LED Driver Circuits

These circuits, again using the MC34063, are designed to drive ultrabright LEDs at constant current. Two suggestions have been made by other designers:

  1. Use the current limiting feature of the MC34063 and ignore the control loop.

  2. Use the control loop with the LEDs in the feedback circuit.

Note that some LEDs may have very limited reverse voltage and so it may be worthwhile protecting them with a reverse biassed diode or zener placed in parallel.

Current Limit

This circuit is very simple but not particularly accurate. The MC34063 will limit current when the voltage across the Vcc-Ipk pins reaches a value between 0.25V and 0.35V. This also varies with temperature. Rlimit can be set accordingly. While the LED doesn't require a dc voltage, the output filter is still necessary to smooth the current flow for the limiter to work effectively.

Feedback Control

This circuit promises greater accuracy, if that is a requirement, as well as more flexibility in choosing the current setting sense resistor value. Another advantage is that the power loss in the sense resistor can be reduced for both heating and power efficiency gains in higher power applications. The disadvantage of course is a more complex circuit.

The trigger voltage at the comparator input ranges between 1.225V and 1.275V. Without the amplifier the power loss in the sense resistor would be excessive. The amplifier gain can be chosen so that the voltage drop across the sense resistor is kept very small, say 100mV or less. Care will need to be taken when high gains are used, as interference and noise will be amplified.

As an example, consider driving a series of high intensity starleds at 1W each. The voltage across each of these is approximately 3.3V so about 350mA current is needed. Any number of LEDs can be connected in series provided the total voltage across them is within the power supply range (less that across any intervening components). If we choose a 1 ohm, half watt R5 then it will dissipate about 0.12W and we will need to amplify the 0.35V across it to 1.25V, i.e a gain of 3.57. An R4 of 3.6K and R2 of 10K will give a gain of 3.8. R3 must be omitted. Measurements showed that the circuit drew 1.8W, an efficiency of 70%.

The circuit can provide a variable intensity by making R2 to be a variable resistor. If R2 is linear then the luminous flux (perceived light intensity) will be approximately linearly variable with the resistor's shaft position.

First created 24 August 2012
Last Modified 6 February 2018
© Ken Sarkies 2012