Hi all,

I'm working on a micro controller project to do race timing for rowing regattas. It's coming along pretty nicely and I want to add one component: A finish light that will be turned on and off by the microcontroller.

The light needs to be seen from a distance in sunny conditions. I want to use an array of 9 Cree XP-E2's with appropriate reflectors (Ledil spot beam).

The main unit will have a 12V supply, and to keep the wire gauge down, I want to use that for the finish light as well.

The red Cree XP-E2 that I plan to use have a forward voltage of 2.2V and need 350 mA.

I'm thinking of using this LED driver: MPS MP3398L, does 4 string @ 350mA, takes 4.5 - 28V

That is as far as I got with Google, and zero experience building anything like this
My questions:
1) Should I add a voltage regulator that steps down my 12V to 5V or 6V so that I will three string of 3 LEDs that. I think they will run fine on a slightly lower voltage? Or use 12V to drive 5, and put the other 4 together with a resistor to get to the right voltage over the remaining LEDs
2) I would like to be able to dim the lights. The LED driver supports both PWM and analog dimming. I want the dimming to be adjustable on the light device itself (ie, not pass a PWM signal from the controller). So the analog dimmer sounds the easiest? Using a potentiometer in a voltage divider to get the desired 0.44V - 1.54V the driver expects.

And the real question, does the above make sense at all? Is it the "right" way to do this?

I'm hoping to design my own PCB later (never done that either, but it sounds like fun) to make it into a nice package.

Any and all info/pointers would be much appreciated!
Bas

 

hi bas,
Welcome to AAC.
The MPS3398 will accept an input voltage range of 4.5V thru 28V, so a voltage regulator is not required, the MPS is a current control device.
Use the analog dimmer method.
What range do you expect the light to be visible in bright sunlight.?
E

 

hi bas,
Welcome to AAC.
The MPS3398 will accept an input voltage range of 4.5V thru 28V, so a voltage regulator is not required, the MPS is a current control device.
Use the analog dimmer method.
What range do you expect the light to be visible in bright sunlight.?
E

Hi Eric,

I was thinking that the voltage regulator made sense to get the voltage down to the level that I'm close for 3 * 2.2V across the three LED's. Is using a resistor in front of each string the more common way to deal with that? I would drop about 2W of power over it I think? ( 0.350 * (12-6.6) = 1.89W)

I only need about 100m of range or so. I'm "cloning" a device I've seen in use that worked well. At full brightness we could sometimes make those out over the length of a rowing course (2km)

Bas

 

hi Bas,
It is the current thru each string of LED's that is set by the MP3398, not the voltage across them.
The voltage you measure across the LED's will be determined by the forward Voltage drops of the individual LED's.
Look at this clip from the d/s.
E

 

hi Bas,
Another clip.
E

 

Hi Eric,

I think I must be missing something basic. I understand that the LED driver is providing a constant current (ie 350 mA). In my mind, let's say that the driver sees a Vin of 12V, the Vout for each channel will be 12V as well. If I have three LED's in the string per channel, they would see 4V each, and let out the magic smoke?

Bas

 

The light needs to be seen from a distance in sunny conditions. I want to use an array of 9 Cree XP-E2's with appropriate reflectors (Ledil spot beam).
The main unit will have a 12V supply, and to keep the wire gauge down, I want to use that for the finish light as well.
The red Cree XP-E2 that I plan to use have a forward voltage of 2.2V and need 350 mA.

hi Bas,
You have it wrong.

Consider that you have one string of LED.s , say 3 in the string.
Across the 3 LED's in series you will measure 6.6V ,when the string has a current of 350mA.
The 350mA current in the string is set by the ISET resistor.
The MP3398 can drive the 3 strings in your total of 9 LED's ie: 3/string.

The total current from the supply will be 3 *350mA = 1.05A.

Do you follow OK.?
E

 

hi Bas,
You have it wrong.

Consider that you have one string of LED.s , say 3 in the string.
Across the 3 LED's in series you will measure 6.6V ,when the string has a current of 350mA.
The 350mA current in the string is set by the ISET resistor.
The MP3398 can drive the 3 strings in your total of 9 LED's ie: 3/string.

The total current from the supply will be 3 *350mA = 1.05A.

Do you follow OK.?
E

Ah, in my mind I knew I blew an LED when I applied to much voltage. But that wasn't to much voltage, it was to much current.

Great, I think I understand it all now. On to ordering some parts

Thank you very much for your help. Once I have it all working, I'll post a picture back here

Bas

 

hi Bas,
I have created a demo simulation so that you can see the results.
All three circuits are passing 350mA, note the voltage drop due to the current thru each string.
E

 

Hi Eric (and others),

I spend some time learning more about how to create a schematic in Fusion 360 (I love that tool for 3D printing). Reading the spec sheet for the LED driver I came up with this:

https://a360.co/2A80NLc (if it's better to post an image, let me know)

The spec sheet has a typical application circuit, and it has a LOT more pieces then I would have guessed from reading the pin functions.

For example:
VIN -> Some capacitors and an extra resistor (R3), is this important to include?
COMP -> Has a resistor R7 in line with the capacitor, pin description just says capacitor. Is that to limit the current that can flow into the capacitor? It has 270 next to the R7 label, would that be 270 Ohm
En -> Turns the chip on if > 1.8V, schematic has several resistor and capacitors on that line. Are they needed
PWD: The pin description says "Internally weakly pulled to GND if this pin is floated. If PWM dimming is not requiredm pull this pin to high 1.25-5V. There is not output on the chip that supplies me with that voltage. So I need something else to get me from 12V to 5V. Or can I let it flow. The description is a bit confusing
OVP -> To disable over voltage protection, I can just attach this to GND?

Any and all feedback would be much appreciated!!

Bas