Machine Design

Driving high-power LEDs

LED output is a function of the current through it. So to drive the device, it would seem simple enough to set the LED current with a series power resistor.

Rick Zarr
National Semiconductor Corp.
Santa Clara, Calif.

But there’s a problem with this idea. First, the resistor will possibly dissipate a large amount of power. For instance, a resistor used to drop 12 V (a common voltage used for halogen task lighting) to the 3.3 V for driving the LED at 350 mA would dissipate over 3 W (3 more than the LED).

You could reduce the power lost in the resistor by stringing multiple LEDs in series, in this case up to three. Three LEDs would drop the power in the resistor to a more reasonable 735 mW. However, the LED forward voltage loss varies with many parameters such as temperature and process variation. All in all, these problems preclude using a simple resistor to make a current source.

The best solution is an active current source which incorporates a feedback loop to monitor and regulate the current flowing into the LED (or multiple LEDs in series). A simple linear regulator serving as a constant current source might use the drop across a series resistor to keep the current at 350 mA regardless of the voltage drop across the LEDs (within the limits of the circuit).

This circuit is adequate, but not efficient. The regulator can dissipate as much as 1.6 W of power and the sense resistor dissipates almost 0.5 W as well. In that mode, the regulation circuit would dissipate over 2 W. The efficiency of the power conversion in this circuit in this case is around 50% — obviously, not good.

A superior circuit uses a National Semiconductor LM3402 buck-mode-switching constantcurrent source. This device is a switching regulator that uses pulse-width modulation to control the output current. It has a built-in amplifier which can monitor the current in an external sense resistor. This sensor goes in series with the LEDs (similar to the linear regulator example).

In operation, the LM3402 will monitor the voltage across the sense resistor and control the duty cycle of the switch to regulate the current. The PWM action gives an estimated efficiency of over 90%. The switch closes only long enough to give the LED sufficient power — it then opens. This cycle continues as the voltage drop across the LED string changes because of temperature and aging.

The losses in the system include the operating current of the LM3402, the internal FET-switch resistance, the external Schottky-diode voltage drop, the inductor conduction loss, and the sense-resistor voltage drop. These losses are all minimal, so the overall efficiency is excellent. In the case of 350 mA of LED current, the power regulator circuit will only dissipate about 420 mW of power. Now the LEDs will be dissipating around 1.4 W each, so they will need thermal management to keep from overheating.

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