Enhancing Backlighting in Portable Devices

Most portable electronic products today, such as mobile phones, personal electronic notebooks, navigation systems, etc., have a small LCD display that requires backlighting as a user interface. People are using these devices to view high-resolution photos, videos, and browse the Internet for longer and longer periods of time. As a result, people's demand for high-quality, bright displays with media storage capabilities is becoming increasingly strong, and the challenges to backlight LED and driver technology are becoming greater. Today, although white light LEDs dominate the market, the emerging red, green, and blue (RGB) backlights can improve the color saturation on the display, so the prospects are unlimited.

Nowadays, most portable devices, especially mobile phones, tend to use personalized designs for keyboard backlighting and other decorative lights. However, the requirements for display backlighting and keyboard backlighting are different, and this will affect the driving methods of the relevant LEDs.

Changes in LEDs and lithium-ion batteries will affect the design of backlight driver circuits. In addition, increasing the number of LEDs in portable devices will create challenges in LED driver design. The most common challenges include power efficiency, control interface/programmability, solution size, electromagnetic interference (EMI) and system cost.

Brightness Control

The brightness control of backlight LEDs can be achieved through pulse width modulation (PWM) or constant current control. PWM brightness control requires a constant current driver to drive the LED, but the on/off time needs to be adjusted to achieve the desired brightness. Therefore, PWM control is more complicated than direct constant current control.

The benefit of constant current control is that there is no continuous switching action, so when adjusting the brightness, the EMI caused by the shift of the LED color spectrum is lower. LED manufacturers group LEDs according to "cluster current" and ensure that the performance of the LED will not be reduced. When the cluster current changes, the brightness of the LED will change more than the set specification, so the naked eye can distinguish the difference in brightness between the backlighting LEDs. This is especially obvious when using very low currents.

If PWM is used to control the brightness, the brightness adjustment will be linear over the entire range and will not produce color changes when adjusted. However, the PWM conversion will generate electromagnetic interference and audible noise. This noise is generated by the piezoelectric effect of ceramic capacitors. In order to avoid this audible noise, the PWM frequency must be high enough to be inaudible to the human ear, such as 20kHz. Another method is to use a very low frequency so that the capacitors and circuit boards in the application will not resonate and ensure that no audible "pop" sound is produced (such as 250Hz). Slowing down the rising/falling edge of the PWM control can help reduce the intensity of electromagnetic interference.

Backlight driver topologies

Driver topologies can be divided into parallel and series. Parallel drivers are used when each LED needs to be controlled individually. In backlighting applications, the brightness of all LEDs should be consistent. However, if parallel drivers are used, there may be a slight mismatch between the LED currents. Fortunately, with the latest drivers, this current mismatch becomes negligible. This is because the typical brightness tolerance of these LEDs is generally much larger than the mismatch in the output current.

When the backlight LEDs are connected in series, the same current flows through all LEDs, resulting in 100% matching between the LED currents. In addition, the series drive eliminates the need for individual driver wiring for each LED, so PCB wiring becomes easier. Since the forward voltage of the driver output already takes into account several LEDs, the series drive method is slightly better than the parallel drive method. The series drive requires a high-voltage boost converter (such as 20V) to extract enough voltage from the lithium-ion battery to drive several series LEDs.

The most common way to drive LEDs is to use a low-side driver output. The LED output pin can be used as a constant current sink. In this case, the LED output and the power supply voltage require independent wiring. If a high-side driver output is used, the LED output pin becomes a current source. At the same time, only the LED pin needs to be wired, and the LED cathode is directly grounded. Usually, there is a ground plane at the PCB, so there is no need for independent wiring.

White LED and battery technology

Portable devices are usually powered by a lithium-ion battery, with a voltage between 2.8 and 4.3V depending on the charge required. The forward voltage of a white LED is usually 3.5V, which is usually not driven by a single lithium-ion battery, so a boost DC/DC converter is required. The converter can be capacitive (charge pump) or inductive (magnetic boost). Due to the small size of the charge pump, it is generally used in parallel LED drivers. As for the magnetic boost converter, it is generally used in high-voltage series drivers because the output voltage that can be achieved by charge pump technology is not high enough. The regulation of the converter output voltage can be performed automatically (adaptively) by sensing the LED forward voltage, or the user can set a constant voltage based on the specification of the LED forward voltage.

In the future, new lithium-ion batteries and LED technologies will bring new challenges to LED drivers. With the latest chemical achievements, the battery voltage range will expand to 2.3-4.7V, and the typical white LED forward voltage will drop to 2.9V. At the same time, the saturation voltage of the output driver will drop. When parallel driving is used, a buck-boost converter is required to efficiently drive a 2.9V LED. Figure 2 shows the effects of advances in battery, driver, and LED technology.

RGB LED backlighting

Generally speaking, the backlighting of small LCD displays is achieved by a set of white LEDs. However, the problem with using white LEDs is that their spectrum is not very ideal for light replication. The reason is that white LEDs are actually blue LEDs with a layer of yellow phosphor added to the surface. This causes the spectrum to have two peaks, one in blue and the other in yellow.

LCD screens are divided into three primary color grids: red, green and blue, and colors are defined by the mixture of these three primary colors. To filter the appropriate color to each color grid, a color filter is needed. Color filters waste most of the optical energy, even after filtering, so the color spectrum after passing through the LCD is not ideal. In this way, using white light LED backlighting can produce up to 75% of the NTSC (National Television Standards Committee) colors on the LCD screen (especially the red edge on traditional LCD screens). However, when using RGBLEDs for LCD screen backlighting, color reproduction can cover 100% of NTSC colors, making the colors brighter and the picture quality higher. If combined with optimized color filters, the energy wasted can be less than that of white light LED backlighting. Figure 4 shows the structure of an LCD screen.

When using RGB backlight, the driver must correct the brightness balance between the three primary colors of red, green and blue to prevent white point shift when the LED temperature changes. In addition, it is necessary to ensure that the driver maintains the correct intensity of light at any operating temperature. In terms of compensation, closed loop or open loop can be used. If closed loop compensation is used, a photosensitive sensor is required to measure the white point and its intensity. On the contrary, if open loop compensation is used, the temperature must be measured in advance and the brightness balance is adjusted through a predefined compensation curve.

An example of an RGB backlight driver is the LP5520 from National Semiconductor, which is an open-loop compensated LED driver. Figure 5 shows the principle of open-loop color compensation. The temperature compensation curves are measured using RGB LEDs in real applications, and these curves are programmed into the EEPROM memory inside the chip. The chip is integrated into an LCD display module, and the manufacturer of the module programs the compensation curves during production. In addition, the RGBLED backlight can also be used as an optimized color filter.

Keyboard backlighting and other decorative lighting

Compared with display backlighting, keyboard backlighting has some special requirements. The color required for keyboard backlighting does not necessarily need to be white, it can be any other color. Nowadays, the design trend of keyboard lighting and other decorative lights in portable devices is to produce more lighting effects. The backlight control of the display is usually a fade-in/fade-out on/off method, but the control of decorative lights is more complicated. By using RGBLED as the keyboard backlight, the color and the entire appearance of the device can be changed by simply changing the brightness balance between the red, green and blue LEDs. In this way, designers can add unique personality to portable phones or other portable devices through software control.

For some complex lighting sequences, such as gradients between different colors, a more sophisticated control method is required in addition to a simple activation control pin. In this regard, the I2C control bus is widely used in various portable devices because it provides great flexibility for controlling LED drivers through only two wires. Furthermore, LED control does not use all the bandwidth of I2C, because real-time control of the LED brightness will generate a certain amount of I2C traffic.

New LED drivers, such as the LP5521 from National Semiconductor, can provide minimal real-time control for sequenced lighting by adding internal memory and execution cores. The lighting sequence is written to an internal memory after power is applied, and then an external trigger pin or I2C write is used to start the lighting sequence. When the lighting sequence is running, no processor control is required. For example, when the phone is in standby mode, the application processor can enter sleep mode, but the LEDs can still perform complex lighting sequences. These sequences can include delays, up and down steps, blinking, toggling, and sending/receiving trigger signals.

To further reduce power consumption, the latest LED drivers have an automatic energy-saving operating mode. The DC/DC converter is only activated when the voltage of the lithium-ion battery is insufficient to supply the LED. In addition, when the lighting sequence is running internally, the driver can also shut down all unnecessary functions when the LED is not active, which can significantly reduce the average current consumption.

With tiny LED drivers, you can create a zoned lighting solution, which means placing the LED driver near the LED. This makes PCB routing easier and reduces electromagnetic interference issues. Drivers for zoned solutions have external control pins to synchronize multiple drivers to create interesting lighting effects.

in conclusion

There are several driver topologies for driving backlight LEDs, and the choice depends on the application. Undoubtedly, the upcoming new lithium-ion battery technology and lower forward voltage white LEDs will bring new challenges to driver design.

RGB LEDs used for LCD display backlighting are generally used in high-end portable phones or other devices that require high-quality image and color reproduction. In addition, with appropriate drivers, LEDs and color filters, RGB backlighting is more energy-efficient than white LEDs.

Programmability is a key feature of today's most advanced keypad backlight drivers, which, in addition to being easier to control, can also save system energy. In addition, programmable LED drivers can also create more interesting lighting effects for personalized cellular phones.