Battery technology for portable electronic devices

Today's portable electronic backup battery technology includes several aspects such as power detection algorithms, battery charging algorithms and battery charging technology. As we all know, the rechargeable battery chemical reaction has four programs: nickel cadmium, nickel hydride, lithium ion and lithium polymer. As a portable electronic device, although these four battery programs have their own characteristics, they are in terms of energy density and safety. Development and practice show that the advantages of lithium-ion batteries and lithium-polymer batteries have become ideal for small long-running equipment, such as notebook computers and hard disk-based PMPs. For portable electronic equipment engineers, the right choice and application Good portable electronic equipment battery technology is very important, this article will discuss this, and an application example analysis.

1. Battery charging algorithm for trickle charging, fast charging and stable charging

Depending on the energy requirements of the final application, a battery pack may contain up to four lithium-ion or lithium-polymer battery cells in a variety of configurations, with a mainstream power adapter: direct adapter, USB interface or car charging Device. These battery packs have the same charging characteristics, regardless of the number of cells, the configuration of the cells, or the type of power adapter. Therefore, their charging algorithms are the same. The best charging algorithms for lithium-ion and lithium-polymer batteries can be divided into three phases: trickle charging, fast charging, and stable charging.

* Fine-flow charging. Used to charge deep-discharged cells. When the cell voltage is below about 2.8V, it is charged with a constant current of 0.1C.
* Fast charging. When the battery voltage exceeds the threshold of trickle charging, increase the charging current for fast charging. The fast charging current should be below 1.0C.
* Stable voltage. During the fast charging process, once the cell voltage reaches 4.2V, the steady voltage phase begins. At this time, the charging can be interrupted by a minimum charging current or a timer or a combination of the two. When the minimum current is lower than about 0.07 C, the charging can be interrupted. The timer relies on a preset timer to trigger an interrupt.

Advanced battery chargers usually come with additional security features. For example, if the cell temperature exceeds a given window, typically 0°C--45°C, charging will be suspended. In addition to some very low-end devices, the current Li-Ion/Li-Polymer battery charging solutions are integrated or have external components to charge according to the charging characteristics, not only for better charging results. It is also for safety.

2, lithium ion / polymer battery charging program

The charging scheme for lithium ion/polymer batteries is different for different numbers of cells, cell configurations, and power types. There are currently three main charging options: linear, Buck (buck) switches and SEPIC (boost and buck) switches.

2.1 linear scheme

When the charger input voltage is greater than the open circuit voltage after fully charged cells plus sufficient headroom, it is best to use a linear scheme, especially if the 1.0C fast charge current is not much larger than 1A. For example, an MP3 player usually has only one battery, with a capacity ranging from 700 to 1500 mAh, and a full charge open circuit voltage of 4.2V. The MP3 player's power supply is usually an AC/DC adapter or a USB interface, and its output is a regular 5V; at this time, the linear solution charger is the simplest and most efficient solution. Figure 2 shows a linear scheme for a Li-Ion/Polymer battery charging scheme with the same basic structure as a linear voltage regulator.

* Linear solution charger application example - dual input Li+ charger and smart power selector MAX8677A. The MAX8677A is a dual-input USB/AC adapter linear charger with a built-in Smart Power Selector for portable devices powered by a rechargeable single-cell Li+ battery. The charger integrates all the power switches required for battery and external power charging and switching loads, eliminating the need for an external MOSFET. The MAX8677A is ideal for portable devices such as smartphones, PDAs, portable multimedia players, GPS navigation devices, digital cameras, and digital video cameras.

The MAX8677A can operate from a separate USB and AC adapter power input or any of two inputs. When connected to an external power source, the Smart Power Selector allows the system to be disconnected from the battery or can be connected to a deep discharge battery. The Smart Power Selector automatically switches the battery to system load and uses the unused input power section of the system to charge the battery, taking advantage of limited USB and adapter input power. All required current sensing circuits, including integrated power switches, are integrated on the chip. The DC input current limit is adjustable up to 2A, while the DC and USB inputs support 100mA, 500mA, and USB suspend modes. The charge current can be adjusted up to 1.5A to support a wide range of battery capacitance. Other features of the MAX8677A include thermal regulation, overvoltage protection, charge status and fault output, power good monitoring, battery thermistor monitoring, and a charge timer. The MAX8677A is available in a space-saving, thermally enhanced, 4mm x 4mm, 24-pin TQFN package and is specified over the extended temperature range (-40 to +85°C).

2.2 Buck (Buck) Switching Scheme

Buck or buck is a better choice when the 1.0C charge current is greater than 1A, or if the input voltage is much higher than the cell's full-fill open-circuit voltage. For example, in a hard disk-based PMP, a single-cell lithium-ion battery is usually used, and the full-fill open-circuit voltage is 4.2V, and the capacity ranges from 1200 to 2400 mAh. Now PMP is usually charged with a car kit, and its output voltage is between 9V and 16V. A relatively high voltage difference (minimum 4.8V) between the input voltage and the battery voltage will reduce the efficiency of the linear scheme. This inefficiency, coupled with a 1C fast charge current greater than 1.2A, can cause serious thermal issues. To avoid this, the Buck scheme is used. Figure 3 is a schematic diagram of a lithium ion/polymer battery Buck charger scheme with the same basic structure as the Buck voltage regulator.

2.3 SEPIC (boost and buck) switching scheme

In some devices that use three or even four lithium ion/polymer cells in series, the input voltage to the charger is not always greater than the battery voltage. For example, the laptop uses a 3-cell lithium-ion battery pack with a full charge open circuit voltage of 12.6V (4.2V x3) and a capacity from 1800mAh to 3600mAh. The input power is either an AC/DC adapter with an output voltage of 16V or a car kit with an output voltage between 9V and 16V. Obviously, neither the linear nor the Buck solution can charge this battery pack. This requires the SEPIC scheme, which works when the output voltage is higher than the battery voltage, and also when the output voltage is lower than the battery.

3, electricity detection algorithm

Many portable products use voltage measurements to estimate the remaining battery power, but the relationship between battery voltage and residual power varies with discharge rate, temperature, and battery aging, making this method the highest error rate. Up to 50%. Market demand for longer-lasting products continues to increase, so system designers need more accurate solutions. Using a fuel gauge to measure the amount of battery charge or power consumed will provide a more accurate battery estimate over a wide range of application power levels.

3.1 One of the application examples of the power detection algorithm, the fully functional single/dual battery portable application battery pack design

* The principle of electricity detection. A good power detector must have at least battery voltage, battery pack temperature and current, measurement method; a micro-processing 9a; and a set of proven power detection algorithms. The bq2650x and bq27x00 are fully functional fuel gauges with an analog-to-digital converter (ADC) for measuring voltage and temperature and an analog-to-digital converter for measuring current and charge sensing. These fuel gauges also have a microprocessor that is responsible for executing Texas Instruments' power detection algorithms. These algorithms compensate for factors such as self-discharge, aging, temperature, and discharge rate of lithium-ion batteries. The microprocessor included in the chip saves these computational burdens for the host system processor. The fuel gauge can provide information such as the remaining battery status. The bq27x00 series also provides the Run Time to Empty. The host can always check the battery. Query this information and notify the user of the battery information via the LED indicator or the on-screen display. The power meter is very convenient to use, and the system processor only needs to be configured with a 12C or HDQ communication driver.

* Battery Pack Circuit Description. Figure 4(a) shows a typical battery pack application circuit with an identification function IC. The battery pack requires at least three to four external terminals depending on the power meter IC used. The VCC and BAT pins are connected to the battery voltage to power the C and measure the battery voltage. A low-resistance sense resistor is connected to the battery ground to allow the high-impedance SRP and SRN inputs of the fuel gauge to monitor the voltage across the sense resistor. The current flowing through the sense resistor can be used to determine the amount of charge that the battery is charging or discharging. When the designer chooses to detect the resistance value, the voltage across the resistor must not exceed 100 mV. Too low a resistance value may cause an error when the current is small. The board layout must ensure that the connections from the SRP and SRN to the sense resistor are as close as possible to the sense resistors; in other words, they should be connected using Kelvin.

The HDQ pin requires an external pull-up resistor that should be on the host or main application so that the gauge can enable sleep when the battery pack is disconnected from the portable device. It is recommended to use 10 kΩ for the pull-up resistor value.

* Battery pack identification. The problem of cheaper counterfeit batteries is getting worse. These batteries may not contain the safety protection circuits required by OEMs. Therefore, the authentic battery pack can include the authentication circuit shown in FIG. 4(a). When the battery is to be authenticated, the host sends a challenge value to the battery pack containing the IC (bq26150, which acts as a cyclic redundancy check (CRC)). The CRC contained in the battery pack will be based on this query value and in the IC. The built-in CRC polynomial calculates this CRC value. The CRC is based on the host-based query command and the secretly defined CRC polynomial in the IC. The host also compares the CRC value calculation well with the battery pack to determine if the authentication is successful. Once the battery has passed the authentication, the bq26150 will issue a command to ensure that the data line communication between the host and the fuel gauge is normal. When the battery connection is interrupted or reconnected, the entire qualification process will be repeated.

3.2 The second example of the power detection algorithm is applicable to the new ICs of various general fuel gauges.

Many manufacturers today offer a wide range of fuel gauge ICs, from which users can select the right functional devices to optimize the price/performance ratio of their products. Using a fuel gauge to store measured battery parameters, this separate architecture allows the user to customize the fuel gauge algorithm within the host. This eliminates the cost of the embedded processor in the battery pack. This is a typical analysis of the DS2762 chip named Dallase semicconductor. A new type of separate fuel gauge IC, the structure of which is shown in Figure 5 (a).

*DS2762 application features

The DS2762 is a single-cell lithium battery fuel gauge and protection circuit that is integrated into a tiny 2.46mm x 2.74mm flip-chip package. Thanks to the integrated high precision resistors for power detection, this device is very space efficient. Its small size and unparalleled high integration are ideal for mobile phone battery packs and other similar handheld products such as PDAs. The integrated protection circuit continuously monitors the battery for overvoltage, undervoltage, and overcurrent faults (during charging or discharging). Unlike an independent protection IC, the DS2762 allows the host processor to monitor/control the conduction state of the protection FET so that system power control can be achieved through the protection circuitry of the DS2762. The DS2762 can also charge a deeply drained battery, providing a current-recovery charging path when the battery voltage is less than 3V.

The DS2762 accurately monitors battery current, voltage and temperature, and its dynamic range and resolution meets the testing standards of any popular mobile communications product. The measured current integrates the internally generated time base to achieve electricity metering. The accuracy of the fuel gauge is improved by real-time, continuous automatic offset correction. The built-in sense resistor eliminates resistance changes due to manufacturing process and temperature, further improving the accuracy of the fuel gauge. Important data is stored in a 32-byte, lockable EEPROM; 16-byte SRAM is used to hold dynamic data. All communication with the DS2762 is via a 1-Wire, multi-node communication interface, minimizing the connection between the battery pack and the host. Its main features are: single-cell lithium battery protector; high-precision current (electrical energy metering), voltage and temperature measurement; optional integrated 25mΩ sense resistor, each DS2762 is individually fine-tuned; 0V battery resumes charging; 32 bytes can be added Lock EEPROM, 16-byte SRAM, 64-bit ROM;

1-Wire, multi-node, digital communication interface; supports multi-battery power management, and implements system power control through protection FET; supply current is only 2?A (maximum) in sleep mode; 90?A in operating mode Maximum); 2.46mm × 2.74mm flip chip package or 16-pin SSOP package, both with or without sense resistors; complex with an evaluation board.

4 Conclusion

The application of portable electronic battery technology is the basis for the selection of lithium-ion batteries and lithium polymer batteries and their chargers. How to choose correctly must also depend on the specific requirements of portable electronic equipment.

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