How to Design an Offline Li-ion Battery Charger

Efficient, low-cost and reliable battery charger designs can be implemented in a variety of ways, but using 8-bit flash MCUs not only reduces design time, reduces costs and provides a safe and reliable product, but also allows designers to perform field upgrades with minimal effort. Considering the cost, design efficiency and importance of safe battery charging, MCU-based solutions can provide many advantages to designers. By selecting an 8-bit MCU with appropriate peripherals and flash memory, engineers can take full advantage of its advantages to design an offline lithium battery charger. The Philips 80C51 MCU with 2KB flash and appropriate peripherals to provide an inexpensive solution is such an example. Integrated flash memory also provides the ability to efficiently and easily debug application code and perform field software upgrades (if necessary).

Because the design community is not only familiar with but also widely accepted 8-bit MCUs, hardware and software development can proceed quickly. Another advantage of this approach is the availability of a wide range of powerful and inexpensive application development tools from a wide range of vendors. Using this approach, the design team can not only greatly shorten the design cycle, but also carry out more complex designs while keeping the overall bill of materials (BOM) of the project within an acceptable range.
For example, when the MCU integrates an internal oscillator, the design of an offline lithium battery charger can benefit in two ways. First, the external oscillator can be eliminated, saving cost and PCB space; second, the internal oscillator can improve the stability of the system at startup.

A quad-channel A/D converter is another valuable peripheral that designers should look to integrate into their chips. In addition to being more cost-effective than using an external A/D converter, it can also be used to sense charge voltage, current, and battery temperature—virtually all the important parameters for safe battery charging operations.

The MCU (P89LPC916) used to implement the design described below not only integrates all of the above features but also has a high-performance processor architecture that can execute instructions on two clocks simultaneously, thereby increasing its performance to 6 times that of the standard 80C51 device. Time0 (Timer 0) can be easily configured as a PWM output, so it is easy to set up and use the PWM function.

Basic battery charging standards

This design is a lithium battery charger solution specifically for rated 700-750mAh, 3.6V discharge voltage and 4.2V voltage limit.

The charging sequence is divided into the following three stages: pre-charge stage, constant current charging stage and constant voltage charging stage. When the battery has very little power left and can therefore only produce a very low output voltage, a pre-charge stage is necessary. In this case, low current charging must be used to protect the battery. However, if the battery being charged can produce a higher voltage (>3V), the pre-charge stage can be omitted. Of course, this is the most common case.

Most of the energy flows from the charger to the battery during the constant current and constant voltage charging phase. The maximum allowable charging current of the battery is determined by the rated capacity of the battery. For fast charging, for example, a battery rated at 700mAh can be charged with a current of 350-400mA.

In the case of lithium batteries, the MCU must maintain the normal charging voltage of the battery while also monitoring the charging current so that the charging process can be terminated when the battery is full.

Temperature monitoring can be used to ensure safe charging procedures are performed because as the battery fills, any additional energy will be converted into heat. Although the MCU must add temperature monitoring to its functions, most lithium batteries on the market today have built-in overcharge protection, so temperature monitoring, although required, is rarely used.

Buck Converter Design

If you want to design a charger with tapered termination characteristics, the most effective and economical way is to use a buck converter as a switching regulator. Buck converters use inductors to store energy. Figures 1a and 1b are schematic diagrams of buck converter operation when the switch is in the on/off position, respectively.

[page]The signal from the PWM controls the charging switch. When the switch is closed (Figure 1a), current flows through the circuit due to the voltage provided by the charger (charger Vin), and the capacitor is charged through the inductor. When the switch is open (as shown in 1b), the inductor tries to keep the current flowing by inducing voltage, but it cannot charge immediately. The current then flows through the Schottky diode and charges the capacitor. This process repeats itself.

; When the switch "on" time is shortened by reducing the PWM duty cycle, the average voltage decreases. On the contrary, when the switch "off" time is extended by increasing the PWM duty cycle, the average voltage increases. Therefore, by controlling the PWM duty cycle to allow the MCU to adjust the charging voltage (or current) to achieve the desired output value. Before discussing the design details, two key points related to inductance and capacitance need to be discussed first:

1. Inductance

It is not difficult to see that determining the size of the buck converter inductor is the key to achieving the appropriate charging voltage and current. The size of the inductor is also related to cost. The inductor capacitance can be calculated using Formula 1: Formula 1 Where: Vi: charger voltage input to the switch; Vsat: voltage loss of the switch when the switch is "on"; Vo: voltage output; T: PWM period; DutyCycle: PWM duty cycle; Io: current output (i.e. constant current charging). Formula 1 shows that the higher the switching frequency of the PWM (i.e. the smaller the switching period T), the smaller the inductor required, which helps to reduce the cost of the device.

2. Capacitor size

&nbsThis design is based on the Philips P89LPC916 MCU. The overall design idea is to achieve the fastest possible charging by first charging with a constant current and then charging with a constant voltage. The MCU also controls the LED that indicates the working status of the charger.

1. Precision power supply

VDD needs to use a precision voltage source because this voltage is used as the voltage reference for the DA-DA converter. A low dropout (LDO) regulator is the best choice for this voltage source, and this design uses a 3-terminal LDO LM1117 to provide a precise 3.31 V power supply for VDD.

2 PMW output solution

One channel of Timer0 is used to generate the PWM signal that controls the buck converter switch. Since the LPC916 has its own on-chip RC oscillator, charging is more stable and efficient - especially in voltage control mode of operation. The required PWM frequency is only about 14kHz, so it can be well controlled within the frequency range of the on-chip oscillator. The PWM duty cycle can be adjusted by changing the "on" time of the buck converter.

System Design

Figure 2 shows a block diagram of the lithium battery charger system. The PWM output controls the charging switch, and its duty cycle can be adjusted as needed using feedback from the charging voltage and current. The LPC916's 8-bit on-chip high-speed A/D converter provides the high accuracy required to monitor the charging voltage. Avoiding overcharging in lithium-ion applications is very important because keeping the charge within its maximum value can extend the battery's life. Table 1 shows the input/output parameter specifications of the circuit.

[page]The next step is to calculate the inductor value. First, it must be pointed out that Equation 1 gives the relationship between duty cycle, output current, PWM period and other variables. The inductor value can be calculated by assuming Vi = 5.1V, the required output voltage Vsat = 0.5V (at Io = 350mA, Vo = 4.25V, the required output current Io = 350mA, 1/T = 14.7kHz and a duty cycle of 50%). Using these values, Equation 1 can be used to calculate an inductor value of no less than 10µH. In this design, the recommended inductor value is 33-10µH. Although input voltages greater than 5.1V can be used, higher input voltages require higher frequency PWM or larger inductors, which increases the cost of the device.

Lithium batteries should be charged in three separate stages. If the battery voltage is below 3V, a pre-charge stage is required and the charge current should be maintained at 65mA. Once the battery voltage reaches 3V+-1%, the fast charge stage begins with a constant charge current of 350mA. The charge current can be kept constant by adjusting the control pulse. When the battery voltage reaches 4V+-1%, the constant voltage charge stage begins. At this time, the voltage is maintained at 4.23V and the charge current is monitored.

After the constant voltage charging phase, the battery is charged for an additional 50 minutes while keeping the charge current less than 30 mA. The charge time can be controlled with a timer, but there are three ways to monitor the end of charge: sensing the charge current, using a timer, and monitoring the temperature (optional).

The charging process is shown in Figure 3. The exact sign of entering from one stage to another is as follows: Pre-charge stage (when needed): If Vbat<3.0(1%, set Iout=10%; Ireg=65mA; Fast charge stage (constant current charging): When Vbat<=4.00+-1%V, set Iout=Ireg=350mA; Timer controlled charging stage (constant voltage charging): When Ibat<60mA, set Vout=Vreg=4.23V(50 minutes) to ensure that the battery is fully charged, but the charging current is less than 30mA. Charging is completed in 4 hours. Considering the end user, the design uses LED status indicators to provide information about the status of the charging sequence.

Testing of Design Solutions

The circuit block diagram that can be used to test the design during the charging process is shown in Figure 4. Use two multimeters to measure the Vout and Vsense_res readings. Vout = Vbat + Vsense_res, and the charging current can be calculated using the formula Iout = Vsense re