LED drivers minimize power dissipation

Fons Janssen, Maxim Integrated Products, Bilthoven, Netherlands

One option for driving highbrightness LEDs uses the standard stepdown buck converter (Figure 1). The sense resistor, RS, generates a feedback voltage, VFB, that sets the desired LED current, ILED, according to the equation RSVFB/ILED. Unfortunately, most buck converters require a relativelyhigh feedback voltage on the order of 1V, which dissipates high power in thesense resistor (PSENSEVFB/ILED). Reducingthe sense resistor’s value and addingan op amp to boost the sensed voltagereduces the power penalty ((Figure 2 ).

In some cases, you can eliminate the op amp by using a stable reference voltage, which is available on some converter ICs, to pull up the sense voltage (Figure 3). The switching converter, a Maxim (www.maxim-ic.com) MAX1951, requires a feedback voltage of 800 mV and provides a 2V reference voltage at the reference pin. Connecting R1, a 50-k resistor, between RS and VFB, and R2, a 100-k resistor, between the reference and the feedback pins shifts the operating point from 200 mV at RS to 800 mV at the feedback pin:

Thus, for VSENSE0.2V, V0.8V. For the cost of two inexpensive resistors, power dissipation in the sense resistor diminishes by a factor of four.

Using the Luxeon K2 LED from Lumileds (www.lumileds.com), power measurements on the circuits of figures 1 and 3 illustrate how the feedback adjustment influences power that the LED driver delivers. Two graphs show LED currents and voltages as a function of input voltage for a half-load of 400 mA (Figure 4) and a full load of 800 mA (Figure 5). As you would expect, the current regulation deteriorates at half-load. The variation of LED current averages approximately 5 mA over an input-voltage range of 4 to 5.5V and 1 mA for the circuit with normal feedback. The input-voltage range, however, increases by more than 0.5V.

Regulation also deteriorates for full load, and the variation increases to approximately 22 mA, versus 6 mA for the circuit with normal feedback (Figure 6). Again, the adjusted-feedback circuit of Figure 3 increases the inputvoltage range.

You can define the improvement in efficiency, , as follows:

The buck converter’s power-conversion efficiency and power dissipated in the sense resistor determine the circuit’s efficiency. As Figure 5 shows, the adjusted feedback of Figure 3 increases the efficiency more than 10% at either half-load or full load. Assuming that the sense voltage doesn’t change, efficiency improves for lower outputcurrent loads because the sense resistor dissipates less power

EDN Europe