Optimizing the design of mobile phone white light LED driver circuit

introduction

One of the main trends driving the transition from monochrome to color displays in mobile phones is the integration of camera functions. Initially, the resolution of these imagers was quite limited, and the image quality was also poor. However, as the technology developed, the resolution progressed from the VGA level of 300,000 pixels to 1-2 million pixels, and is rapidly moving towards the resolution level of 3 million pixels and above. After continuous improvements in imagers, processors and software, consumers now want more digital camera functions, such as flash for low lighting conditions and even autofocus. At such resolutions, good image quality output and picture and video sharing become more practical, and these higher density CMOS imagers need to obtain more reflected light from the target, thus further driving the need for integrated flash function.

1 New requirements for LED driver design

It is very challenging for design engineers to put traditional xenon flash into a compact mobile phone because in addition to the bulky high-voltage capacitor for flash, the bulb and the related transformer and electronic circuit must be added, and the traditional flash is not suitable for video shooting. Fortunately, LED manufacturers have begun to improve the light output of power LEDs by adopting new materials such as indium gallium nitride (InGaN).

Figure 1 Inductive LED drive circuit based on NCP5005

On the other hand, innovations in semiconductor manufacturing technology and improvements in packaging methods have also increased the number of lumens that can be produced and the efficiency of photoelectric conversion. To produce the highest light output, these power LEDs may require a current output capability of 400mA or more and a pulse width of 50 to 200ms. However, for video applications, a smaller current is required, but the time is not limited to a single pulse. In addition to size constraints and ergonomic considerations, these camera modules are usually integrated behind or above the screen so that users can use the LCD as a viewfinder to capture images, which is particularly common on foldable phones. The imaging device and lens mechanism may also have to be able to rotate so that the phone can be used as a face-to-face communication tool in video conferencing mode. This is expected to be more common in phone designs on 3G networks because there is enough bandwidth to use video conferencing.

When discussing these trends, it is clear that the quality and resolution of screen displays are becoming higher and higher, while the size is also getting larger, especially for mobile phones with rich multimedia functions. It is expected that there will be more and more content in the future, such as streaming or broadcasting video, Internet browsing and email, taking pictures and photo viewing, as well as games and information acquisition services. Therefore, when the phone is not on a call, the screen will be used more frequently, but if the battery life of the phone is not enough, these functions will be limited. Therefore, efficient system power management, including backlight power management of the screen and buttons, becomes very important, and functions such as camera flash, which were previously only seen in high-end mobile phones, will gradually become standard configurations.

These will also bring challenges to the design of white light LED backlight driver circuits: larger screens mean more areas require backlighting, so the overall efficiency of the driver device must be improved; due to limited space, more functions must be integrated into a single package; because not only size is considered, thickness must also be reduced, especially for sliding and folding products.

2. Driving circuit solution

Two architectures are often used in white LED driver circuits in mobile phones: an inductive boost converter circuit in which LEDs are connected in series; and a charge pump driver in which each LED is driven by a stable current source. The inductive solution can bring the best overall efficiency, while the charge pump method has the smallest size because it uses small ceramic capacitors as energy conversion devices. Figures 1 and 2 show typical application circuits of the two driver architectures. Currently, the efficiency of power LEDs is constantly improving, reducing the power consumption of backlight LEDs, so fewer LEDs can be used to provide higher light output. This means that a 1.5-inch screen that required four LEDs to provide backlight two or three years ago can now achieve the same performance with only two LEDs that consume half the power.

To meet this demand, a new product that can drive two LEDs and support low-power standby operation of the screen backlight at a relatively low current is needed. To solve this problem, the NCP5602/12 series charge pump LED driver devices are designed to support ultra-low current ICON mode, while providing ordinary backlight functions of two LEDs through a simple single-wire or traditional I2C serial bus. In addition to supporting the latest trend of thin and light packaging, these products are also designed with new ultra-thin LLGA micro-packaging technology (2×2×0.55mm) to support ultra-thin applications. In addition, some improved LED materials and designs have lower forward voltages (from 3.6V to 3.1V), so for inductive solutions, the same power can drive more LEDs. More complex LED driver devices, such as NCP5604A/B, have more voltage conversion mode options, provide more optimized power conversion, and the power loss of the integrated current source is also lower, achieving 85% conversion efficiency (PLED/Pin) during most of the working time of the lithium-ion battery used for mobile phone power.

LED flash has become a must-have configuration for camera phones. The NCP5608 multi-mode charge pump LED driver developed by ON Semiconductor can drive 4 LEDs for the backlight of the main screen and sub-screen, and can provide a high current output of up to 400mA for driving 1W power LEDs. These functions share a high-efficiency charge pump conversion circuit in design to minimize the number of external capacitors.

Figure 2 Charge pump LED driver circuit based on NCP5604A

Due to limited space, especially in new ultra-thin candy-bar and flip phones, this device is packaged in a 4×4×0.75mm QFN package. To minimize the number of connections required to control the driver, the device uses a two-wire I2C data bus as the control configuration interface. Four dedicated channels are provided on the flash LED, allowing the pins to drive a high-power LED in parallel, or to drive several flash LEDs operating at lower currents.

3 Application Prospects

As many emerging multimedia and data services become more common, mobile phone designers will continue to face the challenge of integrating more features (such as larger screens, megapixel imaging devices, video processing and application coprocessors, etc.), while also meeting the demands of mobile phone users for standby and talk time, as well as users' expectations for longer video playback, gaming and web browsing, which will require longer display usage time. Fortunately, improvements in high-brightness LED efficiency and innovative LED driver devices will help designers meet these demands while meeting consumer expectations for weight and size.

In fact, backlight can be applied not only to screens and buttons, but also to colorful RGB (red, green, and blue) LEDs, which can provide some interesting effects and personalized settings based on music, ringtones, etc. For example, through the latest caller identification function, RGB LEDs can be used to visually remind users who is calling. We can see that LEDs have evolved from simple backlight functions to more functions. Edge-emitting devices have begun to be added to buttons to assist in the even distribution of button backlight, while also reducing the overall thickness of the phone and the number of LEDs required to generate backlight. In the future, more innovative applications will be seen in mobile phones and other digital consumer electronics products using LEDs to provide lighting.