Boost Converter - Increases Battery Voltage for White LEDs

White LEDs are making their way into many markets where incandescent lamps were once dominant. Flash lamps are entering newer applications where the reliability, durability, and ability to control the power consumption of LEDs make these devices very attractive. With incandescent lamps, power management of the device is simply switching on and off. However, LEDs cannot be operated directly from the two batteries typical of flash lamps because they require a voltage between 2.8 and 4V, compared to the battery voltage of only 1.8 to 3V. The complexity of power management is increased because the light output of LEDs is related to current, and the characteristics of LEDs show an extremely nonlinear relationship with voltage. One way to solve this problem is to increase the current limit of the power supply. There are many LED application devices available on the market; however, their current ratings are usually too low for the 1 to 5W power required for flash applications.

Figure 1: A boost converter IC is the right choice for increasing the voltage required to drive white LEDs.

Figure 1 illustrates a solution that typically boosts power regulators. A boost converter IC - IC1 generates the higher voltage required for white LEDs. An internal buck power stage connects VIN to PGND, providing current to output pin L. This circuit operates by turning on the high-side switch, which connects the battery voltage across inductor L1. Once inductor L1 has stored enough energy, the high-side switch turns off. The inductor current drives the switch node to the negative pole and drives energy transfer through the low side to the output capacitor C1, creating a virtually lossless switching event. In addition, because the high-side and low-side switches are MOSFETs, the voltage drop is less than a diode implementation; this allows for very high efficiency. The converter IC monitors the current through the LED through a current sense resistor, and compares the current sense voltage to an internal 0.45V reference voltage in the converter IC for regulation. Therefore, the current and illumination are a function of the voltage across the current sense resistor. Although the IC's internal reference voltage is lower than that of most other ICs, it does cause measurable power losses. This reduces efficiency by 10-14% when using LED voltages of 2.8-4 V. This loss can be reduced by reducing the resistor value and using an amplifier to sense the current at low voltages.

Figure 2: Resistive current sensing negatively affects the efficiency of the circuit in Figure 1.

Figure 2 shows load current regulation vs. boost voltage at a 350mA current regulation point. Efficiency reaches over 80% over the normal battery voltage range, but drops as the battery voltage decreases toward end-of-life values. In addition, the graph illustrates the effect of resistive current sensing. At high input voltages, efficiency approaches 95%, while at low input voltages, efficiency drops to 80%. The trend of the curves results from two related effects: At high input voltages, the input current and switch currents are lower. Therefore, conduction and switching losses are lower. Second, much like an autotransformer, the boost power stage does not handle the total input power. The amount of power handled by the power stage is related to the boost voltage, or the voltage difference between the input voltage and the LED voltage. In this design, the LED voltage is approximately 3.7V, so at a high voltage line of 3.2V, the power stage only handles 13% of the power ((3.7-3.2)/3.7). At a lower voltage line with much higher current, the power stage can handle 4 times the power, or 50% of the power.