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Topic: Advise on building a high output LED driver shield for Arduino (Read 593 times) previous topic - next topic

GVCOG

Hello list members,

I have some questions for the experts here that hopefully you can shed some light on.

I am designing a high output multichannel LED driver shield with an Arduino, each of the six PWM channels on an ATMega328P are tied to a CAT4104 LED driver (http://www.onsemi.com/PowerSolutions....do?id=CAT4104) to independently drive at 700mA each high output LED (all of the LED channels on the CAT4140 are tied together to get to that 700mA output rating. The pulse/strobe timing on each LED channel has to be independently controlled via PWM and with minimum ringing or rippling at the LED end.

The common power rail (2.5A or 5A of 12V power) is tied to each of the LED anodes, with the CAT4101 sinking 700mA of PWM pulsed power to each.

According to ON Semi, I should "...calculate the CAT4104 power dissipation based on the formula on Page 14 (Vin*In + VLED*ILED=5V*0.007A + 1.5V*0.7A, leading to 1W when assuming Vin=5V, forward LED voltage=3.5V). The last step is the PCB design assuring the thermal resistance junction to ambient is sufficient low to allow the power generated in the CAT4104."

With designing the PCB, what would be some tips on assuring the thermal resistance junction to ambient is sufficiently low to accommodate all six CAT4104s in a single enclosure without heat dissipation issues? Aluminum clad instead of copper clad PCBs? Heat sinks? Or would active cooling of some sort likely be required?

Lastly I am seeing some significant ringing with views of 20kHz PWM waveforms expanded out to see whole cycles - 5us per division. White LED 650mA, 60cm wire length. I've attached some images of the ringing below, any thoughts on reducing this ringing and rippling would be helpful. I am thinking a smoothing capacitor or in the alternative reducing the wiring length between the CAT4104 (but that would significantly reduce the functionality of the design with short wires).

Any thoughts or help on this would be greatly appreciated!


dhenry

Some suggestions:

1) With high current switching, you want to use a switching mode controller, rather than a linear controller.
2) You want to check the device's thermal resistance and make sure that you aren't off-dissipating. For soic/no thermal pad, you should try to stay within 1/4w, or 1/2w max (indoor, open air flow). Even with thermal pads, I would be extremely concerned pushing the device to 1w of dissipation - those things have thermal resistance of at least 100c/w. So at ambient of 50c, you are at the limit.
3) You should think hard about if you want to switch at 20khz.
4) the ringing is due to reactance. One way to solve it is with slew-limited drivers; or to a bead on the output line.

I would look for those dc/dc converters (SimpleSwitcher from National for example) as an alternative here.

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