Versatile White LED Driver for Multi-Display Backlighting

Introduction
This year, more than 600 million cell phones are expected to be manufactured worldwide. These phones are not only for making calls, but today's newest models can also be used to browse the Web, wirelessly transmit email, take photos, and even watch streaming video. Consumer demand for these "rich features" is putting increasing pressure on phone manufacturers to fit them into smaller and smaller phones while maintaining battery life.
From the block diagram shown in Figure 1, it is easy to understand that more features require more low-voltage output rails at different power levels. In addition, the need to backlight multiple displays and drive multiple "indicator" lights is increasing. For example, it is not uncommon for a cell phone to have a main display for viewing text, graphics, or photos, a secondary display to show the time, date, or caller ID, a white LED that acts as a camera flash or is put in torch mode for video capture, and an LED that emits a red light when the camera is in use.

Tony Armstrong, Product Marketing Manager, Power Business Unit, Linear Technology

Figure 1 Block diagram of the latest cellular phone Linear Technology Power Business Unit products

Figure 2. LTC3206 configured to drive four displays

Why choose white LEDs?
White light-emitting diodes (LEDs) have longer life and lower power consumption than CCFLs and electroluminescent backlighting devices. Most LED manufacturers state that their products have a service life of 100,000 hours under nominal operating conditions, and the efficiency of power input to light output can be greater than 80%. Since the first release of blue LEDs (wavelength 470nm) in 1993, nitride-based LEDs in different colors from ultraviolet to yellow have enriched the application of LEDs. However, it is only in the past five or six years that device manufacturers have developed the chemical methods and process technologies required to produce white light LEDs (wavelength 430~700nm). Typical white light LEDs have a nominal drive current of 15~20mA and a typical forward voltage drop of 2.7~4.0V. For most lighting systems, an LED array is required to provide all the required light intensity, so the accuracy and matching of LED brightness must be considered in any driver design.
At the same time, the use of small color active-matrix TFT liquid crystal displays (LCDs) in cell phones has been proliferating over the past period of time. These LCDs are used to display a variety of textual data as well as images and photos. These displays require small, compact and efficient solutions to drive the white LEDs that provide backlighting for them. Until recently, these requirements have been somewhat contradictory to each other, but recent products have given designers a choice of high-performance backlighting technologies. The two best solutions are to use a low-noise inductorless DC/DC converter (more commonly known as a charge pump) or a step-up DC/DC converter. The main difference between these two solutions is whether an inductor (magnet) is required and whether the LEDs are connected in series or in parallel. The best architecture for LED backlight drivers depends on the specific needs and constraints of each application.

An Ideal White LED Multi-Display Driver
Because many cell phones are in "space-constrained environments," charge pump multi-display drivers are an excellent choice for applications where small to medium load currents must be sufficient and multiple displays must be powered. Moreover, charge pumps offer low height, small solution footprint, and high conversion efficiency to extend battery life and implement at an affordable cost. However, typical charge pump ICs can be noisy and inefficient. For example, gated oscillator charge pumps are simple to design and low cost to implement, but they generate low-frequency noise at both the input and output that is almost impossible to filter out. Noise generated at the power input can interfere with radio frequency (RF) transmission and reception in wireless applications. Noise at the output can couple into sensitive circuits and even generate audible noise. To address these noise issues, designers have had to add bulky and expensive filters. However, Linear Technology's LTC3206 uses an improved charge pump solution. The LTC3206 is a highly integrated multi-display LED controller that includes a high efficiency, low noise fractional boost/direct connect charge pump.
The LTC3206 is a highly integrated multi-display LED controller that can power the main and sub white LED displays and the RGB color LED display at the same time. The LTC3206 requires only four tiny ceramic capacitors and two resistors to form a complete 3-display LED power supply and current controller.
The maximum current of the main, sub and RGB displays is set independently, and the current of each LED is controlled by an internal current source. Dimming and on/off control of all displays are implemented through an I2C serial interface. Two auxiliary LED pins can be independently assigned to the main display or sub display, increasing design flexibility. The dimming range of the main and sub displays is 128:1, and each red, green and blue LED has 16 dimming states, resulting in 4096 color combinations. In addition, to meet the needs of high-current camera flash, the LTC3206 can also provide up to 400mA of continuous current through the output of the charge pump (CPO pin) and can provide up to 800mA of peak current.
The LTC3206 communicates with the master device via a standard I2C two-wire interface. The I2C port operates at speeds up to 400kHz and has built-in timing delays to ensure correct operation when addressed from an I2C-compliant master device. The LTC3206 is a receive-only (slave) device.
To minimize noise, the LTC3206 uses a constant frequency current regulation technique to ensure that the charge and discharge current of the floating capacitor is just enough to provide the load current. The charge pump operates in two phases, and the current remains almost constant on each clock phase, ensuring that there is no audio component. For
optimal efficiency, the power management section of the LTC3206 provides two methods of supplying power to the CPO pin: 1:1 in direct connection mode or 1.5x in boost mode. When any display is started with the LTC3206, the power management system connects the CPO pin directly to VIN using a low impedance switch. If the voltage provided at VIN is high enough to power all LEDs at the proper programmed current, the system will provide the highest efficiency in this “direct connect” mode. Internal circuitry monitors all current sources for the onset of “dropout,” at which point the current source can no longer provide the programmed current. As the battery voltage drops, the LED with the largest forward voltage will reach the dropout threshold first. When any current source that is enabled reaches the dropout threshold, the LTC3206 switches to boost mode and automatically soft-starts the 1.5x boost charge pump. Designers
now have a clear choice when it comes to efficiency and cost effectiveness to drive white LEDs for LCD and keypad backlighting, as well as RGB and camera flash lighting. The LTC3206 fractional charge pump offers high efficiency, small footprint, design flexibility and low noise performance.