Analog design simplifies LED brightness adjustment
Portable devices that use LED-based solid-state lighting (SSL) require efficient drive circuitry to extend battery life, while also requiring some brightness adjustment to adjust the light output to suit the surrounding lighting environment. In applications such as smartphones or portable GPS navigation systems, LED brightness adjustment must be used in order to allow the user to see the screen in both strong sunlight and low light conditions at night. When using a flashlight, users believe that longer battery life is more important than providing the strongest light. We can use analog dimming or pulse width modulation (PWM) dimming methods in these applications. The analog design achieves a higher efficiency than the PWM type design by using an innovative method to establish a reference voltage.
Both the analog and PWM dimming methods control the LED drive current, which is proportional to the light output. The analog brightness adjustment structure is simple, the control power consumption is the lowest, and generally higher than the PWM brightness adjustment method, because the LED forward voltage is lower at low driving current.
However, analog dimming requires an analog voltage to be generated from a separate voltage reference (possibly using an RC filter output for a square wave input signal or using an expensive digital-to-analog converter (DAC)). The circuit shown in Figure 1 eliminates the complexity of these methods by modifying a potentiometer, thus enabling a simple, cost-effective analog brightness adjustment method. This total solution is an efficient, low-cost, low component count LED driver for a single high-current LED, such as Osram's Golden Dragon, which can be used in some small battery-powered devices.