Lighting needs to bring new challenges to the development of the chip

Comparing today's portable consumer electronics with a few years ago, you will understand why lighting has become a major power management challenge. Handheld devices with a single passive LCD panel are rapidly being phased out. Today's devices are equipped with high performance, high resolution, 2.5 to 3 inches diagonal color display, in order to support the entire range of applications ranging from Internet access and mobile TV to video playback.

Typically, these displays require 4 or more LED and drive for backlighting. A number of handheld devices, especially flip designs, have added a smaller sub screen to display basic information about time, date, and connectivity. These sub screens typically require more than two LED for the backlight function.

With the discovery of the important role of fashion designers in product differentiation, many of today's portable electronic devices require an additional power supply to drive the auxiliary RGB status lights and keyboard backlight.

Design of a power supply circuit with higher complexity with built-in camera function. These features are generally provided by a high current in a very short duration to drive a small amount of LED to provide flash. But when handheld device manufacturers began to introduce this feature, they used less than 1 million pixels of the CCD, which requires flash drive current does not exceed 100mA. Today's handheld device design engineers are integrating a much higher resolution of CCD, the CCD requires up to 600mA of the current is only used to drive flash function, and provide enough light to obtain high photo resolution. The new features such as the movie mode and the flash function require a lower level power supply to drive the LED for longer duration, which increases the difficulty of the design of the power supply circuit.

Given these factors, it is easy to understand why lighting is often the most important source of battery power in handheld devices. In the past, the design engineer provided their lighting function by using a boost converter or a charge pump alone. But with the increase in the number of lighting functions and power requirements, design engineers need to better control to deal with light intensity, manage color balance and maximize power. Eventually, these engineers turned to a system microcontroller or a dedicated controller to solve these problems by pulse width modulation (PWM) control.

Simpler design

In recent years, power management integrated circuit (PMIC) manufacturers have developed a variety of IC designed to provide engineers with a higher level of control and simplified design. United States National Semiconductor (NS) Company LM27965, for example, this white LED (WLED) driver can drive up to 9 parallel LED, the total output of up to 180mA. In order to maximize the ability to control the design engineer, the output positive current is divided into two to three independent control group, and the configuration of four to five LED based backlighting display; configuration of two to three LED backlighting for vice display; configure a single independent control of the driver to manage state or indicator LED. Each group of LED is controlled by standard I2C interface.

Although the solution based on the inductive boost converter has shown advantages in many applications. In many cases, however, manufacturers are turning to hybrid models or fractional charge pumps to drive WLED in compact portable applications, and no longer require large sensors. Although the output of the fixed boost charge pump is modulated by a single resistor, the LED current matching and efficiency may be affected. When the mixed mode charge pump is used, the output voltage can be modulated to maintain the constant current of each LED.

Similar to many competing products, linear technology (Linear Technology) in August the company released the LTC3219 multimode charge pump, the device can be conducted in the mode of 1 times, and then when any enabled LED current source close to the voltage drop when the automatic conversion to boost mode or 1.5 times.

Then the device voltage drop will convert to double (2x) mode. In order to support the primary and secondary display and RGB lighting requirements, the device driver can be 9 separate current source.

As with the NS device, adjusting each linear current source in the device through the serial interface using double I2C digital control is gray, brightness, flash and grading.

Early WLED requires relatively high forward voltage and current to achieve the desired luminosity. But recent technological advances have allowed manufacturers to produce WLED capable of operating under 10mA. These devices work at lower forward voltages than before. Recent technical developments have allowed PMIC manufacturers to provide independent current sources with linear matching, which reduces the cost and reduces the occupied area by eliminating the need for most external devices.

For example, the TI TPS75105 provides a four channel LDO, and comes in an extremely compact 1.2 x 1.2mm package with up to four LED matched LED constant current drivers in each of the slots. The product does not need the inductor, the output capacitor and / or the feedback resistor required by the induction boost converter, or the switching capacitor and the input / output capacitor required by the mixed mode charge pump.

Camera flash drive is a more difficult challenge. The design engineer must provide the appropriate amount of light within a given time frame, and does not exceed the system's power consumption limits or LED's thermal limits. Since the basic on / off control is not sufficient, Xu