Analyzing the design of LED lighting driver circuits for mobile phones and PDA applications

This article mainly discusses LEDs and related driver circuits that are 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 light LEDs and provide graphic explanations of the structure and latest functions of driver circuits for LCD backlights, decorative lighting , indicators, and flashlights on camera phones.

How LEDs Improve Lighting and Performance in Mobile Phones and PDAs

Due to its high lighting efficiency, long-lasting effect and small size, LED has become the inevitable choice for portable devices such as mobile phones and PDAs. Low-power white LEDs of about 0.1W are currently widely used in LCD display panel backlights and keyboard lighting. Of course, they can also be used as temporary lighting or flashlights by connecting multiple LEDs to bring higher brightness. High -power LEDs up to 1W are used in camera phones equipped with two million pixels or even higher resolutions to support the camera function in dark environments. In addition to white LEDs, RGB (red, green, blue) LEDs are also often used to enhance the texture of mobile phones. By accurately and appropriately mixing the three colors, RGB LEDs can create rich and diverse colors.

In the indication application, when there is an incoming call or message, the color LED can be made to flash, or the color can be used to indicate the identity of the caller, such as a self-defined group, such as a call from a friend, family or business contact. This function not only brings personalization to the mobile phone, but is also very useful in a very noisy environment. To further enhance the user's audio-visual experience, RGB LED is also used to produce many attractive lighting effects. One example is to synchronize the RGB lighting action with the ringing melody or MP3 music. Another interesting application of RGB lighting is the "Fe EL Talk" function of Panasonic Corporation of Japan. Since the RGB LED is arranged under the casing of the mobile phone, it can display different colors according to the user's mood.

Improvements in LED efficacy and electrical characteristics

After a large amount of money was invested in LED development, the lighting efficiency of white LEDs has been greatly improved compared to when they were first invented. The best white LED lighting efficiency on the market can reach 100lm/W, which is quite close to that of fluorescent tubes. Some leading companies have also tried to use different coating materials on blue LEDs and launched designs with better luminous efficiency. Therefore, the number of LEDs required to provide panel backlight will continue to decline. Currently, the backlight LEDs required for standard LCD panels on mobile phones are about 2 to 4, while the backlight of LCD panels on PDAs or smart phones requires 6 to 10. Before further discussing the driving circuit structure and new functions of LED backlights and flashlights, let's first 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 (Vf) of LEDs is approximately between 2.7 and 4V. High-power LEDs usually have a higher forward voltage of up to 4.9V, so the LED driver circuit must provide sufficient positive voltage to allow the LED to emit light in a forward biased manner. When multiple LEDs are used to provide backlight, the difference between the forward voltages should be considered in the design of the driver circuit. In order to obtain the same lighting intensity, that is, to make different LEDs emit the same color, the design engineer must ensure that the forward current flowing through each LED is the same. Low-power LEDs usually use a forward current of 20mA, with a maximum of about 25mA. The high-power LEDs currently on the market can be driven with a pulse current of up to 1.5A.

Battery electrical characteristics

Currently, the most common batteries in mobile phones and PDAs are lithium-ion or lithium polymer rechargeable batteries. The rated voltage range of rechargeable batteries using lithium materials is 3.6V~3.7V, and the operating voltage is 4.2V~3.2V. To ensure safe operation, this type of lithium battery can only be charged or discharged within the range of 1C, where C is determined by the rated capacity of the battery. For example, the maximum discharge current of a 1,000mAh battery is 1A, and the battery capacity commonly used in mobile phones is approximately between 650~1,000mAh. In order to improve the performance of the battery, new lithium-ion batteries using different cathode materials have begun to be developed. When using this type of battery pack, design engineers should comply with the electrical specification limitations and adjust the drive circuit accordingly.

When using LEDs with a maximum forward voltage of 3.4V to 4V, the input voltage provided 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 drive the LEDs connected in series or parallel.

Charge pump converters are currently widely used in LCD backlight drivers. Compared with inductive boost converter solutions, charge pump drive circuits have become a better choice due to their lower cost, thinner thickness and lower noise characteristics. New integrated circuit designs have gradually improved the efficiency of charge pump drive circuits. The current maximum efficiency can exceed 93%, while the average efficiency is about 80%. Charge pump drive circuits usually operate in 1x and 2x modes, and some devices have added 1.33x and 1.5x modes to improve efficiency. In this type of solution, LEDs are connected in parallel, and the current of each LED is provided by its own independent matching current source. The best driver chip has a matching error of about 0.2% between any two LED currents in the same circuit.

In portable devices, the LED current used is the highest when the keyboard or touch screen is pressed, 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 enable end of the chip. By starting and shutting down the chip, its output current is the average value of the PWM signal duty cycle. For new LED driver chips, since a single (S-Wire) or two-wire I2C interface is used, only one or two I/O ports are needed, so the design is very simple.

Gradual brightness change and mood lighting

Progressive brightness change is mainly used to create a theatrical lighting effect when portable devices are started or shut down. When starting, the backlight current will be gradually amplified to 20mA in a step-by-step manner at a preset time interval. Similarly, when shutting down, the opposite action is used to gradually decrease. With the help of a microprocessor, PWM signals with different frequencies can be sent to the enable end of the LED driver circuit to achieve this effect. The LED current is increased or decreased in multiple steps at specific time intervals. However, the disadvantage of this method is that it consumes real-time processor resources. LED driver chip products such as NCP5602 and NCP5612 have this function, refer to Figure 1.

Figure 1(a): LED driver circuit using I2C control interface

Figure 2(b): LED driver circuit using single-wire S-Wire control interface

These driver chips require two clamping capacitors, located at the output and input ends respectively, and a resistor (R1) to control the maximum output current. The gradual brightness change control command is sent to the driver chip by the processor through the I2C or I/O port. The command itself should contain the starting and final current values ​​and the time interval for the brightness change.

If applied to RGB LED, this function can be used to produce situational lighting effects. Each RGB LED has 32 levels of brightness. LED driver chips such as NCP5623 can achieve an amazing 32,768 color changes. Due to such fine brightness differences and embedded logarithmic algorithms, color changes are linear and quite smooth. The RGB LED driver circuit includes an independently controlled PWM current source for adjusting the output current of the three LEDs to produce the desired color output.

Figure 2: Typical RGB LED driver chip application with I2C control interface.

Since the timing and current size of each current output can be independently controlled, we can use white light or colored LEDs and use different light-emitting modes to achieve colorful decorations or indications. Some circuits with audio input can also synchronize colored LEDs with different frequency bands of internally embedded MP3 or chord ringtones.

ICON Mode

Have you ever looked at the time on your mobile phone in the dark? The strong contrast between the bright backlight and the dark environment is quite uncomfortable for the eyes. If you feel bored and look at the time during a movie, it may disturb the audience next door. This is why the "ICON" mode is used, that is, the time or user-defined pictures are displayed on the external LCD panel with a small current in standby mode. However, if this must be achieved through PWM brightness control, the processor must generate a continuous low-frequency PWM signal throughout the standby mode. In NCP5602, this function is implemented in hardware and is activated through the digital command in Table 1.

Table 1: I2C internal register bit arrangement of NCP5602.

B5 in the data byte sent by the processor to the driver chip represents the state of the ICON mode. When B5 is LOW, it means that the normal backlight mode is used. The current of each LED can be adjusted between 0mA and 30mA. When B5 is HIGH, the ICON mode will be started and only 450μA of current will be sent to one of the two connected LEDs. In this device, the current value of the ICON mode is a fixed value, but in products like NCP5612, this current can be controlled through a single-wire communication protocol. Figure 3 shows the ICON control program through the SCL and SDA connection lines in the I2C communication protocol.

Figure 3: Data sequence on a simple SCL and SDA connection line for ICON mode control.

Linear Regulator/Current Source Solutions

When using cluster LEDs with a lower forward voltage of about 3.3V, a linear regulator can be selected to provide the drive current. Compared with switching converters, linear regulators have the advantages of lower cost and lower electromagnetic interference, because linear regulators only need to add a few resistors to the periphery of the driver chip and do 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 a two-LED driver circuit. Considering the standard 0.2V dropout and 3.3V LED forward voltage, the regulator will leave the regulation mode and enter the saturation mode when the battery voltage is lower than 3.5V, which will cause the regulator output current to drop significantly and the LED brightness to dim. However, if the minimum battery voltage is within an acceptable range, the linear regulator is still the most cost-effective backlight solution for small LCD panels.

Figure 4: The linear regulator NUD4301 is used as a two-LED driver circuit to drive the backlight of a small LCD panel.

Temporary lighting applications on mobile phones

The LED lighting function provided by mobile phones is generally considered to be a very sophisticated design, consisting of many ...

High power flash LED

Different currents and driving times are usually used in lighting and flash modes. For example, a continuous current of 200mA can be used in lighting mode, while a pulse current of 400mA to 1A is used in flash mode. The duration of the flash pulse depends on the characteristics of the camera module. Usually, the width of the flash pulse is between 20ms and 200ms. The flash driving circuit can support a driving current of about 1A for the flash LED, providing an output of up to 4.9W for the LED. In order to maintain the junction temperature of the LED within the highest tolerable range, a good temperature management strategy must be used. Reducing the pulse width helps reduce unnecessary consumption, and a larger ground area is also a recommended method for exporting heat from the LED.

Single high power flash lamp driver circuit

A boost converter is necessary to support the forward voltage of up to 4.9V in high-power LEDs, but even for the same LED chip , the forward voltage can vary under different conditions. When the LED heats up, the forward voltage may drop below the input battery voltage, so a buck converter is needed. Technically, a buck-boost converter is the most suitable solution to drive a single high-power LED, but such driver chips are usually expensive and require an external inductor that increases the overall cost and size. The advantage of a buck-boost converter is its high overall efficiency, mainly because it fully uses the battery's energy, while providing ultra-high output currents of more than 1A or even higher. The newly introduced high-current charge pump driver circuit is a low-cost alternative to the buck-boost converter. However, the output current of the charge pump converter is limited to about 700mA at most, mainly due to its low efficiency and the current drawn from the battery.

Integrated lighting management chip

Integrated lighting management chips (LMICs) with backlight and flash functions, and some even with RGB and other audio and video functions, are now available. They include boost converters that can be designed with charge pumps or inductors. Each output is provided by an adjustable current source. Such a solution is particularly useful in flip-top or slide-top phones 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 with 8 outputs that can provide up to 500mA of total current. Its output current can be adjusted by the processor through the I2C port, and different LED configurations can also be formed to meet the needs of various platforms. Currently, digital control through a single line has been widely used on independent backlight LED driver chips, but such control protocols are too slow and too complex for LMICs because various different control combinations are required in the integrated driver circuit. Instead, I2C or other dedicated control protocols with clock and data lines are usually used on LMICs.

Figure 5: Integrated LED driver ICs offer a variety of LED combinations, from four LEDs at 25mA for backlighting and four 100mA LEDs for flash, all the way to flash applications that combine all outputs to drive a high-power LED.

Figure 6: 4.5W flash driver circuit with internal switch and time limit protection.

Real flash functionality in camera phones

Mobile phones with 3 million or even higher pixels have appeared on the market to support high-quality photography. In order to enable LED to provide lighting intensity comparable to that of xenon flash, two or more high-power LEDs can be driven as flashes; an inductive boost converter with a 4.5W high-power drive capability can drive two series-connected LEDs at a drive current of 500mA. Note that a time limit protection circuit must be added to this type of drive circuit to prevent the LED from being damaged by long-term operation, and a switch should also be added to the driver chip to change the current size during lighting and flash applications.

Conclusion

The mass supply of LEDs has made the unit price of low-power LEDs used in LCD panel backlights on mobile phones and PDAs lower and lower. The newly launched backlight driver chips also have built-in step-by-step brightness control and situational lighting control functions that do not require any software design and do not consume any microprocessor resources. These LED driver circuits can help portable product manufacturers shorten development time. For lower-cost solutions, linear regulators can be used to drive LEDs with lower forward voltages. On the other hand, several flash drive solutions have also appeared on the market. They are independent buck-boost converters, high-current charge pump driver 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 is that the unit price is high. Two LEDs are used in some high-end mobile phones to provide higher-brightness flashes to enhance the camera effect of camera phones. As camera phones gradually replace digital cameras, higher-power flash solutions will become more and more popular, providing users with a real photo experience. (Text/Lin Xinxin, Marketing Manager of Low Voltage Power Management Products of ON Semiconductor)