This article mainly discusses LEDs and related driver circuits widely used in portable devices such as mobile phones and PDAs. In addition to focusing on the latest LED applications, we also examine the development of white LEDs and will focus on LCD backlights and decorative lighting. Graphical descriptions of the structure and up-to-date functions of the drive circuits such as flashing lights on the instruction and camera phones.
How LEDs improve lighting effects and performance in mobile phones and PDAs
Due to its high illumination efficiency, long-term efficiency and small size, LED has become an inevitable choice for portable devices such as mobile phones and PDAs. A low-power white LED of about 0.1W is currently widely used in the backlight and keyboard of LCD display panels. Lighting, of course, can also be connected to multiple LEDs to bring higher brightness for temporary lighting or flash applications, while high-power LEDs up to 1W are used in camera phones with 2 megapixels or even higher resolution. Medium to support camera functions in dark environments. In addition to white LEDs, RGB (red, green, blue) LEDs are often used to enhance the texture of mobile phones. RGB LEDs create a rich variety of colors by precisely mixing them in three colors.
In the indicator application, when there is an incoming call or message, the color LED can be flashed, or the color can be used to display the identity of the caller, such as a self-defined group, such as a friend, family member or a business call, this function is not only for mobile Personalization of the phone is also very useful in very noisy environments. In order to further enhance the user's audio and video experience, RGB LEDs are also used to generate a lot of attractive lighting effects. One example is to synchronize the RGB lighting action with the ringing melody or MP3 music, and another interesting application of RGB lighting. It is the "Feel Talk" function of Panasonic Corporation of Japan. Since the RGB LEDs are arranged under the casing of the mobile phone, different colors can be displayed according to the user's mood.
LED performance improvement and electrical characteristics
After the development of a large number of funds betting LEDs, the lighting efficiency of white LEDs has been greatly improved compared to the original invention. The best white LED lighting efficiency on the market can reach 100lm/W, which is quite close to fluorescent tubes, and some leading companies. Attempts have also been made to use different coating materials on blue LEDs and to introduce a design with better luminous efficiency, so the number of LEDs required to provide panel backlighting will continue to decline. Currently, the backlight LED required for standard LCD panels on mobile phones is approximately 2~4, and the backlight of the LCD panel on PDA or smart phone needs 6~10. Before discussing the drive circuit structure and new functions of LED backlights and flash lamps, let's review the electrical characteristics of LEDs and batteries widely used in mobile phones and PDAs.
Depending on the technology used by different manufacturers, the forward voltage (V f ) of the LED is between 2.7 and 4V. Usually high-power LEDs have a high forward voltage of up to 4.9V, so the LED driver circuit must provide enough The positive voltage is such that the LED illuminates in a forward biased manner. When multiple LEDs are used to provide backlighting, the difference in forward voltage should be considered in the design of the driver circuit. In order to achieve the same illumination intensity, that is, different LEDs emit the same color, the design engineer must ensure that each pixel flows through each The forward current of the LED can be the same. The low-power LED usually uses a forward current of 20mA, and the maximum is about 25mA. The high-power LED on the market can be driven with a pulse current of up to 1.5A.
Battery electrical characteristics
At present, the most common battery in mobile phones and PDAs is lithium ion or lithium polymer rechargeable batteries. The rechargeable battery with lithium material has a rated voltage range of 3.6V~3.7V and the working voltage is 4.2V~3.2V. It can work safely. This type of lithium battery can only be charged or discharged in the range of 1C, where C is determined by the rated capacity of the battery. For example, the maximum discharge current of a battery of 1,000mAh is 1A, and the battery capacity normally used by mobile phones is about Between 650 and 1,000 mAh. In order to improve the performance of the battery, a new lithium ion battery using different cathode materials has been developed. When using this type of battery pack, the design engineer should follow the electrical specification limits and adjust the drive circuit accordingly.
When using an LED with a maximum forward voltage of 3.4V to 4V, the input voltage supplied by the battery must be equal to or higher than the required driving voltage, so a boost converter with a stable current function is required to push in series or LEDs connected in parallel.
Charge pump converters are now widely used in backlighting of LCDs. Compared to inductive boost converter solutions, charge pump driver circuits are characterized by lower cost, thinner thickness, and lower noise characteristics. A better choice, the new integrated circuit design has gradually improved the efficiency of the charge pump drive circuit, the current maximum efficiency can exceed 93%, and the average efficiency is about 80%. Charge pump drive circuits typically operate in 1x and 2x modes, and some devices incorporate 1.33x and 1.5x modes to improve efficiency. In this type of solution, the LEDs are connected in parallel, while the current of each LED is independent. The matching current source provides that the optimum driver chip has a matching error of approximately 0.2% between any two LED currents in the same circuit.
In portable devices, when the keyboard or touch screen is pressed, the LED current is the highest, and after a few seconds of no action, the LED current will be reduced to reduce power consumption. A common way to control the LED current is to use PWM pulses to drive the chip. The enable terminal, through the startup and shutdown of the chip, its output current is the average of the duty cycle of the PWM signal. For the new LED driver chip, since the single-chip (S-Wire) or two-wire I 2 C interface is used, only one or two I/O ports are needed, and the design is very simple.
Progressive brightness change and contextual illumination
Progressive brightness changes are mainly used to create a theater-like lighting effect when the portable device is turned on or off. At startup, the backlight current is stepped up to 20 mA in a step-by-step manner at the preset time interval. The action is gradually reduced. With the help of the microprocessor, the PWM signal with different frequencies can be sent to the enable end of the LED drive circuit to achieve such an effect, and the LED current is increased by multiple steps at specific time intervals. Or lower, but the disadvantage of this method is that it consumes real-time processor resources, which is available on LED driver chips such as NCP5602 and NCP5612.
These driver chips require two clamp capacitors at the output and input and a resistor (R1) to control the highest output current. The progressive brightness change control command is sent by the processor through the I 2 C or I/O port. To the driver chip, the instruction itself should contain the time between the start and end current values ​​and the change in brightness.
If applied to RGB LEDs, this feature can be used to create contextual lighting effects. Each RGB LED has 32 levels of brightness, and an LED driver chip like the NCP5623 can achieve an amazing 32,768 color variations. Due to such a fine brightness level difference and embedded logarithmic algorithm, the color change is linear and quite compliant, and the RGB LED driver circuit contains an independent control PWM current source for adjusting the output current of the three LEDs to produce the desired color. Output.
Since the timing and current of each current output can be independently controlled and adjusted, we can use white light or colored LEDs, and use different illumination modes to achieve colorful decoration or indication, and some circuits with audio input. It also enables color LEDs to be synchronized with different bands of internally embedded MP3 or polyphonic ring tones.
ICON mode
Have you ever seen time on the mobile phone in the dark, when the contrast between the bright backlight and the dark environment is quite uncomfortable for the eyes, if you are bored watching the movie during the movie, it may interfere with the next door. Audience, which is why the "ICON" mode is used to display time or user-defined pictures on the external LCD panel with a small current in standby mode. However, if this must be achieved by PWM brightness control, the processor must generate a continuous low frequency PWM signal in the entire standby mode. In the NCP5602, this function is implemented in hardware and is implemented by the digital commands in Table 1. start up.
B5 in the data byte sent from the processor to the driver chip represents the state of ICON mode. When B5 is LOW, it indicates that the normal backlight mode is used, and the current of each LED can be adjusted between 0mA and 30mA. When B5 is HIGH, then ICON mode will be started, and only 450μA current will be sent to one of the two connected LEDs. In this device, the current value of ICON mode is fixed, but similar On the NCP5612's products, this current can be controlled by a single-wire communication protocol. Figure 3 shows the ICON control program for the SCL and SDA lines in the I 2 C communication protocol.
Linear Regulator / Current Source Solutions
When using a clustered LED with a low forward voltage of about 3.3V, a linear regulator can be selected to provide the drive current. Linear regulators are compared to switching converters with the advantages of lower cost and lower electromagnetic interference due to linearity. The regulator only needs to add several resistors on the periphery of the driver chip, and does not need to use switching devices. However, the disadvantage of this type of solution is that the battery voltage operating range is reduced. Figure 4 shows the use of the NUD4301 low-dropout linear regulator as two. In the case of an LED driver circuit, considering the standard 0.2V voltage drop and 3.3V LED forward voltage, the regulator will leave the regulation mode and enter saturation mode when the battery voltage is lower than 3.5V, which will cause the regulator output. The current drops sharply and the LED brightness dims. But if the minimum battery voltage is within acceptable limits, linear regulators are also the most cost-effective backlight solution for small LCD panels.
Temporary lighting application on mobile phones
The LED lighting capabilities offered by mobile phones are generally considered to be quite subtle designs, and many flashlights are now composed of several low-power LEDs and can be seen by the relatively low 20mA to 60mA current drive trend. This kind of lighting can be used as a portable flashlight, but its weak illumination intensity is insufficient to support the camera action in the dark environment. In fact, one or more high-power LEDs must be supported to support the object lighting of 1 meter or more. The main reason that hinders engineers from adopting high-power LEDs is cost. However, after Taiwan and Korean manufacturers gradually increase the capacity of power LEDs, the unit price is expected to start to decline.
High power flash LED
Different currents and driving times are usually used in illumination and flash mode. For example, 200mA continuous current can be used in illumination mode, and 400mA to 1A pulse current is used in flash mode. The length of flash pulse depends on the camera module. Depending on the characteristics, usually the width of the flash pulse is between 20ms and 200ms. The flash drive circuit can support the drive current of the flash LED by about 1A, and provide the output of the LED up to 4.9W, in order to maintain the junction temperature of the LED at the highest capacity. Scope, must use a good temperature management strategy, reduce the pulse width to help reduce unnecessary consumption, and a larger grounding area is also a recommended method to derive heat from the LED.
Single high power flash drive circuit
The boost converter is a prerequisite for supporting up to 4.9V forward voltage in high power LEDs, but even with the same LED chip, the forward voltage will be different under different conditions. When the LED is warming up, the forward voltage may slip below the input battery voltage, so a buck converter is required. Technically, buck-boost converters are the most suitable solution for driving a single high-power LED, but such driver chips are usually costly and require external inductors that increase overall cost and size. The advantage of the buck-boost converter is the higher overall efficiency, mainly due to the full use of the battery's energy, while providing ultra-high output current of more than 1A or higher. The new high current charge pump drive circuit is a low cost alternative to buck-boost converters. However, the output current of the charge pump converter is up to about 700 mA, which is mainly limited by the lower efficiency and current drawn from the battery.
Integrated lighting management chip
With integrated backlighting and flashing capabilities, some integrated lighting management chips (LMICs) with RGB and other audio and video capabilities are now available, including boost converters with charge pump or inductive design. Each output is provided by an adjustable current source, and such a solution is particularly useful in flip-top or slide-type handsets because it eliminates the long path required to pull from the power management unit to the other side of the phone. The NCP5608 is an integrated charge pump driver chip that delivers up to 500mA total current with 8 outputs. The output current can be adjusted by the processor through the I 2 C port. It can also be configured with different LED configurations to meet various requirements. The needs of the platform. At present, single-wire digital control has been widely used in stand-alone backlight LED driver chips. However, such a control protocol is too slow and too complicated for the LMIC, because various control combinations are required in the integrated driver circuit. Instead, I 2 C with clock and data lines or other proprietary control protocols are typically used on the LMIC.
The real flash function in the camera phone
There are 3 million or higher pixel mobile phones on the market to support high-quality camera functions. In order to make LEDs provide illumination intensity comparable to Xenon flash, two or more high-power LEDs can be used as flash lamps. An inductive boost converter with 4.5W high power drive capability can drive two LEDs in series with a drive current of 500mA. Note that a time-limited protection circuit must be added to this type of driver circuit to avoid damage to the LED for long periods of time, and a switch should be added to the driver chip to change the current during illumination and flash applications.
Summary of this article
The mass supply of LEDs has made the price of low-power LEDs for LCD panel backlights on mobile phones and PDAs lower and lower. The newly-launched backlight driver chip also incorporates step-by-step brightness control and does not require any software design. Situational lighting control that consumes any microprocessor resources, and these LED driver circuits can help portable product manufacturers reduce development time. On a lower cost solution, a linear regulator can be used to drive the LED with a lower forward voltage. On the other hand, several flash drive solutions have emerged on the market. They are independent buck-boost converters, high-current charge pump drive circuits, and lighting management chips. Most power flashes may contain several standard LEDs or one high-power LED. The main reason why high-power LEDs are not popular in camera phones today is the higher unit price. Two LEDs are used in some high-end mobile phones to provide a higher-brightness flash to enhance the camera's camera. With the trend of camera phones gradually replacing digital cameras, higher power flash solutions will become more popular, providing users with a true camera experience.
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