Selecting LED Drivers for Wireless Products

Mobile phone lighting has changed dramatically in the last few years. From the end customer's point of view, not much has changed - they may just notice a bigger display, more indicator lights or a flash when taking a photo. From a technical point of view, the challenges of creating lighting features are tough and will become more so in the future. Today, the biggest technical challenges in mobile phone lighting are:
● Proper partitioning of LED drivers in the system
● Limiting power consumption
● Radiated noise
● Control interface and programmability
● Safety issues
● Size of external components
● System cost

Display backlighting
Depending on the phone, the display backlight LED driver requirements vary from simple 4-LED drivers to more complex dual display (main and sub) backlight drivers that can drive up to 10 LEDs. Depending on the phone manufacturer, the LED driver is either part of the display or mounted on the phone motherboard. A small number of phones have organic LED (OLED) displays, which do not have backlight LEDs but do require a bias voltage for illumination.
Most of the discussion is about choosing between parallel and series. Unfortunately, there is no simple answer to this question. Usually, a charge pump AC/DC converter with LED current sink or source can provide the smallest solution (no inductor required). The efficiency of parallel LED drivers varies with the input voltage range. Multimode charge pumps can achieve 75% to 90% LED efficiency over the lithium-ion battery voltage range. The disadvantage of paralleling is that it increases the amount of wiring on the PWC. In the past, matching was a major parameter, but new circuit structures can now achieve 0.2% matching between LED branches. Series connection requires a magnetic DC-DC converter to increase the battery voltage to 16 to 20V depending on the number of LEDs and the maximum forward voltage specification. The size of the inductor in this application is generally 3.5mm × 3.5mm × 1mm, and the output capacitor voltage is specified to reach 25V. Therefore, the system size is larger than similar charge pump applications. Magnetic solutions provide excellent matching (similar current for each LED) and higher total LED efficiency (80% to 95%).
Some mobile phone manufacturers have added LED driver features to the baseband power management unit. In some cases, it is an economical solution, but it is not flexible enough. Lighting requirements in phone designs change rapidly, and the LED driver driven by the PMU is generally designed to turn on or off a certain number of LEDs. If the PMU cannot drive the backlight, a separate driver is needed - and the PMU function will not be used if it increases system cost.
If the LED driver is located in the display module, system height becomes a determining factor. By far, the charge pump solution is the best, but new inductors will be as thick as 0.8mm. At the same time, it is also possible to use chip inductors, or use charge pumps or magnetic DC-DCs to create thin displays. Another factor to consider is the control interface - single start, dual start, I2C or SPI interface. Serial interfaces are available in most phones, allowing for the most convenient and flexible control functions.
In the near future, backlight color saturation will become a major differentiator. High-resolution cameras and digital TVs in handheld devices require better display and backlight technology. Large OLED displays once promised to change display technology, but so far they have not. New RGB LED backlight technology, separated from high-quality TVs, will also provide brighter colors in small displays. Although the technology is new, it can provide up to 110% NTSC color saturation compared to white LEDs (NTSC ratio is 70%).
Efficient display backlighting can be achieved by more than just choosing the right driver or LED. Backlight intensity should be controlled based on the ambient light environment to reduce power consumption. Ambient light feedback control techniques depend heavily on sensor layout (front, back), sensor type (photodiode, phototransistor, sensor) and required features (direct feedback, filtered feedback, end customer specified feedback). Ambient light controlled displays are not only more efficient, but also provide a more natural look and feel to the end user.

Color LED Drivers on Cell Phones
Color LEDs are commonly used as indicator lights - an incoming call turns the LED red, a text message turns purple, etc. Such LED driver products range from simple controllers (without a boost converter) to complete LED drivers (with a boost converter). Many new power management units have integrated GPIOs that can be used to drive color LEDs.
Indicators are not only used for color LEDs. They can also be used for entertainment, warning signs, keypad guidance or phone decoration. A good example is inventory tracking lights, where the user can program the inventory with the corresponding colors according to their preferences. On keyboards, RGB LEDs can be used to illuminate specific areas of the keyboard for easy navigation on the keyboard. In MP3 mode, the phone keypad controlling the volume or tracking light will be displayed in a different color than the other keypads.

Fig. Flash LED driver - the problem.

The drawbacks of having more color LEDs are power consumption, wiring difficulties (space and noise) and increased control software. In many cases, discrete color LED drivers built into the DSP and boost circuits can be used. The advantages are maximum flexibility, local wiring, existing serial interfaces (I2C or SPI) and reduced power consumption. The power consumption of color LED drivers is particularly important because the indicator LEDs are always on in the standby mode of the mobile phone, which reduces the standby time. Usually, the LED driver is controlled by the baseband or power management IC, which increases the standby current. The more control, the higher the power consumption. New color LED drivers can achieve minimal control through built-in command sequences. Users can program time delays, decay, blinking, loops and trigger signals with simple C-like coding. These commands are stored in the LED driver and automatically turn on and off according to the program. Because the control is predetermined, the LED driver can optimize the output power and turn on or off the boost converter accordingly. This type of LED driver can achieve single output (single indicator), triple output (RGB LED), six outputs (dual RGB) or twelve outputs (quadruple RGB) according to the requirements. Typically, the device is controlled by a serial interface, so no additional GPIO is required for control. Also, the layout of the LED driver is simple, as the package size is generally less than 2mm×2mm.
Camera phones are common. High-resolution camera phones have a flash function, which generally does not change the picture quality unless used in a very dark environment. New flash LEDs with integrated flash LED drivers provide quite high flight intensity, which does improve the picture quality. Unfortunately, the higher the flash power, the more difficult it is to complete the design. Typically, the power specification of new LEDs is about 3~5W, which reflects the worst-case battery current (2~3Amp). If not designed properly, such a large current will cause the battery voltage to drop sharply. The best methods include minimizing wiring constraints, shielding EMI, using radio TX (if the radio is on, the flash is turned off), and applying supercapacitors. Ultra-low capacitors are plate capacitors with a capacitance of up to 1F. Capacitors can store energy for high current applications such as flash LEDs. Flash LED drivers generally also have integrated indicator LED drivers, flash timers for safety, TX disable pins, flash trigger pins, and torch adjustments. In the future, when the battery voltage range drops below 2.3V (new battery technology), flash LED drivers will require a supercapacitor approach.

Conclusion Backlight LEDs, keypad LEDs, flash LEDs, and indicator/color LEDs are a very important part of today’s phone designs. Well-designed lighting functions and effects can make a big difference - especially to the end user. Designing natural, power-efficient lighting systems is challenging, but can be overcome with the right products. Lighting design starts with system design and proper partitioning, and product selection plays a big role in the success of a product. Future challenges include: ● Larger, brighter, more colorful, and more efficient displays ● Brighter camera flashes require higher power and new power architectures ● Keypad/color/indicator LED drivers will have built-in features to achieve the highest power efficiency, thereby extending standby time ● Higher integration and space constraints will increase the popularity of integrated lighting management units Meeting future technology requirements will not be easy, but fortunately, technology leaders can improve solutions to achieve higher performance today.