Reference Design for High-resolution camera cell phone LED flashlight super capacitor

Reference Design for High-resolution camera cell phone LED flashlight super capacitor

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Source: Today's electronics/21IC Author: Advanced Analogic Technologies Company Thomas Delurio

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Mobile phones are becoming the ultimate consumer electronics platform. Its performance includes capturing high-quality images, Wi-Fi network access, crisp audio, longer call times, and longer battery life. However, a major design challenge is also emerging. In order to adapt to highly complex mobile applications, mobile phone battery still requires a lot of effort to provide enough peak power, which promotes the need to provide the required power circuit for high-performance operations, this type of circuit can store large currents in a short period of time without overload battery.

The most important challenge for High-brightness camera mobile phone manufacturers is to provide the peak current required for high-brightness camera flash. As the resolution of camera phones increases to 3 million pixels or more, the luminous flux required to produce high-quality images has also increased dramatically. To match the photo quality of a digital camera, the LED flash must be driven with up to 2A current, or the xenon flash tube must be charged to more than 330V. Other applications of mobile phones, including RF amplifiers, GPS navigation, Internet access, music, and video, may also exceed the supply capability of battery current.

Design challenges
A high-brightness flashlight is required for a photo mobile phone to produce a picture with sufficient exposure in a moderate to low-light environment. Designers can choose LED or xenon flash tubes, but both of them have corresponding challenges.

A high-current LED flashlight requires 4 times the power provided by the battery to generate the brightness required for a high-resolution image. To overcome the power limit, some camera phones have used a longer flash exposure time to compensate for the lack of luminous flux, which leads to blurred pictures.

Xenon flash tubes provide good illumination, but their flash exposure time is short, so they cannot be used in video capture/movie mode. The electrolytic storage capacity required for the SLIM design is too large, and the operating voltage is very high. It takes a long time between the two flashes to be fully charged, it cannot be used for other applications that require peak power.

Solution: provide 1 ~ for each LED flashlight ~ One way to solve the 2A drive current problem is to use a capacitor to store the current and quickly supply power without distributing the main battery. However, to use traditional capacitors to store large currents, you do not need a large capacity capacitor, or multiple parallel medium capacity capacitors. For portable systems with limited space, a more practical solution is to use super capacitors with very high capacities. By using a super capacitor, the designer can provide the large current needed for these short continuous interval events and recharge them between these events through the battery. To support battery, designers can add a very thin super capacitor, this allows you to cope with peak power requirements for your phone without sacrificing the slim cell phone design (for example, flashing for photos, audio and video, wireless transmission, and GPS reading ). It also allows designers to best adjust the size of battery and power circuits by satisfying only average power consumption rather than peak power consumption, reducing the footprint of the system.

Design a super capacitor
What is super capacitor (SC? Like any capacitor, a super capacitor is basically made up of two parallel conductive boards, separated by an insulating material called dielectric. The capacity of the capacitor is proportional to the area of the conductive board and inversely proportional to the thickness of the dielectric. Manufacturers that develop super capacitors make conductive panels with porous carbon materials to maximize the surface area and use electrolyte as thin as molecules as the dielectric to minimize the distance between the two conductive panels, A higher capacitance value is achieved when the size is minimized.

This method can be used to produce a capacity of 16mF ~ 2.3F capacitance. These capacitors can be equivalent to very low internal resistance or ESR (equivalent series resistance), which enables them to provide peak current pulses while minimizing the output voltage. By providing very high capacitance values with relatively small shape sizes, these super capacitors reduce the system's demand for PCB Area. They can be made in any size and shape and recharged in seconds, extending battery life by 5 times and allowing designers to use smaller, lighter, and cheaper batteries.

Inherent challenges
However, this low esr poses a problem for designers during the charging process. In any system, the capacitor is left empty before it is initially used. Then, when the power supply voltage is applied, the super capacitor looks like a low-value resistor. If the current is not controlled or limited, it will lead to a large peak current. Therefore, the designer must implement some peak current limit to ensure that the battery will not be exhausted immediately. Short circuit, overvoltage, and overcurrent protection are usually required for any type of circuit.

The challenge for designers is how to efficiently connect battery, DC/DC converter and super capacitor to limit the peak charging current of super capacitor, the super capacitor can be recharged continuously between loads. LED flash drives that meet super capacitor charging requirements are now available on the market, making it easier for designers to work and saving PCB space, costs, and time to market. The LED flash of a digital camera needs 1 ~ 2A current can continue flashing for 120 ms.

The super capacitor can be used to store the necessary current and quickly supply power without distributing the main battery. When working with the battery, the super capacitor releases its storage current during the peak load and recharges between two peak loads. Compared with battery-powered devices, a power supply system with super capacitor can provide up to 2 times of power and prolong the battery life. Obviously, designers must limit peak currents no matter when they use super capacitors. In addition, when the voltage falls below the operating voltage of the LED flashlight, the super capacitor needs to be recharged. When the super capacitor is fully charged, it must be disconnected from the power-on. In addition, it also requires short-circuit protection, source overvoltage protection, and overcurrent protection.

Benefits of super capacitor
The super capacitor can be recharged within several seconds in a period greater than kb, and the power is stored in the electrostatic field. Because only large load current can reduce the voltage too low when fully powered, the use of super capacitor also reduces the ESR and impedance.

Super Capacitors can be made into any size and shape, whether flat or small size. Super capacitor also has a long life (10 ~ 12 years ). Unlike batteries, they have non-destructive open-circuit (high ESR) failure modes. If an over-high voltage is applied to the device, the only consequence is a slight increase in the ESR and eventually enters the open circuit state. The whole process will not cause fire, smoke, or explosion.

Design solutions
The super capacitor-powered LED flashlight unit can drive high-current LEDs, which provide many times the flash brightness of the standard battery powered LED flashlight unit, or lasts longer than the xenon lamp. As shown in 1, AAT1282 contains a boost converter, which is used to convert 3.2 ~ 4.2V the input voltage of the battery increases to 5.5 V. If the battery voltage is 3.5 V and the boost converter efficiency is 90%, the battery must provide a current above 3A to maintain a 2A flash pulse. This will not cause the battery protection circuit to shut down the battery, or cause a low-voltage shutdown, and the battery still has a lot of energy.

The solution also provides flash management functions, such as movie mode and super capacitor charging. This solution controls and adjusts the current from the cell phone battery source, increases the battery voltage, and manages super capacitor charging to control and provide the large current required by the LED flash in the terminal application.

Figure 1 power the LED with a super capacitor

Figure 2 Detailed circuit principle of AAT1282

To better achieve this goal, the AAT1282 has a built-in circuit to prevent high peak current impact during startup, a fixed 800mA input current limiter, and a load disconnection circuit after the super capacitor is fully powered. The output voltage is limited by the Internal Overvoltage protection circuit, which can prevent the converter and super capacitor from the damage of an open-circuit LED (under open-circuit conditions.

When the output voltage rises and reaches 5.5 V (typical value) during the open circuit, the overvoltage protection circuit closes the switch circuit to prevent the output voltage from rising further. Once the open circuit is removed, the switch circuit immediately resumes operation. The Controller returns to normal operation and maintains an average output voltage. An industrial standard I2C serial digital input interface enables/locks LEDs, and sets the Cine mode current with up to 16 Cine modes for lower light output requirements.

A detailed schematic diagram (see figure 2) shows that only a few components are needed around the super capacitor. A super capacitor of 0.55F and 85mΩ can be used together with the AAT1282 LED flash driver to provide of LED Burst Power. In order to achieve high brightness, the drive current of the LED flashlight is between 1 ~ Between 2A. The forward voltage (VF) on the LED can rise to 4.8 V. If we include 200mV overhead of the current control circuit, you can easily see how the total load voltage rises to 5 V during the flash event, which also proves that the 5.5 V output voltage is necessary.

Figure 3 LED flash test results under different driving conditions

Figure 4 digital control of the flash function and movie mode options

Figure 3 shows the test results of two 1A-current-driven LED flash and one 2A-current-driven LED flash. As we have seen, super capacitors can easily provide the current needed for a flash lasting ms while keeping the power supply voltage sufficiently above the led vf voltage. Between two flashes, the super capacitor is recharged at a slow rate, and the charging time is set externally. Different battery sizes and chemical principles can be optimized. Figure 4 shows the digital control of the flash function and movie mode options.

Conclusion
Super Capacitors are rarely used in portable systems. Their applications are generally confined to backup or standby, which use relatively low current and require a relatively long charging time. By integrating the latest boost converter and super capacitor, designers can now create a compact solution.