Economical circuit drives white LEDs

Eddy Wells, Texas Instruments -- EDN, 11/23/2000

Newly available white LEDs are replacing CCFLs (cold-cathode fluorescent lamps) in handheld applications using a back-lit LCD. These applications include PDAs (personal digital assistants), digital cameras, and cellular telephones, to name a few. Advantages of white LEDs over CCFLs include longer life, higher efficiency, and significantly lower operating voltages. Regulating the current in the LED (typically 10 to 30 mA) controls the brightness; the forward voltage in each LED is approximately 3V. The circuit in Figure 1 provides a means of efficiently controlling LED current in a series-connected string. The TL5001 PWM-controller IC is an older, industry-standard, inexpensive driver. The boost topology of the circuit allows operation from a single or dual lithium-ion cell. The ratings of Q₃, SD₁, and the maximum allowed duty cycle of the IC (programmed with pin DTC) determine the maximum output voltage of the circuit. V_CC comes from a separate 5V supply.

Trade-offs in the selection of inductor L₁ include size, dc resistance, and inductance value. An 82-µH inductor (with 200-m? dc resistance) results in continuous inductor current at higher LED currents, but the current becomes discontinuous at lower levels. The RT pin programs the oscillation frequency at approximately 200 kHz. Because the TL001 has a relatively weak (20-mA) open-collector output driver and is intended to drive a buck-topology circuit, the circuit uses a low-cost inverter stage comprising Q₁ and Q₂ to efficiently drive Q₃. R₆ provides controlled turn-on and fast turn-off for Q₃. The reverse-breakdown voltage of SD₁ must be greater than the C₂-filtered V_OUT.

R₇ senses current in the white-LED string; the error amplifier at the FB pin of the IC controls the feedback signal at this pin to 1V. You can control the LEDs' intensity by summing in a control voltage via R₄. Figure 2 shows the efficiency of a four-LED string. You could obtain approximately 2% higher efficiency by adding a gain-of-5 op-amp stage between R₇ and R₃, resulting in a lower voltage drop across R₇. Of course, this slight efficiency improvement adds to the system cost. For higher power applications, such as notebook computers, you can attach additional LED strings to V_OUT. To maintain uniform intensity in each string, you should add a dummy resistor of the same value as R₇ to each string.