Design to protect lithium batteries

In order to prevent the lithium battery from being damaged by abnormal conditions such as overcharge, over discharge, and overcurrent, a lithium battery protection device is usually used to prevent abnormal conditions from damaging the battery. Currently, lithium batteries are used more and more widely, from portable devices such as mobile phones, MP3, MP4, GPS, toys to gas meters that need to continuously save data, and the market capacity has reached hundreds of millions per month.

The circuit principle of the lithium battery protection device is shown in Figure 1, which is mainly realized by the battery protection control IC and the external discharge switch M1 and the charging switch M2. When the P+/P- terminal is connected to the charger and the battery is charged normally, M1 and M2 are both in the on state; when the control IC detects abnormal charging, M2 is turned off to terminate charging. When the P+/P- terminal is connected to the load and the battery is discharged normally, M1 and M2 are both turned on; when the control IC detects abnormal discharge, M1 is turned off to terminate discharge.

Figure 1: Circuit principle of lithium battery protection device

1 Several existing lithium battery protection solutions

Figure 2 is a commonly used lithium battery protection board designed based on the above lithium battery protection principle. The SOT23-6L package in the figure is the control IC, and the SOP8 package is the dual switch tube M1, M2. Since the manufacturing process of the control IC is different from the manufacturing process of the switch tube, the two chips in Figure 2 are manufactured from different process flows, and usually these two chips are also provided by different chip manufacturers.

Figure 2: Traditional battery protection solution

In recent years, the industry has shown a trend of packaging several chips together to improve integration and reduce the final solution area. The lithium battery protection market is no exception. The two lithium battery protection solutions A and B in Figure 3 appear to integrate the two chips in Figure 2 into one chip, but in fact, the controller IC and switch tube chip inside the package are still separate and come from different manufacturers. This solution simply seals the two together, commonly known as "two chips in one".

Since the two chips inside actually come from different manufacturers, the appearance cannot match well, resulting in different final package shapes, and in many cases, universal packaging cannot be used. This package is relatively large and cannot save peripheral components, so this "two-chip-in-one" solution does not actually save much space. In terms of cost, although the cost of two packages is reduced to the cost of one package, since this package is usually larger, some are not universal packages, and some require chip stacking packaging to reduce the package size, so compared with the traditional two-chip solution, its cost advantage is not obvious.

Figure 3

Figure 4 is a true single-chip solution that integrates the controller chip and the switch tube chip on the same wafer. The switch tube in the traditional solution schematic diagram 1 is an N-type tube, connected between B- and P- in Figure 1, commonly known as negative electrode protection. Due to technical reasons, the switch tube in the solution in Figure 4 can only be changed to a P-type tube, connected between B+ and P+, commonly known as positive electrode protection. After using this chip to complete the protection board solution, the user needs to change the test equipment and concept when testing the protection board. Although this solution reduces a certain packaging cost, the chip cost has not been reduced, and there is no real cost advantage when competing with large-scale and mature traditional solutions. On the contrary, its positive electrode protection concept that is incompatible with traditional solutions has become a huge obstacle to its promotion process.

Figure 4: Lithium battery protection scheme for positive electrode protection

Although the above “two-core-in-one” solution and single-chip positive protection solution bring certain advantages to users in terms of solution area and cost, the advantages are still not obvious. These solutions also bring some disadvantages. Therefore, in the process of competing for customers with mature traditional solutions, they can only fight a price war by reducing gross profit margins. Since the real original cost of these solutions does not have a clear advantage, with the price reduction of control ICs and switch tube chips of traditional solutions, these “two-core-in-one” solutions or positive protection solutions have not been able to shake the market dominance of traditional solutions.

In recent years, many new switch tube chip manufacturers have appeared on the market. In order to reduce costs, they changed the original gold wire to copper wire during packaging, and the switch tubes did not have ESD protection. Although these products have certain differences in performance compared with branded switch tubes, they quickly occupied the secondary market due to their cost advantages, and also made great contributions to the traditional solutions in the market competition with the "two-core-in-one" and positive protection solutions.

2 Fully integrated lithium battery protection solution

SyChip Microelectronics has integrated the control IC and the switch tube into the same chip through a number of independently developed device and circuit patents combined with unique process technology, and launched the world's smallest lithium battery protection solution XB430X series products. This series of products uses traditional N-type switch tubes, which is consistent with the negative electrode protection principle of traditional solutions. Protection board manufacturers or lithium battery manufacturers do not need to change any test equipment or concepts. This series of chips itself is a complete lithium battery protection solution, and the lithium battery protection function can be realized without any external components. In order to prevent noise on the Vcc line, it is recommended to connect a capacitor between VCC and the negative terminal of the battery when using the XB430X series chips, as shown in Figure 5.

Figure 5: XB430X series lithium battery protection solution

The XB430X series chip is highly integrated. It not only integrates the traditional control IC and switch tube, but also integrates R1 and R2 in the schematic diagram 1 into the same chip. The integrated chip is very small, and the smallest can use the common SOT23-5L package on the market. The internal resistance of the switch tube of this chip series is extremely low, and the minimum internal resistance can reach below 40m?, which is equivalent to the internal resistance of the best switch tube on the market. When the smallest package SOT23-5L is used, the continuous charging and discharging current can reach 2.5 amperes without heat dissipation problems. If the continuous charging and discharging current is greater than 2.5 amperes, it is recommended to use the SOP8 package product in the XB430X series.

XB430X has all the protection functions in the traditional protection scheme: overcharge protection, over-discharge protection, overcurrent protection and short circuit protection. In addition, since the control IC and the switch tube are integrated into the same chip, the control IC can detect the temperature of the switch tube chip at any time. When the chip temperature is too high due to long-term use of the battery in a high temperature environment, or the current exceeds the normal charge and discharge current during charging and discharging, but does not reach the overcurrent protection threshold, the overtemperature protection function will be activated to protect the chip and battery. In addition, the built-in switch tube has an ESD protection function, which can greatly improve the yield of the protection board and battery during the processing process.

The internal control IC and switch tube of XB430X come from the same production process and the same manufacturer, and the packaging uses the most mature and universal packaging form. Therefore, the consistent performance is much higher than the traditional solution, the "two-core-in-one" solution, and the positive electrode protection solution.

By using the XB4301 series chips, only two components are needed to complete the final battery protection solution (as shown in Figure 5). Compared with the five components of the traditional solution, the production capacity and efficiency of each placement machine can be increased to 2.5 times. Compared with the traditional solution, the protection board manufacturer not only does not need to purchase resistors and switch tube chips, streamlining the resource chain, but also reduces the pads of two resistors and the eight solder joints of the switch tube when making the protection board, thereby greatly reducing the production cost of the protection board.