LED displays are now ubiquitous on the streets, along with decorative LED lights and LED car lights; LEDs have become an integral part of every aspect of life. Undoubtedly, LEDs are currently one of the hottest applications, used in handheld devices, game consoles, neon lights, billboards, and more. Their dazzling colors and high-quality light always attract attention immediately. With so many LED controllers available, choosing a feature-rich yet cost-effective product to suit one's design needs is a challenge for every designer.
The simplest LED driver can be implemented using ordinary I/O. However, I/O control can only turn LEDs ON and OFF, and cannot be used for functions such as light mixing or flashing. Moreover, each LED requires a separate I/O resource, which is undoubtedly inefficient. We can also design a dedicated high-current LED controller, but the high cost will be a problem, and the design complexity will increase accordingly with the occurrence of various interferences.
Based on these features, NXP has launched a series of LED drivers using an I2C interface. These drivers can simultaneously control the ON/OFF, blinking, and RGB mixing of 4 to 24 LEDs via just two lines of the I2C interface. In the mixing scheme, each LED is driven by an independent 8-bit/256-level PWM. Currently, the chip can drive each LED with a current range of 25mA to 100mA. Of course, for some high-current applications, external MOSFETs can be used.
This I2C-based LED control method increases design convenience and flexibility, and reduces investment in both hardware and software, making the once mysterious LED appear simple and exciting. Below, we will use the NXP LED driver PCA9633 as an example to fully illustrate the advantages of this type of LED driver through several simple applications. The PCA9633 is a four-channel LED driver, with each channel capable of driving a maximum current of 25mA. Depending on the package, it offers selectable fixed I2C addresses and hardware addresses with 4 or 7 bits (Figure 1).
As shown in Figure 1, each LED is controlled by a separate 8-bit/256-level PWM. Because the PWM is fast enough, it can theoretically mix any color of light from the four LEDs it drives. In addition to each individual PWM, the PCA9633 also provides a GroupPWM, which can be used to control the brightness and frequency of the mixed light, compensating for some functions that cannot be achieved by controlling only a single PWM. So how exactly does the PCA9633 achieve dimming? The secret lies in the PWM. Without PWM, it can only perform on/off actions; low-speed PWM can only achieve LED blinking, not sufficient for color mixing; high-speed PWM can achieve RGB color mixing; if the PWM speed is controllable, then both blinking and color mixing functions can be achieved. Furthermore, the controllable 8-bit/256-level PWM increases the color gradation and enhances the sense of color depth (see Figure 2).
Having understood the principles of color mixing, how is a specific color generated? We know that the human eye perceives color as the sum of the average brightness values of various colors. We can control the brightness of the driven LEDs by controlling the duty cycle of each PWM of the PCA9633. According to the principle of three primary colors, if we are driving RGB (or RGBA) LEDs, then by adjusting the different brightness levels of these three LEDs, we can obtain the desired color. Figure 3 shows an example of using the PCA9633 to control three RGB LEDs to adjust pink light.
Through the above description, we have a basic understanding of the internal structure and driving principle of the PCA9633. Below, we will use several applications of the PCA9633 with a fixed I2C address to further understand the advantages of this LED controller.
In our first application, we'll use the PCA9633 to control a brightness bar. We know that applications like brightness bars typically require a large number of LEDs connected in series. Controlling each LED with a single interface would significantly increase cost and software complexity. However, with I2C, only two control lines are needed in hardware, and a single byte command is required in software for easy control. Furthermore, due to the uniqueness of I2C device addresses, multiple PCA9633s can be used to control the number of LEDs being driven. If the PCA9633's drive current is insufficient in a practical application, simply adding an external FET can easily solve the problem. Additionally, the PCA9633's unique GroupPWM makes controlling the brightness and flicker of the entire brightness bar effortless. Below is its schematic diagram (see Figure 4), where the I2C master is provided by the system and can be an MCU or a logic circuit.
The left half of Figure 4 shows the I2C master, which will not be described in detail. The top right corner shows the LED current-limiting resistor. Typically, the forward voltage of an LED is around 3V, though this can vary depending on the color and manufacturing process. We can calculate the value of this current-limiting resistor using the required LED current: R = (Vsupply - Vfsum) / If. If the required LED current is greater than 25mA, the FET added in the figure can easily solve this problem. After adding the FET, simply set the OUTDRV register of the PCA9633 high to distinguish it from its default value.
Now we can see that controlling so many LEDs with the PCA9633 is quite simple in schematic diagram, and equally convenient in software register settings. The PCA9633 provides a simple yet complete set of internal registers, such as LED output structure settings, power-saving mode settings, chip enable mode settings, LED output status settings, and control register settings for each PWM and GroupPWM. In addition, the PCA9633 provides a register setting increment bit. This means that if we set this bit, we can complete the sequential configuration of all internal registers through a single instruction sequence. This is very useful in certain applications, maximizing the saving of software and system resources. Below, we will illustrate the internal register settings through another example.
The second example is using the PCA9633 to implement the breathing light function. Although the PCA9633 does not have an internal breathing light module, we can achieve this function through some simple register settings, which is undoubtedly a significant cost advantage compared to a dedicated breathing light chip. For ease of explanation, we will only use the PCA9633 to control the breathing action of one LED. The schematic diagram is very simple and will be omitted here. The breathing effect is achieved by controlling the gradual brightening and dimming of this LED. To achieve this function, the independent PWM of the PCA9633 is the most important factor. As mentioned earlier, each LED is controlled by an 8-bit/256-level PWM, which means that each LED has 256 adjustable brightness levels, which can perfectly realize the breathing function. Specifically, we achieve this by controlling the duty cycle of the PWM. If our LED is controlled by PWM0 of the PCA9633, then the duty cycle of PWM0 will determine the brightness of the LED: Bright(dutycycle) = PWM0 [7:0]/256.
Thus, a complete breathing process is finished. With just a few simple register settings, something that seems only possible with complex systems or dedicated chips can be accomplished. From the two examples above, we can see that using NXP's I2C LED driver is very simple and easy to operate, both in hardware and software, and the functions that can be achieved with such devices are in no way inferior to some systems and proprietary chips.
In summary, NXP's I2C LED driver offers a cost-effective LED design solution. Compared to using GPIO or dedicated LED drivers, it not only saves system resources but also significantly reduces design cost and complexity, effectively improving design reliability and the uniformity of driven light. Furthermore, using this type of LED driver can effectively help shorten design cycles and increase design flexibility.
NXP currently offers I2C LED drivers ranging from 4 to 24 channels, and these drivers are used in various fields such as automotive, home appliances, and communications. Although LEDs are ubiquitous in daily life, they also have some shortcomings that require designers to have more specialized knowledge in order to design products that better meet our needs.
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