High-power LED driver circuit without external switch

As new generations of LEDs achieve higher power and efficiency, the use of these devices is gradually expanding into new areas, such as flashlights or vehicle applications. High-power LEDs are used in ambient lighting together with incandescent bulbs and fluorescent tubes. A current source is the best way to power an LED. Since most energy sources, including batteries, generators and industrial mains power supplies, are increasingly acting like voltage sources rather than current sources, LEDs require some electronic circuitry to be inserted between them and the power source. This circuit can be as simple as a series resistor. But considering energy efficiency and other factors, the best is an efficient voltage-fed current source. For LED currents greater than 0.35A, an inductive switching regulator is usually the best choice.

This design example provides a series of single-supply integrated circuit switching regulator circuits, mainly to improve efficiency and reduce size. In order to achieve this goal, the circuit designer minimizes the use of larger components, such as external power transistors, switches, large capacitors, and current sensing resistors, and uses a continuous high-intensity light source to extend the illumination range as much as possible to maintain normal operation of the circuit.

The circuits in Figures 1, 2, and 3 are suitable for powering from a power source consisting of three or four alkaline, nickel metal hydride (NiMH), or nickel cadmium (NiCd) batteries. The circuits in Figures 4 and 5 can be used in automobiles where the power distribution system has a nominal line voltage of 12V, 24V, or 42V. The circuits in Figures 4 and 5 can also be used in industrial systems and emergency subsystems and telecommunications applications that include a 24V distribution line for control, where the system power supply is a –48V line voltage.

The designers of these circuits use the same concept: a fully integrated single-chip IC switching regulator and a micropower op amp. The op amp drives the 1.25V feedback terminal on the IC. Although this node is targeted at a standard voltage regulator topology, the op amp matches it to a much smaller current sense voltage and a slightly different current regulator topology. None of these circuits require the use of an external power switch. Because there is no need to smooth the high-frequency ripple in the LED current, this design avoids the large value filter capacitors commonly used in switching regulators. Common to all circuits is the optional dimming function, which is achieved by introducing a resistor- and potentiometer-adjustable bias at the op amp input. Depending on the IC, the resistor and potentiometer can be powered by the VD or CVL terminal of the internal regulator.

A high-frequency switching regulator is used to power the basic LED regulation circuit (Figure 1). It operates from an input voltage of 3.6V to 6.5V, drives a single LED at up to 1A, and uses a current sense resistor to control the current regulation loop. The circuit in Figure 2 is also similar, but uses the parasitic resistance of the inductor instead of the current sense resistor. Similar to the circuit in Figure 1, it operates from an input voltage of 3.6V to 6.5V and drives the LED at 1A.

For the single-LED circuit in Figure 3, the MAX1685's starting voltage defines an input voltage range down to 2.7V. The maximum currents for the circuits in Figure 1 and Figure 2 are 0.5A and 1A, respectively. The maximum operating voltage limit is again 6.5V. Once the circuit is operational, it powers the LED for input voltages as low as 1.7V. Applications for the circuits in Figures 1, 2, and 3 include headlamps, flashlights, and any other portable lighting powered by three or four alkaline primary cells, NiMH/NiCd secondary cells, or a single lithium secondary cell.

The circuits in Figures 4 and 5 operate from 8V to 50V. Assuming a 12V system where all components are specified, this circuit can withstand load shedding due to the 76V absolute maximum voltage rating of the IC input power supply terminal VIN. The maximum available current is 1A, and the circuit can drive three series-connected LEDs by simply increasing the lower limit of the operating voltage to 11.5V. The two circuits are very similar, except that the circuit in Figure 5 uses an inductor resistor as the current sensor. The disadvantage of using an inductor resistor is the temperature dependence of the output current due to the large temperature coefficient of copper resistivity. The inductor winding is made of copper, and its DC resistance has a first-order temperature coefficient of 3.9 parts/1000/°C. The result is that the regulated current drops by 4% for every 10°C increase in temperature over the operating temperature range.