Selecting the right LED driver solution for low voltage backlight or flash applications

White LEDs are widely used in small liquid crystal display ( LCD ) panels and keyboard backlight and indicator applications. High-brightness LEDs are used as flash sources in mobile phones and digital cameras. These applications require optimized driver solutions that can extend battery life and reduce printed circuit board ( PCB ) area and height. In these application areas, common LED driver solutions involve different topologies of linear, inductive or charge pump types, each with its own characteristics. For example, the inductive solution has the best overall energy efficiency; the charge pump solution uses low-profile ceramic capacitors, which occupies a very small circuit board area and height; the linear solution is very suitable for color indicators and simple backlight applications. ON Semiconductor provides LED driver solutions for all three types of topologies (see Figure 1) to meet users' different application needs.

Figure 1: Examples of different LED driver topologies for low voltage portable device applications

In terms of charge pump solutions, ON Semiconductor provides products that support different dimming types, such as single-mode, dual-mode, tri-mode or quad-mode charge pump solutions, such as CAT3200, NCP5602, NCP5612, NCP5623, CAT3606, CAT3616, CAT3626, CAT3603, CAT3604, CAT3614, NCP5603, etc. Take NCP5623 as an example. This is a high-efficiency LED driver with a 2.0 mm × 2.0 mm × 0.55 mm LLGA-12 lead-free package, with an I2C interface and built-in progressive dimming function. It is specially designed to drive RGB LED decorative lights and enhanced LCD backlights in portable products such as mobile phones . NCP5623 achieves 94% peak efficiency and a standby current of less than 1 microampere, extending the battery life of portable devices to the maximum. In addition to the advantages of a very small IC package, the device requires only four passive components for typical applications . The device also features short-circuit and overvoltage protection to protect the system in the event of LED failure.

It is worth mentioning that ON Semiconductor also provides a variety of quad-mode charge pump LED drivers, such as CAT3636, CAT3637, CAT3604V, CAT3643, CAT3644, CAT3647, CAT3648 and NCP5604A/B. Take CAT3648 as an example. This is a patented high-efficiency quad-mode adaptive fractional LED driver from ON Semiconductor. It can drive up to 4 LEDs at 25 mA (see Figure 1) with an efficiency of up to 92%. This quad-mode driver provides four modes: 1x, 1.33x, 1.5x and 2x. Compared with most charge pump drivers that provide three modes of 1x, 1.5x and 2x, it has a 10% higher efficiency and does not require additional capacitors, bringing the performance of LED drivers to a new level. These drivers are suitable for driving white LEDs in small-size LCD backlights and LED flash applications in low-voltage portable devices.

In the inductive boost solution, ON Semiconductor provides different products using PWM dimming, such as CAT32, CAT37, CAT4137, CAT4139, CAT4237, CAT4238, NCP5005, NCP5010 with output currents between 20 mA and 50 mA, as well as CAT4240 (250 mA), NCP5050 (600 mA) and NCP1422 (800 mA) with higher output currents. These inductive boost drivers are suitable for driving white LEDs in low-voltage portable device backlight and flash applications.

In terms of linear backlight driver solutions, ON Semiconductor provides a variety of single-mode LED drivers with 2 to 4 channels, such as CAT4002A, CAT4002B, CAT4003B, CAT4004A and CAT4004B. These backlight drivers provide 32-level dimming control, fixed or adjustable output current of 25 mA and extremely low shutdown current of less than 1 μA, without switching power supply noise issues. These single-mode LED drivers are usually used as part of a system-level approach to design backlights that integrate low-voltage LEDs and simple LED drivers . These driver circuits are simple, helping to extend battery life, reduce costs (such as saving external capacitors) and reduce noise, providing a simple solution for entry-level portable products and low-cost mobile phone markets.

Figure 2: CAT4002B application circuit diagram

LED driver solution specifically for camera flash

It is worth mentioning that in camera flash applications, in addition to inductive boost drivers such as NCP5005 and CAT4134, charge pump drivers such as NCP5680 and CAT3224 can also be used to support high-megapixel camera flashes and replace xenon flashes with slim designs. Among them, NCP5680 and CAT3224 are both supercapacitor-based LED drivers that can provide large flash currents of 10 A and 4 A respectively.

In fact, today’s cameras with 5 megapixels or higher resolution require a high- brightness flash to take high-resolution photos in low light. Today’s white LEDs can provide this level of light energy, but require nearly 400% more energy than the camera battery can provide. Take ON Semiconductor’s NCP5680, for example. This device manages a supercapacitor in conjunction with the battery to drive the LED flash to full brightness, providing a high peak current of up to 10 A. The integrated driver in the NCP5680 also manages the supercapacitor and handles other peak power functions such as zoom , autofocus, audio , video, wireless transmission, GPS data reading and radio frequency ( RF ) amplification, extending battery life without giving up a slim design. The NCP5680 integrates all the circuits required for supercapacitor charging, inrush current management and LED current control, saving designers development time, board space and component costs.

Figure 3: Application circuit diagram of the industry’s first single-chip 4 A supercapacitor LED driver, the CAT3224.

The CAT3224 is the industry's first 4 A single-chip supercapacitor LED driver (see Figure 3), which integrates a dual-mode 1x/2x charge pump and provides three key functions: precise supercapacitor charging control, current discharge to LED flash management, and constant current for LED flashlight mode. The operating current of these three modes can be easily programmed with three external resistors and can absorb up to 4 A of LED flash pulse current. The high peak current advantage of supercapacitor technology, combined with the simple parallel logic interface of the CAT3224, makes this device an excellent choice for applications that use LEDs to replace xenon lamps.

Other novel LED drivers/ controllers

In addition to the above LED drivers, ON Semiconductor also offers some other novel products for low-voltage portable device applications, such as NCP5890 and CAT3661. Among them, NCP5890 is a unique lighting management integrated circuit (LMIC) that integrates liquid crystal display (LCD) backlight, decorative light control and ambient light sensing functions in an extremely small package of 3 mm x 3 mm x 0.5 mm .

As we all know, today's portable electronics are popular with larger LCD screens and LED lighting effects. To meet all these lighting requirements, hardware designers usually need to use several LED drivers. Due to limited board space, implementing more advanced lighting effects usually requires a lot of software programming and microcontroller ( MCU ) resources. ON Semiconductor provides simpler single-chip silicon solutions such as NCP5890, which has a variety of command-controlled lighting effects to help hardware design engineers meet their specific lighting and power design goals. This lighting management IC has a 30 V output voltage capability and drives series LEDs to achieve balanced backlighting of LCD screens. In addition, this device controls three groups of white LEDs or RGB LEDs to create decorative light patterns on the keyboard or chassis, forming a unique combination with the backlight. The driver also adjusts the backlight current according to the brightness of the ambient light, thereby extending battery life. NCP5890 is a dedicated solution for applications such as compact smartphones.

Figure 4: Application circuit diagram of the CAT3661 LED driver optimized for button cells.

Today, more and more innovative portable devices are powered by coin cells, such as blood glucose meters, digital thermometers, oximeters, respiratory analyzers and physiological monitors in medical applications. Due to the unique characteristics of this battery and the need for a long working life, these compact applications require customized LED drivers that not only manage the backlight but also monitor the battery power. In such applications, the CAT3661 2 to 2.5 V single LED driver, which ON Semiconductor plans to launch in the second half of 2010, can be used. This device also uses ON Semiconductor's patented Quad Mode charge pump architecture, with energy efficiency up to 92%, quiescent current as low as about 150 μA, and provides adjustable low battery detection function, as well as strong LED fault monitoring, soft start and short circuit limiting protection functions. It uses a low-profile 3 x 3 mm T QFN -16 package, which is very suitable for these portable device applications.

summary

As the leading supplier of high-performance, energy-efficient silicon solutions for green electronics, ON Semiconductor provides comprehensive LED driver or control solutions using its expertise in low-voltage and high-voltage technologies and power management solutions. This article focuses on various white or RGB LED drivers from ON Semiconductor that use different topologies for low-voltage portable device backlight or indicator applications, and specifically introduces ON Semiconductor's drivers for portable device flash applications that require high current capabilities, as well as lighting management integrated circuits that integrate multiple functions and LED drivers optimized for button batteries, so that engineers can choose the right product for their specific application.