Design of charging integrated circuit based on AT89C52 single chip

With the rapid development of microelectronics technology, various electronic products are constantly emerging, and the trend toward portability and small size and light weight is developing. In order to use these electronic products more effectively, rechargeable batteries are rapidly developed. . Common rechargeable batteries include nickel-metal hydride batteries, nickel-cadmium batteries, lithium batteries, and polymer batteries. Among them, lithium batteries are widely used for their high energy density, stable discharge characteristics, no memory effect and long service life. At present, most mobile phones and digital cameras use lithium batteries. Battery life and single cycle time are closely related to the charger maintenance process and usage. A good charger not only can fully charge the battery in a short period of time, but also can play a certain role in maintaining the battery, repairing the memory effect caused by improper use, that is, the battery activity decline phenomenon.

2 main characteristics of lithium battery

1) High energy density, the weight of lithium-ion battery is half of the same capacity of nickel-cadmium or nickel-hydrogen battery, the volume is 40%~50% of nickel-cadmium, and the nickel-hydrogen is 20%~30%. Therefore, lithium battery has more High weight-to-energy ratio, volumetric energy ratio.

2) The high voltage, single-cell lithium battery voltage averages 3.6V, which is equal to the series voltage of three nickel-cadmium or nickel-hydrogen rechargeable batteries.

3) Self-discharge is small and can be stored for a long time.

4) No memory effect, lithium battery does not have the so-called memory effect of nickel-cadmium battery, so lithium battery does not need to be discharged before charging.

5) Long life, under normal working conditions, the number of cycles of charge and discharge of lithium batteries is much more than 500 times.

6) Multiple lithium batteries can be used in parallel.

7) No pollution, because the lithium battery does not contain heavy metal elements such as cadmium, lead, mercury, etc., it has no pollution to the environment.

8) Fast charging, using a constant current constant voltage charger with a rated voltage of 4.2V, can make the lithium battery fully charged within 1-2 hours. Lithium batteries are relatively expensive compared to other rechargeable batteries. However, with the development of technology, the cost performance of lithium batteries is getting higher and higher, and it has been widely used in various portable mobile devices.

3 lithium battery charger hardware design

3.1 System structure block diagram

The system hardware circuit consists of three parts: single-chip circuit, voltage conversion, optical coupling isolation circuit and charging control circuit. The charging process of pre-charging, fast charging, full charging, power-off, alarm, etc. is realized by the control of the single-chip microcomputer.

Figure 1 System block diagram

3.2 52 single chip circuit schematic

The MCU chip is AT89C52 MCU of Atmel Company, B1 is the buzzer, and the P2.0 port output of the MCU controls the optocoupler device, which can turn off the charging power supply when needed.

3.3 Charging circuit control section

The charge status output pin /CHG is connected to the P3.2 port of the microcontroller via the inverter 74LS04 to trigger an external interrupt. The PNP is a P-channel field effect transistor or a triode. D1 is a green LED that lights up when it is energized; D2 is a red LED that illuminates when the power is turned on. R1 sets the resistance of the charging current, the resistance is 2.8 kΩ, and the maximum charging current is set to 500 mA; C2 is the capacitance for setting the charging time, the capacitance is 100 μF, and the maximum charging time is set to 3 hours.

4 lithium battery smart charger function realization

In this design, functions such as pre-charge, fast charge, full charge, power-off and alarm are required. These functions mainly rely on the built-in charging state control of the intelligent charging management chip MAX1898 and the control of the peripheral microcontroller AT89C52. The following is introduced separately.

4.1 Precharge

After the battery is installed, the input DC power is turned on. When the charger detects the battery, the timer is reset to enter the pre-charging process. During this period, the charger charges the battery with 10% of the fast charging current to make the battery voltage. The temperature returns to normal.

The precharge time is controlled by an external capacitor. Here, 100nF is selected, that is, the precharge time is 45 minutes. If the battery voltage reaches 2.5V or above within the specified charging time, and the lithium battery temperature is normal, the fast charging process is entered; if the lithium battery voltage is still lower than 2.5V during the pre-charging time, the lithium battery is considered to be unchargeable, / The LED connected to the CHG pin flashes to indicate a charging failure.

4.2 Fast charge

When the precharge is over, the lithium battery voltage rises above 2.5V, and the MAX1898 quickly charges the lithium battery with a constant circuit. Fast charge is also called cross current charging. During the fast charge process, the lithium battery voltage gradually rises until the lithium battery voltage reaches the set termination voltage.

4.3 Full charge

When the fast charge process, the lithium battery voltage reaches the set termination voltage, the fast charge ends, the charge current decreases rapidly, and the charge enters the full charge process. At this time, when the charging rate falls below the set value or the full charge time expires, the top end of the charging process is entered.

At the top turn-off charge, the MAX1898 replenishes the battery with minimal charge current. Under normal circumstances, full charge and top cut-off charging process can extend the use time of lithium batteries by 5%-10%.

4.4 Power failure

When the battery is fully charged, the pulse level sent by the second pin/CHG of the MAX1898 will be low to high, which will be detected by the MCU, causing the interrupt of the MCU, and the status of the charging is judged in the interrupt. At this point, the MCU will control 6N137 through the P2.0 port, cut off the power supply of the LM7805 to the MAX1898, thus ensuring the safety of the chip and the lithium battery, and also reducing the loss.

4.5 Alarm

When the battery is fully charged, the MAX1898 chip itself will extinguish the external LED green light. However, for the sake of safety, after detecting the pulse of full state, the MCU will not only automatically cut off the power supply of the MAX1898, but also alarm through the buzzer to remind the user to remove the battery in time. When the charging error occurs, the MAX1898 itself will control the LED green light to flash at a frequency of about 1.5Hz. Do not turn off the power supply of the MAX1898 at this time, let the user see this prompt.

5 Summary

The design of the charger for the AT89C52 single-chip microcomputer and the charging integrated circuit used in this design can not only realize the charging of the lithium battery, but also realize the corresponding overvoltage and temperature protection, so that the performance of the lithium battery can be fully utilized and avoided. The charger may cause damage to the battery during charging, and has certain intelligent functions. The program effectively protects the battery, shortens the charging time and maximizes the service life of the lithium battery, in line with the current environmental protection trend.