Luxeon LED current drive step down converter back to projects

The following circuit is designed to let an Luxeon LED work from a battery of other power supply without dissipating power in a resistor to limit the current. The circuit operates between 6 and 20Volts. Operating up to 40 volts is possible however, but operation has not been tested by the me.

explanation of the circuit: The LM2578 is a switching voltage regulator from National Semiconductor. It has a built in drive transistor designed for currents up to 750mA, enough for this purpose. a functional description is given below.

The voltage regulator option is not used here, the regulator is simply enabled by connectin a resistor of 15k(R2) from pin3 to ground. the capacitor from pin3 to ground is a timing capacitor C2 and sets the oscillator frequency to approx. 100KHz. As seen in the functional block circuit above the current to the Coil is switched by the internal transistor of the regulator thrue the shunt resistor of 0.25 Ohms. The shunt resistor(R1) takes care that the current limit is at 400mA (the LM2578 switches off at 0.1V from supply lines). the Coil of 10uH stores energy during the periods that the transistor is switched off and current is sustained thrue the schttky diode connected from the ground to the coil. The other schottky diode(D2) is to prevent the internal transistor of the regulator chip to turn on accidentally. C3 is to store the oscillating current to a more stable voltage, it can be omitted however this will rise the lowest working voltage a little. Capacitor C1 is for filtering the supply line from the peaks generated in this device and is necesary for stable operating of the device.

A Luxeon power led can be connected as D3, the current thrue the device will be approx 400mA independent of the power supply voltage. the LM2578 can become a little warm(up to 50degreed Celcius), this is normal, however with external transistor options(see the data sheet of the LM2578) this can be reduced further (and also the efficiency can be improved).

The circuit is used for a powerful microscope light. A proper housing of aluminium, a focusing lens and a cooling fin complete the design. (the circuit is not inside this lamp, but built in to a small housing somewhere down the cable, not shown in the photo). the luxeon LED is designed for a maximum current of 1Amps at 5Volts, however for a longer lifetime lower currents are advised. This LED light runs at 500mA, which still gives a lot of light.