The Critical Role of LED Drivers in Lighting Applications

Incandescent and other older bulbs use electricity to heat a filament or gas, which then glows to emit light. A light-emitting diode (LED), on the other hand, is made of a special semiconductor material that directly converts electricity passed through it into light, a phenomenon known as electroluminescence.

Each LED material emits light in a narrow frequency range when supplied with a specific voltage and current. Deviations from these values can cause the LED to stop producing light or change its color intensity.

Designers typically rely on either constant current reduction (CCR) or pulse width modulation (PWM) to control LED intensity. Both aim to adjust the light output, but operate in fundamentally different ways, each with its own set of design trade-offs:

• CCR, often referred to as analog dimming, works by reducing the current flowing to the LED. It’s a straightforward and low-noise approach that doesn’t introduce flicker, making it suitable for basic applications. However, reducing the current can slightly shift an LED’s color and limit the dimming range, especially at very low light levels.
• PWM dims LEDs by rapidly switching them on and off while maintaining a constant current during each pulse. This technique preserves color consistency and enables a much wider dimming range, often down to less than 1%. This makes it ideal for tunable lighting or displays. The trade-off is that PWM can introduce electromagnetic interference (EMI) and visible flicker if the switching frequency isn’t high enough. Designers must carefully balance these factors.

PWM may require more complex drivers and attention to EMI filtering, while CCR may fall short in applications demanding color precision or ultra-low dimming. In some cases, a hybrid approach combining CCR and PWM offers the best of both worlds.

Design considerations

Designers can overcome the limitations of either CCR or PWM dimming through smart design choices. For CCR, designers can select LEDs with stable color performance across a wide current range and apply gamma correction or logarithmic dimming curves to adjust the dimming response to match how humans perceive changes in brightness. This results in smoother and more natural transitions. Careful driver selection and thermal management can also aid in maintaining color stability and extending dimming performance without the need for additional circuitry.

For PWM dimming, the key challenges are flicker, EMI, and design complexity. These can be overcome by using high PWM frequencies—typically above 20 kHz to 25 kHz—to avoid visible flicker and minimize interference with audio or camera systems. EMI can be effectively controlled by carefully designing the circuit board, utilizing filters, and selecting LED drivers that provide features such as adjustable signal rates. Drivers that incorporate built-in PWM capabilities simplify the process by generating signals internally, eliminating the need for precise timing management outside the driver.

CCR may be preferred for applications that demand minimal EMI, such as healthcare settings, laboratories, or environments with sensitive electronics. This option provides reliably smooth and flicker-free dimming over a limited range, and its relative simplicity also lends itself to general illumination in homes, restaurants, and large venues, particularly when simplicity and cost are top priorities.

PWM is often preferred for high color consistency and a wide dimming range, such as for stage lighting or environments requiring very subtle lighting control. PWM drivers with integrated signal sources further simplify the design process by handling timing internally, reducing design complexity.

When choosing the PWM route

Applications that require multi-channel control, color consistency, and automotive reliability benefit from the PWM approach.

A good example is the AL5887Q Advanced 36-Channel Automotive LED Driver from Diodes Inc., which offers dual-mode capabilities. It dims with deep PWM by modulating the duty cycle of the constant current from 100% to 3%. However, under 3% it transitions to an analog dimming mode that produces the same functionality of CCR through programmable digital controls rather than a classic analog CCR-only circuit.

With a built-in 16 MHz oscillator, the AL5887Q eliminates the need for an external clock, allowing for simplified board design and layout, smaller printed circuit board footprints, and a lower cost bill of materials (BOM). It uses a 12-bit PWM addressable register and a 30 kHz internal PWM generator for better color mixing and reduced noise.

Designers can use this targeted toward:

• Automotive interior and exterior lighting
• Infotainment displays
• Status indicator lights
• Touch panels and LCD display backlights

These applications require controlling the color and the intensity of the LEDs, which are the key functions the AL5887Q driver (Figure 1) facilitates.

Figure 1: The AL5887Q LED driver from Diodes Inc. simplifies automotive display and lighting applications. (Image source: Diodes Inc.)

Controlling the color of an RGB LED

Controlling the color of an RGB LED involves adjusting the current flow to each of the three different colored die contained within the LED package (Figure 2). In simple terms, to produce a bright yellow color, the red and green die are driven up to their specified maximum, and the blue LED is dimmed or turned off. Similarly, a large range of colors can be produced by controlling the intensity of the individual LEDs.

Figure 2: To control the color of an RGB LED, the application needs to adjust the current flow to each of the three different colored die within the LED package. (Image source: Broadcom)

The architecture of a typical lighting application

A lighting application could consist of hundreds of LEDs and other components, all commanded by a program running on a single small computer or microcontroller.

The pinout of the AL5887Q is shown in Figure 3. An application might contain multiple drivers; each of which can connect to up to 12 RGB LEDs or up to 36 individual LEDs, with dedicated pins OUT0 to OUT35.

Figure 3: The pinout of the AL5887Q. It can connect to up to 12 RGB LEDs or up to 36 individual LEDs, with dedicated pins OUT0 to OUT35 (Image source: Diodes Inc.)

Why the AL5887Q makes a good LED driver for lighting applications

The AL5887Q can mix and match channels, with some used for color blending and others for status indicators or similar single-color LED applications. Each channel serves as a programmable constant-current source to deliver uniform brightness and color—an important consideration for retail, automotive, or architectural lighting, where inconsistency is easily noticed.

Error detection

In addition to mixing and dimming LED lights, the AL5887Q detects various error conditions, such as short circuits, which it records in an internal “flags” register. The AL5887Q alerts the microcontroller about the problems using its FAULT pin. The LED driver’s error detection mechanism allows the application running on the microcontroller to react and provide diagnostics for repairs.

Application developer simplifications

The AL5887Q removes some of the overhead from the microcontroller that is controlling it, such as intensity color mapping, simplifying the job of the application developer. It makes the program running on the microcontroller simpler, decreases development time, and makes the system more robust by eliminating possible sources of errors.

Banking

Some LED animation effects like blinking and “breathing” (which is like slow pulsing) involve simultaneous actions on many LEDs. Instead of the microcontroller program identifying each LED separately to send the same commands over and over, the AL5887Q can be configured to group the LEDs into “banks.” The entire group can then follow a single command.

I²C and SPI support

A high-level LED application with many peripherals and configurations requires flexible logic that discovers and modifies behavior accordingly. It might choose I²C, which uses two wires to discover and communicate with dozens of peripherals.

On the other hand, a low-level application with a fixed configuration and simple logic can be simplified using SPI, which uses more wires but communicates directly with known peripherals.

The AL5887Q can fit into both kinds of application architectures. It can support either the I²C or the SPI communication method. An “interface selection” (INT_SEL) pin allows the microcontroller application to command the driver circuit to use one or the other communication method during start-up.

Power-saving mode

When the LEDs are off, the AL5887Q automatically enters power-saving mode, in which it consumes only 25 micro-amperes. If the microcontroller sends any command to it, it resumes normal mode. This feature can be disabled with a configuration command.

Conclusion

The AL5887Q’s 12-bit PWM control attribute combines precision and flexibility, making it a versatile and reliable choice for advanced LED lighting applications. Its small footprint and integrated features reduce the need for additional components, lowering overall cost and streamlining development.