Incandescent lamps and other old-fashioned light bulbs use electrical energy to heat the filament or gas, causing it to become hot and emit light. Light emitting diodes (LEDs) are made of special semiconductor materials, which directly convert the electrical energy passing through their interior into light through electroluminescence.
Each LED material emits light within a narrow wavelength range when subjected to specific voltages and currents. By adjusting these values, the LED can stop emitting light or change its brightness.
Designers typically rely on constant current reduction (CCR) or pulse width modulation (PWM) to control the brightness of LEDs. Both methods are used to regulate light output, but their working principles are completely different. The following are the trade-offs in the design of each method:
CCR， Often referred to as analog dimming, its working principle is to achieve dimming by reducing the current flowing to the LED. This is a simple, intuitive, and low-noise method that does not produce flickering, making it suitable for basic application scenarios. However, reducing the current may slightly alter the color of the LED and limit the dimming range, especially in very dim light conditions.
PWM achieves dimming by quickly switching the LED while maintaining a constant current during each pulse period. This technology can maintain color consistency and achieve a wider dimming range, typically adjustable to less than 1%. Therefore, it is very suitable for dimmable lighting or display screens. The trade-off is that if the switching frequency is not high enough, PWM may cause electromagnetic interference (EMI) and visible flicker. Designers must carefully weigh these factors.
PWM may require more complex drivers and special attention to EMI filtering, while CCR may not perform well in applications that require color accuracy or ultra-low dimming. In some cases, a hybrid approach combining CCR and PWM can balance the advantages of both.
Design considerations
Designers can overcome the limitations of CCR or PWM dimming by choosing intelligent design. For CCR, designers can choose LEDs with stable color performance over a wide current range and apply gamma correction or logarithmic dimming curves to adjust the dimming response to match human perception of brightness changes. This can make the transition smoother and more natural. Careful selection of drivers and optimization of thermal management can also help maintain color stability and extend dimming performance without the need for additional circuitry.
For PWM dimming, key challenges include flicker, EMI, and design complexity. These issues can be overcome by using high PWM frequencies, typically between 20 kHz and 25 kHz, to avoid visible flicker and minimize interference with audio or camera systems. EMI can be effectively controlled by carefully designing circuit boards, using filters, and selecting LED drivers with adjustable signal rates and other features. The driver with integrated built-in PWM function simplifies the process by generating signals internally, eliminating the need for precise timing management of the driver externally.
CCR may be more suitable for application scenarios that require the lowest EMI, such as medical environments, laboratories, or places containing sensitive electronic devices. This option can provide reliable smooth and flicker free dimming effects within a limited range, and its relatively simple structure makes it suitable for general lighting in scenarios such as homes, restaurants, and large venues, especially in situations where simplicity and cost-effectiveness are emphasized.
PWM dimming technology is often used in stage lighting or situations that require extremely fine light control due to its excellent color consistency and wide dimming range. The PWM driver with integrated signal source further simplifies the design process and reduces design complexity through internal processing timing function.
When choosing PWM scheme
The PWM dimming method is suitable for applications that require multi-channel control, color consistency, and automotive grade reliability.
For example, the AL5887Q advanced 36 channel automotive grade LED driver from Dior Inc. has dual-mode functionality. By adjusting the duty cycle of the constant current (from 100% to 3%), deep PWM dimming can be achieved. However, when it reaches below 3%, it will switch to analog dimming mode and achieve the same dimming function as CCR through programmable digital control, instead of using traditional analog CCR dedicated circuits.
AL5887Q has a built-in 16 MHz oscillator, eliminating the need for an external clock, simplifying circuit board design and layout, reducing printed circuit board footprint, and lowering bill of materials (BOM) costs. It uses a 12 bit PWM addressable register and a 30 kHz internal PWM generator to achieve better color mixing and reduce noise.
Designers can use this feature for the following applications:
Automotive interior and exterior lighting
Information entertainment display screen
status indicator light
Touch screen and LCD display backlight
These applications require controlling the color and intensity of the LED, which is the key function that the AL5887Q driver (Figure 1) can achieve.