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Posted on 01 July 2019

Super Capacitor Reference Design

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LED flash power issues for high resolution camera phones solved

Cell phones are becoming the ultimate converged consumer portable appliance. Capabilities include capturing high-quality photography, Wi-Fi web access, crisp audio and extended talk time along with longer battery life. A major design challenge is now emerging. A phone battery still struggles to provide enough peak power to drive some highly-complex mobile applications, driving the demand for circuits that can store high currents for short periods without overloading the battery to provide the power required for high performance operation.

By Thomas Delurio, Applications Manager, Advanced Analogic Technologies, Inc

 

For makers of advanced camera phones, delivering the high peak current consumed by high-intensity camera flash is of utmost importance. As resolution of camera phones grows to three megapixels and beyond, the amount of light required to achieve a high quality image has sharply increased. In order to match the photo quality of digital still cameras, cell phones must either drive flash LEDs at currents as high as 2A or Xenon flash tubes charged to over 330V. Other applications in the phone such as the RF power amplifier, GPS mapping, internet access, music and video can also exceed source current availability.

Design Challenges

Camera phones require a high intensity flash in medium to low light conditions to produce good pictures. Designers can choose either LED or xenon flash units, but there are challenges with each. High-current Flash LEDs need up to 400% more power than a battery can provide to achieve the light intensity needed for highresolution images. To overcome this power limitation some camera phones have used long flash exposure times to compensate for the lack of light, resulting in blurry photos. Xenon flash tubes deliver good light power, but have a short flash exposure and can’t be used for a video capture/movie-mode functions. The required electrolytic storage capacitors are very bulky for slim-line designs, operate at high voltages, take a long time to recharge between flashes and cannot be used for other peak-power needs in the phone.

One way to solve the problem with Flash LEDs driven at 1 to 2A each is to use a capacitor to store the current and deliver it quickly without draining the main battery. However, the use of conventional capacitors would require either a very large case size or multiple devices connected in parallel. A more practical solution for space-constrained portable systems is to use very high value “super” capacitors. These devices offer high levels of capacitance in a relatively small, flat case size. By using a super capacitor, designers can deliver the high current levels needed for these short duration events and then recharge from the battery between events. To support the battery, designers can add a thin super capacitor to handle the phone’s peak-power needs – flash photos, audio and video, wireless transmissions and GPS readings – without compromising slim handset design. It also allows the designer to reduce the system footprint by optimally sizing the battery and power circuitry to cover just the average power consumption instead of peak levels.

Defining a super capacitor

What is a super capacitor (SC)? Like any capacitor, a super capacitor is basically two parallel conducting plates separated by an insulating material known as a dielectric. The value of the capacitor is directly proportional to the area of the plates and inversely proportional to the thickness of the dielectric. Manufacturers building “super” capacitors achieve higher levels of capacitance while minimizing size by using a porous carbon material for the plates to maximize the surface area and deploying a molecularly thin electrolyte as the dielectric to minimize the distance between the plates. Using this approach they can manufacture capacitors with values from 16mF up to 2.3F. The construction of these devices results in a very low internal resistance or ESR (Equivalent Series Resistance) allowing them to deliver high peak current pulses with minimal droop in the output voltage. These super capacitors reduce system footprint requirements by delivering a very high capacitance in a relatively small case size. They can be manufactured in any size and shape and recharge in seconds. By averaging high power demands, they extend battery life by up to a factor of five and allow designers to specify much smaller, lighter and less expensive batteries.

Inherent challenges

That low ESR presents designers with an inherent problem during the charge cycle, however. In any system the capacitor is initially discharged. When the supply voltage is then applied, the super capacitor looks like a low value resistor. This can result in a huge in-rush current if the current is not controlled or limited. Therefore, designers must implement some sort of in-rush current limit to ensure the battery does not shut down. Any circuit of this type also typically requires short-circuit, overvoltage and current flow protection. The challenge for designers is how to efficiently interconnect the battery, DC/DC converter and super capacitor in a way that will limit the super capacitor inrush charge current and continually recharge the cap between load events. LED flash drivers that can manage super capacitor charging requirements have appeared on the market to make the designers’ jobs easier, integrating the circuitry to save space, cost and time to market. Flash LEDs for digital still cameras require 1 to 2A for up to 120mS. A super capacitor can be used to store the required current and deliver it quickly without draining the main battery. Working together with the battery, the super capacitor discharges its power during peak loads and recharges between peaks, providing the power needed to operate systems from battery operated hosts up to 200% longer while extending the life of the battery. Clearly, any time designers use a super capacitor, they must limit in-rush current. Moreover, the super capacitor needs to be recharged when the voltage drop or droops below the operational limit of the LEDs. When the SC is fully charged, it has to be disconnected from the source. Short circuit protection, source over voltage protection and current flow protection are also required.

Benefits

Conventional capacitor technology would require either a very large case size or multiple devices connected in parallel to achieve high capacitance values. Super capacitors recharge in seconds with >500k Cycles and store energy in an electrostatic field as opposed to a chemical state like a battery. Since voltage does not droop excessively until heavy load currents when fully charged, the use of a super capacitor also reduces ESR and impedance. Super capacitors can be manufactured in any size and shape, flat and small size. They can be used to extend battery life by five times by ‘averaging out' high power demands so they allow smaller, lighter and cheaper batteries. Super capacitors also have a long life (10 to 12 years). Unlike a battery, they have an open-circuit (high ESR) failure mode that is not destructive. Similarly, if over-voltage is applied to the device, the only consequence will be a slight swelling and a rise in ESR, eventually progressing to an open circuit. There will be no fire, smoke or explosion.

Design Solution

Super capacitor-powered LED flash units can drive high-current LEDs to provide light intensity that is many times greater than standard battery-powered LED flash units or longer than xenon strobes. In the block diagram shown in Figure 1, the AAT1282 contains a step-up converter used to boost the 3.2V-to-4.2V battery input voltage up to a constant 5.5V. If the battery voltage is 3.5V and the boost converter is 90% efficient, the battery would need to supply over 3A for the duration of a 2A flash pulse. This will either cause the battery protection circuit to shut the battery down or cause a low voltage shutdown while plenty of energy still remains in the battery. This solution also offers flash management capabilities, such as movie-mode, and super-capacitor charging capabilities. The solution controls and regulates the current from a cell phone’s battery source, steps up the battery voltage, and manages the charging of a super cap, for the control and supply of high-current to flash LEDs In the end application.

Using a super capacitor, it is possible to drive very high LED currents for an ultra bright LED flash

To better achieve this, the step-up converter features built-in circuitry to prevent excessive inrush current during start-up as well as a fixed input current limiter of 800mA and true load disconnect after the super capacitor is charged. The AAT1282 boost converter’s output voltage is limited by internal overvoltage protection circuitry, which prevents damage to the AAT1282 converter and the super capacitor from an open LED (open circuit conditions). During an open circuit, the output voltage rises and reaches 5.5V (typical), and the OVP circuit disables the switching, preventing the output voltage from rising higher. Once the open circuit condition is removed, switching will resume. The controller will return to normal operation and maintain an average output voltage. An industry-standard I2C serial digital input enables, disables LEDs and sets the movie-mode current with up to 16 movie-mode settings for lower light output.

In figure 2, a detailed schematic illustrates that few components are required. An 0.55F 85mOhm super capacitor delivers 9W LED power-bursts using the AAT1282 LED Flash driver which has the SC charger integrated with the Boost DC/DC LED Driver. To achieve high light levels, the flash LEDs are driven at currents of between 1A and 2A. The forward voltage (VF) across the LED at these high currents can range up to 4.8V. If we include 200mV of overhead for the current control circuitry, it’s easy to see how the total load voltage during a flash event can range up to 5V, demonstrating the need for the 5.5V step-up voltage.

Detailed Circuit Schematic

Figure 3 shows test results using two flash LEDs at 1A each and one LED at 2A. As can be seen, the super capacitor can easily supply the necessary current for 120ms while holding up the supply voltage sufficiently above the VF of the LEDs. Between flash events, the super capacitor is recharged at a slower rate to be ready for the next picture. The time to charge the super capacitor between flashes is set externally and can be optimized for different battery sizes/chemistries. Figure 4 illustrates the digital control of the Flash function and movie-mode option.

Performance results, two LEDS at 1A each or one LED at 2A

Flash and movie-mode control

Conclusions

Super capacitors have rarely been used in portable systems. Their use has been typically limited to back-up or standby functions that use relatively low currents and offer fairly long charge times. By combining newly available boost converters with super capacitors, designers can now create compact solutions that supply high levels of current for short durations and, in the process, extend battery life or allow the use of smaller, lighter and less expensive power sources.

By using a super capacitor and a Flash LED controller in a complete reference design, it is possible to drive very high LED currents for an ultra bright LED flash. For example, 2x Lumiled’s PWF4 Flash LEDs can be driven at 1A each to deliver more light than a K800i xenon strobe. The super capacitor is less than 2mm thick and can provide other benefits such as extending talk time and improving audio quality.

 

References:

1) Comparison of xenon flash and high current LEDs for photo flash in camera phones.
2) Use of Supercapacitors to Improve Performance of GPRS Mobile Stations.

Pierre Mars
CAP-XX Ltd.
9/12 Mars Road
Lane Cove NSW 2066 Australia
http://www.cap-xx.com

 

 

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