High Efficiency Battery life is critical in portable applications. If the LED driver is practical, it must be efficient. The efficiency measurement of an LED driver is different from the efficiency measurement of a typical power supply. A typical power efficiency measurement is defined as the output power divided by the input power. For LED drivers, the output power is not a relevant parameter. What matters is the amount of input power required to produce the expected LED brightness. This can be determined simply by dividing the LED power by the input power. If efficiency is defined in this way, the power dissipated in the current-sense resistor causes the power supply to dissipate. Compared with the power supply of 1V reference voltage, the efficiency of this type of 0.25V reference voltage is improved. A lower current sensing voltage power supply is more effective. Regardless of the input voltage or LED current, as long as other conditions are the same, a lower reference voltage can improve efficiency and extend battery life.
PWM Dimming Many portable LED applications require dimming. In applications such as LCD backlighting, the dimming function can provide brightness and contrast adjustments. Analog and PWM dimming methods can be used. With analog dimming, 50% of brightness can be achieved by applying 50% of the maximum current to the LED. The disadvantage of this method is that LED color shift will occur and an analog control signal is required, so the utilization rate is generally not high. Applying full current to the LED with lower busyness can achieve PWM dimming, and applying full current at 50% busyness can achieve 50% brightness. To ensure that the human eye cannot see the PWM pulse, the frequency of the PWM signal must be higher than 100Hz. The maximum PWM frequency depends on the power-up and response times. To provide maximum flexibility and ease of integration, the LED driver should be able to accept PWM frequencies up to 50kHz.