accept 3V to 18V input and it is a boost regulator, so it will step the voltage up, allowing you to run pretty much as many LED's as you want.  And if you ran, say, 10 LED's in a string, you will get into the 90-95% efficiency range so the driver circuit will run nice and cool.  And not only is the driver cheap, TI will send you some for free  and their datasheets are awesome (IMO) and they basically give you a parts list and a PCB layout.  You can run the LED's at lower current (like 100mA or even a bit less) for more efficiency as well as more even light distribution within your housing.  Then you could just put the driver circuitry on the back of the same board, fill it with copper pours, do some testing to see what you can get away with for max drive current and you're good to go.

The other nice thing about this setup is that LED current is set with a resistor so you can use a precision 10-turn pot if you like, or use a trimpot in conjunction with a normal resistor so you can adjust LED current precisely in small increments, and then test it in the housing while you let it blink on your bench for several minutes at a time and see what temperature it reaches.

Keep in mind it's the LED junction temperature that matters - on the MLE's it's probably 150C but you won't want to get anywhere close to that - but even staying below 50C board temp, I bet you get massively more brightness than a 3W incandescent bulb. 



 

[agilly1989]

That looks like a good and simple chip

I will be doing Copper Fill on both sides of the board to help with heat dissipation and the housing will be part of the thermal-path as well (just to help), so thermals wont be too much of an issue AND since its an indicator, it'll be flashing, so its not on ALL the time.

Thanks for the tips guys I'll post pictures when I make it

[Corporate666]

I've opened up hundreds OEM LED headlights, tail lights and turn signals .....

Hundreds !! 

Just curious how many 12 volt based led lights you've built ? 

Thanks for the vote of confidence   Just to clarify, the ones I have opened were not ones I designed myself, but I have designed dozens of automotive turn signals and tail lights that we have sold tens of thousands of and we don't just use resistors , but what do I know 

The other bad part about using resistors is that in the case of the LED cited - the Cree ML-E... all of these LED's are thermally limited. So a Cree ML-E may have a Vf of 2.4 and an If of 0.15A.  So that's 0.36 watts.  Some of that turns to light, but even in an efficient LED, about half of that will be heat - that is about 0.2 watts of heat to handle.

In the proposed 6 volt system, 2 ML-E's in series would be 4.8V, so we have to drop 1.2 volts at 0.15A.  That's an additional almost 0.2 watts of heat.  Well, considering lighting designers biggest challenge is heat management and the 0.2 watts is quite a challenge to overcome, then adding another completely unnecessary 0.2 watts for absolutely no reason other than pure laziness makes no sense at all.

Yellow LED's are the worst when it comes to degradation of light output from temperature.  Page 7 of the Cree ML-E datasheet illustrates this perfectly - the amber ML-E loses 90% of it's light output at 150C that is has at 25C.  So keeping the LED as cool as possible is the #1 priority. 

Using resistors to control current which burn up the voltage as heat is a fabulously bad idea 

[agilly1989]

Either I'm thinking straight or I just don't know but if I was to run 3x 100mA LEDs in series, would I need 300mA or 100mA or some other amount?

[mariush]

3 in series =  3 x voltage, same current

3 in parallel = 1x voltage, 3 x current

[agilly1989]

thanks mariush I thought that was the case.

Just to make sure, if I was running 6 (2 sets of 3) it would be 3 x voltage and 2 x current, right?

[agilly1989]

I can see what you mean by TI having awesome data sheets it basically tells you exactly how to configure it.

It was a little confusing at start but I think I have got the equations right.

[agilly1989]

I am now confused :/ I'm working out the "Thermal Considerations" for the TPS61165 and the equation is as follows

P_D(MAX) = (125-T_A) / R_(JA)

P_D(MAX) = Maximum power dissipation limit
T_A = Maximum Ambient Temp = 80^oc
R_(JA) = Thermal resistance Junction-to-ambient from the table = 210.1

which gives me the answer for P_D(MAX) as 0.21418372

I don't know if that's a good answer or not

[pickle9000]

These are for flashlights, yes I know ebay sucks.

http://www.ebay.ca/itm/Cree-XLamp-XM-L-XML-EasyWhite-LEDs-T6-7B-3200K-Warm-White-6V-16mm-Board-/321145897333?pt=Motors_Car_Truck_Parts_Accessories&hash=item4ac5c98175&vxp=mtr

[agilly1989]

It really doesn't matter about what LED (or LEDs) I use as long as they are amber or orange and can give off 300ish Lumens of output (total).

The driving of said LEDs is the thing I'm mostly worried about. Making sure the driver is stable, efficient and suitable for this application. Having it in the enclosure on a hot Aussie day, the internal temp could hit 80^oC or higher so making sure the components wont melt and let the magic black smoke out is a priority.

Now... I'm no thermal design professional or anything, but I do know, the more metal connected to a hot device the better.

More LEDs would probably be better (mainly for better light distribution) but it possibly wont matter as much because the lens is diffused... So yea

The 6 Cree XP-C LEDs when running at 250mA (100mA less then the tested current) will still give 231lm which I'm happy with. Having them run at that temp will make them run a bit cooler hence last longer.