A lithium battery power monitoring circuit design method

With the rapid development of portable electronic devices such as mobile phones, digital cameras, audio and video players, their working and standby time has gradually become a bottleneck technology that restricts their development. Extending the working time of equipment generally starts from two aspects: improving battery capacity and efficiency; strengthening battery power management. At present, most portable electronic devices use rechargeable lithium batteries. Lithium batteries refer to the general term for chemical batteries with lithium as the negative electrode material, including lithium-ion batteries, lithium polymer batteries, etc. This type of battery has no memory effect, can be recharged many times, has a long service life, and has a high energy/mass ratio and energy/volume ratio. At the same time, the disadvantage of this type of battery is that it has relatively harsh requirements during charging and use. Charging overcurrent, overvoltage, discharge short circuit, overheating, etc. will seriously affect the life and performance of the battery, and it is necessary to use circuits for control and management during use. At the same time, in order to improve the utilization rate of the battery and fully grasp the status of the battery, it is necessary to monitor the battery voltage, remaining power, estimated power supply time, temperature and many other signals of the battery in real time, and determine the control strategy based on this type, so as to improve the reliability of system operation. Usually, during the battery discharge process, there is no linear relationship between the battery voltage and the remaining power and working time, so the remaining power cannot be simply calculated by voltage sampling and function. In response to this requirement, a chip-based lithium battery monitoring and protection circuit is designed, and at the same time, it cooperates with the main controller (MCU) to complete the reliable management and effective protection of the lithium battery. Actual use and testing show that the design is reasonable and feasible.

1 Introduction to DS2762 chip
DS2762 is a high-precision Li+ battery monitor launched by MAXIM that integrates data acquisition, information storage and security protection. It is designed for cost-sensitive battery pack applications. This low-power device integrates accurate temperature, voltage, and current measurement, non-volatile (NV) data storage, and Li+ battery protection circuit, and adopts a small-sized TSSOP package or flip chip package. For applications such as remaining capacity estimation, safety monitoring, and battery characteristic data storage, using DS2762 for battery monitoring is a more appropriate solution. The chip can monitor the battery voltage, current, charge and discharge status, and remaining power parameters in real time, and can store these data and provide them to the microcontroller for corresponding processing. Its main features are:
(1) Provides Li+ battery safety circuits, including overvoltage protection, overcurrent/short circuit protection, and undervoltage protection.
(2) Sends a warning signal to the host when the accumulated current or temperature exceeds the user-set threshold.
(3) Current detection can use the built-in 25 mΩ current sense resistor or an external user-selectable current sense resistor.
(4) Current measurement. 12-bit bidirectional measurement.
Internal current sense resistor: 0.625 mA LSB and ±1.9 A dynamic range;
external current sense resistor: 15.625μV LSB and ±64 mV dynamic range.
(5) Current accumulation. Internal current sense resistor: 0.25 mAhr LSB;
external current sense resistor: 6.25μVhr LSB.
(6) Voltage measurement resolution is 4.88 mV.
(7) Uses an integrated sensor to measure temperature with a resolution of 0.125°C.
(8) Supports system power management and control functions.
(9) 32 B lockable E2PROM, 16 B general-purpose SRAM.
(10) Dallas 1-Wire interface, with a unique 64-bit device address.
(11) Low power consumption. Operating current: typical 60μA, maximum 90μA; sleep current: typical 1μA, maximum 2μA.
The chip uses TSSOP package, and its pin assignment and definition are shown in Table 1.

DS2762 is mainly composed of internal registers, accumulators, sensors, ADC and other parts. Figure 1 shows its internal functional block diagram.

2 Hardware Circuit Design
2.1 Monitoring Protection Circuit
Since DS2762 can judge the battery overvoltage, overdischarge, overcurrent and other states, and can output control levels through CC and DC terminals, it only needs to add a control switch MOS tube in the external circuit for control, and cut off the battery output for protection in the overvoltage, overdischarge, overcurrent and other states. At the same time, DS2762 and the external main control unit only need one-wire connection to transmit commands and data. In order to ensure reliable connection of reliable data, a pull-up resistor of about 4.7 kΩ needs to be connected on the data line. Figure 2 shows the specific circuit connection relationship and the electrical performance parameters of each device.

In Figure 2, an external resistor Rsns is used for current sampling. Its resistance value is selected according to actual needs and is generally in the mΩ level. You can also choose its built-in current sampling resistor, as shown in Figure 3. The input terminals IS1 and IS2 of the current sensor are connected to the SNS terminal and VSS respectively through pull-up resistors, and the resistance value of its Rsns-int is 25 mΩ at room temperature. When the built-in sensor is used, a capacitor is connected to IS1 and IS2 to form a low-pass filter with the internal resistor.
According to the circuit connection in Figure 2, when the battery voltage VCELL changes between VOV and VLV, the charging protection output terminal CC and the discharge protection output terminal DC output high and low levels according to the changes in VCELL. Its logic and timing relationship are shown in Figure 4.

2.2 Main controller and display
The main controller controls and accesses DS2762 according to the working status to obtain real-time information on the battery status. According to the specific application of portable electronic devices, the main controller can be DSP, ARM, MCU, etc. The experimental circuit in this article uses Atmel's AT89S52, a low-voltage, high-performance CMOS microcontroller with 8 KB flash programmable and erasable system memory (PEROM). Because the multi-functional 8-bit CPU and flash memory are combined in a single chip. The single-chip microcomputer is used to directly drive the LED display screen to display various status information of the battery in real time.

3 Software design
Because the DS2672 chip is powerful, most of the work is done by it independently, and only relevant communication is required between it and the main controller. The chip and the main controller are connected through a one-wire interface, which can transmit and receive signals respectively. The registers of the DS2672 chip mainly include: current register, current accumulation register, current bias compensation register, voltage register, temperature register, current accumulation high threshold register, current accumulation low threshold register, temperature protection high threshold register, temperature protection low threshold register, etc. During use, the main controller strictly follows the read/write timing of the DS2672 chip to read or set the contents of each register. Its main control commands are shown in Table 2.

4 Conclusion
The lithium battery in portable electronic equipment needs to be controlled and protected against charging overcurrent, overvoltage, discharge short circuit, overheating, etc. during charging and use. At the same time, it is necessary to monitor the battery voltage, remaining power, estimated power supply time, temperature and other signals of the battery in real time, and make judgments and processes according to the monitoring results. This paper proposes a battery power monitoring circuit design method based on the DS2762 chip, which can automatically protect the lithium battery from overvoltage, overcurrent, short circuit and other states, and cooperate with the main controller to read the real-time voltage, remaining power, temperature and other values ​​of the battery and process them. The circuit has excellent performance and strong scalability, and can be widely used in various electronic devices to complete the comprehensive management of batteries.