Research on the design and balancing strategy with multi-section lithium battery protection IC

This study uses 0.5um BCD technology to design a multi section lithium battery protection chip suitable for small mobile electronic devices such as Bluetooth earphones and smart bracelets. By providing a complete system structure diagram, the principles and design methods of several key circuits in the chip were introduced. This chip can effectively monitor the voltage of lithium batteries, take protective measures against abnormal situations such as overcharging, discharging, and overcurrent, and has a balancing module with dynamic power consumption management function. The balancing module adopts a passive balancing architecture, and the balancing algorithm used is an average based voltage detection algorithm. Using this algorithm can make the balancing system structure simple and reduce system power consumption, reduce the differences between batteries within the group, and thus improve the uniformity of the battery pack. The overcharging detection accuracy of the designed chip is ±25mV, the overcharging detection accuracy is 50mV, and the static working current of the chip is 7uA.

many foreign integrated circuit design companies, such as Texas Instrument, Maxim, ADI, and Ricoh, have continuously researched and developed lithium-ion battery protection systems on a large scale in order to improve their efficiency while ensuring their safe operation.They have also launched a series of lithium-ion battery power management system chips.Foreign lithium battery protection chips have reached a very high level in terms of product functionality, reliability, and accuracy.However, due to limitations such as a late start and technological strength, the research and development level in China lags far behind that of foreign countries.In recent years, with the vigorous development of the domestic lithium battery industry and continuous support from national policies, academia and various chip companies have also achieved a series of achievements in the field of lithium battery protection chips.
This article proposes a four cells lithium battery protection chip with equalization function, which is used master the voltage situation of each battery and prevent abnormal situations such as overcharging, discharging, and overcurrent in the lithium battery.The chip has an equalization module and adopts an average based voltage detection algorithm.Compared to traditional voltage detection algorithms, it can improve the accuracy of equalization, and the timing logic of the system is simple, while reducing the operating current of the system, Effectively advancing the consistency of the battery pack, simulation results show that the various protection and balancing functions of the chip are effective.

2.Four lithium battery protection system block diagram
Figure 1.Block diagram of four lithium battery protection system Figure 1 shows the peripheral appliance circuit of the designed four lithium battery protection chip, the chip is used to protect four lithium batteries (VC1, VC2, VC3, VC4), the positive electrode of each lithium battery is joined to the series pin of VC, Rvc and Cvc for the battery filtering capacitor resistor, which can filter out the ripple of the power supply of the lithium battery, P+ and P-connect the load and charger, when the terminals are connected to the load, the current flow of the application circuit is clockwise, this is the discharge state, when connected to the charger, the current flow of the application circuit is counterclockwise, this is the charging state [3].OC and OC terminals are connected to charge control FET and discharge control FET respectively adjusting the charging and discharging status state of the battery pack, resistor Rsense is a current sampling resistor, one end of which is grounded, and the other end of which follows the current in the loop, CS The CS port detects this voltage and sends it to the chip for overcurrent detection, and the VM port detects whether the load and charger are connected.

Chip System Framework
The overall block diagram of the chip is shown in Figure .2, from which it can be seen that it mainly consists of modules such as bandgap reference circuits, voltage detection comparators, current detection modules, current bias circuits, oscillators, load/charger detection circuits, logic circuits and so on.

Figure 2. Overall chip diagram
The bandgap reference circuit mainly provides reference voltage and bias current for overcharging and overcharging comparators.The sampling circuit samples and compares the voltage of each battery.The current bias circuit provides bias current for other circuit modules.The charger/load detection circuit detects the connection status of the charger and load, and the oscillator is used to generate a stable periodic clock signal.The frequency divider is used to divide the clock signal generated by the oscillator to generate accurate delay time.The temperature protection circuit provides high and low temperature protection for discharge and low temperature protection for charging of the chip.Overcurrent protection includes charging overcurrent protection, discharging overcurrent protection 1, discharging overcurrent protection 2, and short circuit protection [4].

Voltage Detection Module
Voltage detection module can control voltage status of lithium battery pack in real time, complete the charging over-voltage, discharge under-voltage logic judgment.The voltage detection module mainly consists of a battery voltage sampling circuit and a voltage detection comparator.The voltage detection comparator is shown in Figure 3 as a two-stage open-loop comparator [5].
The input VIN and VIP are connected to the voltage detection reference voltage and the sampling voltage of the lithium battery, respectively.The two voltage values are compared by the comparator, and the output level is used to determine whether the lithium battery is overcharged and overdischarged.M1, M2, M3, M4 and M5 constitute a simple five-tube operational amplifier, M8, M9 and M10 are the enable control terminal to control the start and stop of the comparator's work, the sampling voltage of the battery is input to the negative end of the comparator, the reference voltage is input to the positive end of the comparator, M1 and M2 constitute a pair of current mirrors [6].M3 and M4 are differential input pairs, with M5 providing the tail current.
 If VIN+ is much smaller than VIN-, then M1, M2, and M3 are all turned off.Since no current flows out of VDD, M4 and M5 all work in the deep linear region, and the transmission current is zero, so the VOUT output is 0.  When VIN+ is larger than VIN-, finally M2 enters the linear region.If VIN+ is larger than VIN-, M4 is turned off, the current of M2 is 0, and it works in the linear region, then VOUT=VDD and VOUT is flipped to 1.

Current Detection Module
After the discharge current passes through the sampling resistor Rsense, a discharge overcurrent monitoring voltage VI is generated.The voltage VI is compared with the discharge overcurrent threshold voltage through a discharge overcurrent comparator to provide discharge overcurrent protection.

Figure 4. Discharge overcurrent detection circuit
As the current flow in the charging circuit of the lithium battery pack is from the ground terminal to the VI terminal, the VI port potential is negative, so it is necessary to resistor Ru and current source Iu to convert the detected negative voltage VI to positive voltage, when the voltage exceeds the reference voltage triggers the charging overcurrent protection, the charging overcurrent circuit is shown in Figure 5.

Equalization module algorithm
The equalization circuit of lithium-ion batteries mainly consists of peripheral circuits, internal equalization modules, and equalization algorithms.The equilibrium algorithm is the basis for determining the equilibrium system.The traditional voltage balancing algorithm starts balancing by requiring the battery voltage to exceed the threshold voltage for balancing, and the logic is simple.However, when the battery is balanced and balanced, the consistency of the battery is still poor.The improved equalization algorithm proposed by Ti compared the differential pressure between each battery [6].When the voltage difference between any two batteries exceeds the threshold, the battery pack begins to equalize.This method improves the accuracy of equalization, but requires relatively complex non overlapping clocks or multiple comparison module [7].The algorithm implementation is relatively complex and increases the power consumption of the system.Based on the above shortcomings, this article adopts an average based voltage detection algorithm [8].
The balancing process is as follows: when the voltage of any battery reaches the balancing threshold voltage Vbla, the balancing module is activated.At the beginning of balancing, the voltage of each battery is compared with the average voltage of the battery.The balancing action is initiated for batteries above the average voltage, and batteries below the average voltage are charged normally.When the balancing is turned off, the battery voltage is detected.The two processes alternate until the charging overvoltage protection is triggered.The algorithm used is shown in Figure 6.

Figure 6. Balancing Algorithm Block Diagram
VBi is the voltage of each battery, and is the average voltage of the four batteries in the battery pack.The algorithm uses a two-phase non overlapping clock to calculate the voltage of each battery and the average voltage of the four batteries in the battery pack,among them, voltage detection clock clk_det and equalization clock clk_bla The high-level time of bla accounts for 1/4 of the entire cycle, so during equalization, the duty cycle of the system's voltage detection module is reduced from 100% to 25%, thus reducing power consumption to 25%.

Balanced peripheral circuit
Li-ion battery equalization circuit mainly consists of peripheral circuits and internal equalization module, the internal equalization module includes equalization driver circuit, equalization sampling detection circuit and equalization clock logic circuit.The designed equalization circuit adopts the passive equalization [9], using a resistor shunt to slow down the charging speed of a single-cell Li-ion battery with too much power, and the peripheral circuit schematic is shown in Figure 7, using the battery filter resistor as a shunt resistor, and the equalization switch is integrated in the internal equalization mode of the chip [10].

The average battery voltage sampling circuit
When the equalization clock is at low level, the switch M_ S off, the average voltage sampling circuit of the battery pack composed of resistors Rx and Ry is open, and the enabling end of the equalization comparator COMP5-COMP8 is inputted at a low level.The comparator is turned off to reduce the power consumption during equalization.When the equalization clock is at a high level, the average voltage sampling circuit of the battery pack and the equalization comparator are turned on, and the negative input voltage of the comparator is： VCC is the total voltage of four batteries,The average battery voltage sampling circuit is shown in Figure 8.When clk_bla is high level, the bla_on enable signal is high level, and the balancing operation is performed.When the voltage of the second battery is balanced, the b2 enable signal is turned over to high level when the bla_on signal is high level, so that the equalizer control MOS tube is turned on, and the VC1 and VC2 signals are pulled to the same potential.Indicates that the second battery balancing action is being performed.

5.Conclusions
In this paper, a four-section lithium battery protection chip is designed by 0.5um BCD process.Simulation results show that the designed chip can effectively detect the lithium battery voltage and take protective measures against abnormal conditions such as overcharge, overdischarge and overcurrent.The chip also has an internal equalization module, which adopts the voltage detection algorithm based on average value.Compared with the traditional voltage detection algorithm, the timing logic and structure are simpler, and the algorithm adopts two-phase non-overlapping clock, and the duty cycle of the equalization clock and the voltage detection clock are reduced to 25% of the original, so the power consumption of the voltage detection module can be reduced.

Figure 7 .
Figure 7. Equalization peripheral circuit schematic When the second battery B2 triggers equalization, the equalization module outputs a low-level signal to the switching MOS tube MP2 to make MP2 conductive and the resulting discharge current is： 2 2 s 2

Figure 11 .
Figure 11.Lithium battery overcharge protection simulation diagram Figure.11shows the simulation diagram of overcharge voltage in Section 4. It can be seen from the waveform diagram that the lithium battery protection chip can effectively detect the lithium battery voltage and take protective measures against abnormal overcharge when the battery voltage exceeds the overcharge threshold, where OC_COMP is the output signal of the voltage detection circuit and OC is the charge control terminal.The chip operates at a current of 7uA.