Programmability is implemented in the integrated circuit of the battery charger

It is the invisible programmable devices under the user's fingers that make handheld devices so convenient and fun. These handheld devices are equipped with a battery capacity that is sufficient for a child's concentration period or one business day, so that as long as the battery has a chance to recharge it, people can enjoy it again and again. Chargers in some of the more advanced devices have powerful programmability, which not only shortens the charging cycle, but also extends battery life.

Nowadays, the development of charger programmability has far exceeded the above two basic requirements of handheld devices. While monitoring the charging voltage and current, the charger can also monitor the battery temperature at any time to accurately control the battery charging speed. This results in optimal battery capacity recovery and safety. The charger also continuously monitors the battery voltage during battery use, not only to alert when the battery voltage is low, but also to tell the user the remaining time that can be used before the battery must be recharged.

This programmability is primarily implemented in an integrated circuit of a battery charger located within the device. The charger can communicate some of the information it obtains with a semi-dedicated microprocessor that manages the operation of the device, shutting down unused or idle portions of the system. , thus effectively extending battery life.

While this smart charger will bring significant benefits to handheld devices, it cannot be ignored as the most basic form of stand-alone charger. These stand-alone chargers will be the main battery maintenance occasions where there is intentional or unintentional lack of microprocessor control. power. These occasions must be taken seriously, because for some reason the processor will not be able to intelligently control the charger, at which point the charger must have a certain degree of self-reliance. For example, the fact that the battery is nearly exhausted and the microprocessor does not have enough operating voltage is one of the situations in which the charger needs to be "self-standing."

Another situation is that the battery pack is disconnected from the processor internally due to some non-destructive fault. At this point, the charger directly powered by the external charging device must be able to continue to safely drain or open the battery according to the strict charging requirements specified by the battery chemistry in the shortest possible time without the help of a microprocessor. Charging batteries.

After roughly defining a relatively ideal battery charger based on the handheld device environment, let us turn our attention to the external charging device. As the work progressed further, the electrical engineering background gradually lost its effect, because after specifying the voltage and current requirements, we found that the most difficult task was to select the package, cable length and other tasks.

Of course, the above situation assumes that the charging device is powered by an AC power source or a charging adapter in a car. It is mainly hoped that the charger design will withstand the occasional overcharge test. The design has been modified to provide self-protection and prevent over-voltage of the battery. Another device that can be used to charge a handheld device is the USB port on the computer. The USB port can output a constant current of up to 500mA, and a charging circuit between the USB port and the battery of the handheld device can be established with only one cable.

Many computers have a rich USB port, so the USB port has become the preferred solution for charging devices, and the AC-powered wall socket is the second option. Even if the computer has only one USB port, there is no problem, because the port is only used for uploading photos and downloading MP3s. Of course, the importance of wall sockets cannot be ignored, but wall sockets have many disadvantages compared to USB ports, at least for unnecessary unnecessary costs. The cheapest wall socket consists of a 50Hz transformer, rectifier bridge and filter capacitor. The output voltage characteristics of the rectifier are quite poor, just like the AC voltage. Adding a linear rectifier improves the rectification effect but increases power consumption. In short, the AC-powered low-end charging device is cheap, but bulky and inefficient. High-end AC adapters are smaller in size and more efficient due to the use of switching methods, but the cost is greatly increased, and there are conduction and radiation problems. If you want to meet the requirements of the FCC and other regulatory agencies, it will further increase the size and cost.

Since many people have personal computers and workplace computers are ubiquitous, the ultimate low-cost charging solution is definitely the USB port. For those who travel frequently to and from the two places, it is only necessary to add a charging cable and a ready-made continuous charging source to the vehicle. The battery charger will be described in detail below, as well as how to obtain the maximum capacity charger without violating the maximum load regulations of the USB port.

The USB port with power supply can output 5V and up to 500mA. When using a linear charger with a constant charging current of 500 mA, a battery with a capacity of 1,000 mAh can be fully charged in just 2 hours. Using a switch mode charger can further increase efficiency and reduce the size of passive components. Figure 1 is a general lithium ion battery charging curve.

Figure 1: Programmable Charging Curve for Lithium Batteries

It is worth noting that the precharge current is only a small part of the normal charging current. Only when the battery voltage exceeds the float voltage can the battery be charged with a full charge stream instead of the precharge current.

Switching chargers must operate at the highest possible frequency to maintain their high efficiency and miniaturization of SMD inductors and capacitors. An integrated power switch further reduces charger volume and cost and offers specific performance benefits such as current limit and current mode control per cycle. Otherwise, the cost will be much higher if the switch is outside the integrated charger. Although the integrated average charge current detection circuit is very attractive, it is best implemented externally to the charge controller and can be placed in series with the battery.

Temperature sensing with a low-cost thermistor provides additional protection and longer life for the battery. By adding a two-wire interface communication with the charger, the user can be provided with a programmable configuration of charging/detection parameters, while forming a smart charger that can be operated from the USB port, which can be pre-determined and programmable. Charge and charge the battery at full charge rate.

Programmable performance is still effective even after the device is turned off. This can be done using on-board EEPROM, which can be programmed by the user via the I2C interface and the Windows GUI interface. Programmable performance includes some key parameters during the charging process, such as pre-charge current, pre-charge to fast charge conversion voltage, fast charge current, fast charge to minus charge conversion voltage, charge termination current and float voltage setting.

There are also auxiliary settings designed to prevent battery damage by terminating the charging process, such as precharge timeout, fast charge timeout, and low temperature/high temperature alarm set points. The temperature can be monitored by three different standard value thermistors, each corresponding to a unique constant current. The charging process can be activated by an I2C command or a programmable polarity enable. The status output pin can be programmed as a flashing or steady-state open drain low signal to indicate that the battery is charging.

Battery and charger status can be read via the I2C interface. The status includes charger status such as idle, precharge, fast charge and end of charge, and battery status such as battery not installed, over voltage or under voltage, over temperature and under temperature. The user can also obtain the charging timer status of the three charging phases through the two-wire interface. In addition, the charge controller is programmed to automatically initiate another recharge cycle and to inform the user if the current charge cycle is the first cycle after the controller has started operating, or if the charge cycle has ended after the controller has started operating.

Once the battery is selected, the actual design work can begin. In this case, a lithium battery with a full charge voltage of 4.2V and a capacity of 1,000mAh was selected. Use the USB port as the charging source and charge the battery as fast as possible. For the convenience of design, you can run the Windows GUI tool shown in Figure 2 and get all the parameters that need to be set by the user by clicking the "Standard Settings" button above.

Figure 2: Programming the charging parameters easily via the Windows GUI

In order to prevent the USB port output current from exceeding the limit, according to the maximum charging current regulation, the charging current can be set to be slightly smaller than the USB maximum output current minus the tens of milliamps current required by the battery charger itself and the peripheral circuit bias circuit. The reference GUI tool can be found that 450mA is an ideal charging current, even if the consumption of the bias circuit is added, it will not exceed the maximum output current limit allowed by the USB port. The charging current detection resistor is 100mΩ.

Although the charger current can be set higher, considering the current consumption of the charger operating current and other peripheral support circuits (such as pull-up resistors and visible LED indicators), there must be a margin to ensure the maximum output current of the USB port. Not overtaken. After completing the complete charging curve, the precharge and termination currents were set at 100 mA and 25 mA, respectively. At the same time, 100uA/10k thermistor is selected to increase the thermal protection function. The thresholds of floating charge and precharge to fast charge are set to 4.2V and 3.0V respectively.

The next step is to choose the switching inductor and the large output capacitor. The selection criteria for the inductor is to handle 20% to 25% of the ripple current at the selected 1.25MHz switching frequency. The value of L is roughly calculated according to the following formula:

According to the calculation results, an inductance of 15 uH can be selected. Ceramic capacitors with low equivalent series resistance are ideal for bulk capacitors. A 10uF capacitor with an equivalent series resistance of 8mΩ can be selected. This capacitor is cost-effective and has enough capacity to filter the ripple voltage, which is the product of the ripple current flowing through the inductor and the equivalent series resistance. Adding a large capacitor and a bypass capacitor to the input and adding a pair of bypass circuits to the charger bias pin completes the design.

The USB standard specifies the value of the input large capacitor. In principle, the USB port voltage must not fall below the specified value during hot plugging. The charger bias capacitor is used to maintain the integrity of the internal voltage reference and analog circuitry. Add a 500mA, 10V Schottky flywheel diode and use a simple RC and charger transimpedance amplifier combination to frequency compensate the switching circuit for further stable operation.

With a two-wire serial data interface and the help of a low-level microprocessor, the charger can greatly expand its use. When the battery supplier changes, the charging curve can be redefined via the serial interface and all other parameters required for battery maintenance so far, without the need to replace the charger or change any external components. Switching to the "Status/Control" column in the GUI shown in Figure 3, a number of volatile states and fault registers can be found that can be read and reacted by the microprocessor during the charging process.

Figure 3: Status and fault registers showing the state of charge. Command register start/stop charging

The charging process can begin or end as shown. There are other operations, including switching the charger to the USB hub operating mode, where the operating current is 100mA. Figure 4 shows the complete schematic of a battery charger with a two-wire serial interface.

Figure 4: Schematic diagram of a user-programmable USB-powered switch-mode lithium battery charger

Technological innovation is once again successful because those devices used to enhance PC functionality can now be used as a battery charging source for many handheld devices. The USB port communication capability further enhances the performance of the charger, allowing the charger to evolve from the original stand-alone charger to a smart, fully functional extended charging platform with a low-cost USB cable.

The bulky AC adapter, which is still a common device in the home, will become an optional accessory for battery-powered devices, giving the dominant position to the more popular USB charger.