Power Management Trends in 3G Mobile Phones

3G phones can browse the Web, send e-mails, take digital photos and even stream video, and handset makers are under increasing pressure to pack all of these features into ever-shrinking form factors while keeping the phones powered on longer.

As can be seen in Figure 1, the increasing number of functions has led to the need for more low-voltage output levels at different power levels in mobile phones. An example is the application processor for image processing, which requires up to 360mW of power during video capture. When running at full capacity, the peak power required by the load of the internal system of the mobile phone will typically exceed 4W. Such high power will quickly deplete the energy of the battery. Another important factor affecting battery run time is power efficiency and system power management.

Low power conversion efficiency will cause heat. This heat is generated by the power loss in the voltage regulator during the energy conversion process. However, there is no fan or heat sink for cooling in the mobile phone, only densely packed printed circuit boards. Therefore, there is no channel for heat to dissipate. This heat will shorten the battery life and reduce the reliability of the product.

The heat generated during voltage conversion has caused the industry to rethink what regulators should be used. Currently, manufacturers are replacing simple but inefficient linear low-dropout (LDO) regulators with switching regulators because switching regulators have higher efficiency.

We must carefully consider the pros and cons of the different voltage regulators available to meet the power conversion needs within a mobile phone (see Table 1). There are three options: linear LDO regulators, inductorless switching regulators (also known as charge pumps), and traditional switching regulators (inductor-based).

The linear LDO regulator is considered the simplest solution, which can only convert the input voltage to a lower voltage. Its most significant disadvantage is heat management, because its conversion efficiency is close to the ratio of output voltage to input voltage. For example, an LDO input is a single-cell lithium-ion battery with a nominal 3.6V, and it provides an output voltage of 1.8V at an output current of 200mA to drive an image processor. Then, its conversion efficiency is only 50%, so it will cause hot spots inside the phone and shorten the battery life.

Switching regulators avoid all of the efficiency drawbacks of linear regulators. By using low impedance switches and magnetic storage components, switching regulators can achieve efficiencies of up to 96%, significantly reducing power losses in the conversion process. Operating at very high switching frequencies (greater than 2MHz) reduces the size of external inductors and capacitors. The drawbacks of switching regulators are relatively few and can be overcome with good design techniques.

Between linear regulators and traditional switching regulators is the charge pump. In a charge pump, the external energy storage element is a capacitor rather than an inductor. The absence of an inductor mitigates potential EMI issues that could affect sensitive RF receivers or Bluetooth chipsets. The disadvantages of a charge pump are limited input-to-output voltage ratios and limited output current capability.

Challenges of the camera function

Many 3G phones can take pictures and even stream video. However, as consumers begin to accept these phones with built-in cameras, they are demanding higher-quality video capabilities. The industry is ready to adopt improved image sensors and optical systems, but engineers are paying special attention to high-quality "flash" lighting. Flash is the key to good camera performance, but you must consider it carefully when implementing it in a compact 3G phone.

For a 2-megapixel camera phone, the size and performance of the built-in flash are two key factors that system designers need to consider. Today, there are two practical choices for flash illumination: white LEDs (light emitting diodes) and flash. Table 2 compares the performance of LEDs and flash. The advantages of LEDs are their ability to operate continuously and the low density of support circuits required.

However, flash has some properties that are particularly important for high-quality video. Its line-source light output is hundreds of times that of a point-source LED, so it can produce dense and easily diffused light over a large area. In addition, the color temperature of the flash is 5500°K to 6000°K, which is very close to natural light, eliminating the color correction required by white light LEDs at their peak blue light output.

Most designers want the performance of a xenon flash lamp, but they must ensure that circuit size and complexity do not negatively impact the implementation. To better understand the design difficulties associated with this task, the designer must carefully consider the physical size and operation of the flash lamp and the supporting circuitry required to ensure it operates safely and correctly.

Flashlight and its supporting circuits

The flash lamp is usually a cylindrical glass tube filled with xenon gas, with the anode and cathode in direct contact with the xenon gas, while the trigger electrode distributed along the outer surface of the lamp tube is not in contact with the xenon gas. The gas breakdown voltage is generally in the range of several thousand volts. Once the breakdown occurs, the lamp impedance will drop below 1W, and the high current flowing through the breakdown gas will produce strong visible light.

The flash is controlled by a trigger circuit and a storage capacitor that can produce high transient currents. When operating, the flash capacitor is typically charged to 300V. Initially, the capacitor cannot discharge because the flash lamp is in a high impedance state. However, when the trigger circuit receives the command, it applies a high voltage of several thousand volts to the lamp, which breaks down and discharges the capacitor. The main limitation on the flash repetition rate is whether the lamp can be safely cooled. The second limitation is the time required for the charging circuit to fully charge the flash capacitor. Depending on the current input power, capacitor value and charging circuit characteristics, the charging time is generally between 1 and 5 seconds.

A flash capacitor charger is basically a transformer-coupled boost converter with some special capabilities. When its "charge" control line is high, the regulator synchronizes the internal power switch to generate high-voltage pulses from the boost transformer. These pulses are then rectified and filtered to produce a 300V DC output with up to 80% efficiency.

Linear Technology provides a complete flash circuit that meets all of the above technical and performance requirements. By utilizing the LT3468-1, a flash capacitor charger in a SOT-23 package, the solution offers a compact form factor suitable for 3G mobile phones.

The upper left of Figure 2 shows the capacitor charging circuit. A Schottky diode (D2) is used to safely clamp the reverse transient voltage caused by T1. Step-up transformer T2 is used to generate the high voltage trigger pulse. Assuming C1 is fully charged, when Q1-Q2 turns Q3 on, C2 stores current in the primary terminal of T2. The secondary terminal of T2 then provides the high voltage trigger pulse to the flash lamp, causing it to ionize and conduct. C1 discharges through the flash lamp, causing it to emit light. The entire circuit occupies an area of ​​less than 400mm2 and is no more than 6mm high (including the flash lamp).

Battery Charging Solutions

Virtually all 3G mobile phones use lithium-ion batteries as their primary power source. Due to thermal and space constraints, designers must carefully consider what type of battery charger to use and what features are required to ensure safe and accurate charging of the battery.

A clear trend in linear Li-ion battery chargers is that package size continues to decrease. However, a concern is the board space or ventilation required to cool the IC during the charge cycle, especially during the high current phase . The power consumption of the charger will increase the junction temperature of the IC. Combined with the ambient temperature, it can reach a high enough level to overheat the IC and reduce circuit reliability. In addition, if overheated, many chargers will stop the charge cycle and only resume operation when the junction temperature drops. If this high temperature persists, the repeated cycle of charger "stop and start" will continue to occur, thereby extending the charging time. To reduce these risks, users can only choose to reduce the charge current to extend the charging time or increase the board area to dissipate the heat. Therefore, the overall system cost will also increase due to the increase in PCB heat dissipation area and thermal protection materials.

There are two solutions to this problem. First, a smart linear Li-Ion battery charger is needed that does not have to sacrifice PCB area to worry about heat dissipation and uses a small thermally enhanced package that allows it to monitor its own junction temperature to prevent overheating. If a preset temperature threshold is reached, the charger can automatically reduce the charge current to limit power consumption, thereby keeping the chip temperature at a safe level. The second solution is to use a charger that generates almost no heat even when the charge current is high. This requires the use of a pulse charger, which is a completely different technology from linear chargers. Pulse chargers rely on a well-regulated and current-limited wall adapter to charge.

Solution 1: LTC4059A Linear Battery Charger

The LTC4059A is a linear charger for single-cell lithium-ion batteries that eliminates the need for three discrete power devices and allows for fast charging without worrying about overheating the system. The monitor reports the charge current value and indicates when the charger is connected to the input power supply. It uses the smallest possible package without sacrificing thermal performance. The entire solution requires only two discrete components (input capacitor and a charge current programming resistor) and occupies an area of ​​2.5mm x 2.7mm.

The LTC4059A uses a 2mm x 2mm DFN package, which occupies only half the area of ​​the SOT-23 package and provides a low thermal resistance of about 60°C/W to improve heat dissipation efficiency. Through proper PCB layout and heat dissipation design, the LTC4059A can safely charge a single lithium-ion battery at a maximum current of 900mA at an input voltage of 5V. In addition, there is no need to consider the worst-case power consumption when designing, because the LTC4059A uses patented thermal management technology to automatically reduce the charging current under high power conditions (such as excessive ambient temperature).

Solution 2: LTC4052 pulse charger with overcurrent protection

The LTC4052 is a fully integrated pulse charger for single-cell 4.2V Li-ion/Li-polymer batteries. When the input voltage is 5.25V and the current is fast charged at 0.8A, the LTC4052 consumes approximately 280mW, while the linear charger solution consumes up to 1.8W. Unlike switching chargers that use inductors for high efficiency and low heat dissipation, the LTC4052 uses an inductorless design. A 700mA to 2A Li-ion/Li-polymer battery charger circuit designed with the LTC4052 occupies only 70mm2 and is less than 1.7mm in height. By minimizing power dissipation, the LTC4052 relaxes the thermal design requirements of the terminal equipment, allowing for smaller packages, smaller heat sinks, and smaller PCB areas, and eliminating hot spots, eliminating the need for heat sinks or fans.

The LTC4052 requires a current-limited wall adapter to control the charge current. It also requires overcurrent protection circuitry to protect against accidental use of higher currents or wall adapter failures. The LTC4052 is a fully integrated pulse charger that does not require external MOSFETs or blocking diodes (see Figure 3). This standalone charger IC features C/10 detection, charge status indication, end-of-charge timer, wall adapter detection, and overcurrent protection. The LTC4052's input supply can be 4.5V to 12V with 1% drift voltage accuracy.

Summarize

As the functionality of 3G mobile phones continues to increase, Linear Technology will continue to introduce innovative ICs that will enable these phones to maintain their current form factors without sacrificing performance, reliability or battery life.