Until furter notice: This is a work in progress
This circuit DOES NOT WORK with the currently suplied BOM!
30.04-2019 All parts have finaly arrived and i've finished two LED driver sircuits. Solderjob have been veryfied with magnifying lens and multimeter. Results are pretty much the same as before. Seems to me that the power MOSFET inside the LM3403 briefly swithes on and then turns off.
04.05-2019 Found that Texas Instrument has this neat web tool for choosing and designing driver circuits: Webench Power Designer I put in the same values in this tool and out come a totally different values on the passives controlling on and off time of the MOSFET. I'm not to confident in the vallues i've initially set for this sircuit. I might have misunderstood a thing or two in regard to LED forward voltage and how this value is used in the calculation of the passives.
08.05-2019 Using TI's web tool i played around with different inputs (Vf, Vin, If ) and did some more reading. I then realized what got me stuck. An LED's voltage forward has been described all around on the internet as the voltage needed to start letting current flow trough the diode matterial. This was the voltage i previously used for calculating the circuit. I now realize that the LED Vf needs to be picked from an IV curve diagram (or by testing) and must always be used in relation to the LED If. Figuring this out I have now printed out a graph paper and plotted the IV-curve. A reasonable If and Vf could be set at 0.8A @ 11.6V. Putting in these numbers in the TI's Webench i was no longer able to choose the LM3404 as controll unit in my sircuit. However lowering the If to 500mA @ 11.2V finally let me pick the LM3404. Seems like my initial pick of the LM3404 was a bit hasty and it's not capable driving full effect on the LED with only 2 Volts or so in headroom.
I'm currently considering bying other high power LED's with lower Vf to make use of my prototype PCB's
If anyone is interested and uses the TI Webench i have made the 500mA design public at this link: Design # 6 LED driver 500mA
This project is a DC-DC constant current regulator circuit. It uses the Texas Instruments LM3404MR switching regulator which can opperate under varying input voltages and are designed to drive a series string of high power LEDs at forward currents up to 1.0A. This circuit is intended to be driven from 12volt lead-acid batteries and it's connected charging systems. The LM3404's allow for PWM dimming control input on pin 3 but i've chosen to only regulate the forward current with the current setting resistor Rsns.
The COB LED i planned to drive with this circuit was baught from ebay with no datasheet and specs other then what was stated on the ebay-page: 12-24volt 1000LM 10w This is obviously not very reliable so i hooked them up to my bench psu for testing. At around 12volts the COB LED's draw fairly accurate 1 Ampere giving of a fairly bright light. The LED array consists of 4 LED's in series x 25 so we are probably safe at 12volt and 1A.
In real life condition this circuit will have an input voltage between 13 and 15 Volt. On a rare occations under low charging conditions (solar power) we might come as low as 12 Volt. This circuit is designed to be mounted inside the lamp next to the LED chip with minimal lenght of wiring to reduce the effects of parasitic inductance. In case longer saparation between the regulating circuit and the LED, design guidlines recomended placing the output capasitor on or as close as possible to the LED chip.
Note! Currently in development! PCB's are ordred and need to be tested to verify that the design works.
I will update this project as soon as i'm able to when all components has arrived and a prototype is tested out. See ya !
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