A design scheme for lithium-ion battery charging circuit with USB interface

summary:

Personal computers (PCs) have been widely used, and the USB interface has become one of the standard peripheral interfaces on PCs. On the other hand, portable devices such as mobile phones, digital cameras, and MP3 players that use lithium-ion batteries have emerged in large numbers. How to use the ubiquitous computer USB interface to charge the lithium-ion batteries used in these devices is currently a hot topic in the application of USB interfaces.

Lithium-ion battery and USB interface overview

Lithium-ion (Li-ion) battery, referred to as lithium battery , is a new type of battery that has gradually become popular in recent years. It has the advantages of small size, light weight, large capacity (high energy density), low self-discharge rate and no memory effect. However, it also has some fatal defects: it has strict requirements on charging and discharging, and cannot be overcharged or over-discharged, otherwise it is easy to cause irreversible damage. In extreme cases such as short circuit and overcharging, it may explode and cause danger.

Generally, the nominal voltage of a single lithium battery is 316~317V. When charging, it is generally required to adopt the voltage and current limiting method. First, constant current charging, that is, the current is constant, and the charging current is 0.2C (arbitration charging system) according to the low rate charging specified by the national standard, and the maximum does not exceed 1C; and the battery voltage gradually increases with the charging process. When the battery terminal voltage reaches the termination voltage of 412±0105V, the constant current charging should be changed to constant voltage and low current (about 011C) charging. This state is generally called trickle charging state. The charging current gradually decreases as the charging process continues according to the saturation of the battery cell. When it decreases to 0101C, the charging is considered to be terminated.

USB interface is the abbreviation of Universal Serial Bus in English, and its Chinese meaning is "Universal Serial Bus". It is a new interface technology used in the PC field. USB uses a 4-pin plug as a standard plug to connect to external devices. In this 4-pin plug, two pins are data communication lines, and the two outer pins provide power for external devices. According to the USB specification, each USB interface should be able to provide a current output of 500mA; the USB interface provided by the USB host or a powered hub has a minimum available voltage of 4.5V at the connected peripheral end, while the minimum voltage that can be provided by the hub driven by the USB bus should be 4.35V. It is worth noting that although the USB specification defines that the upper limit of the current provided shall not exceed 0.15A, in fact, the current output by the USB port often exceeds several amperes.

From the above analysis, it can be seen that the current should be sufficient when charging lithium batteries using the USB interface. The key is how to control the current. In addition, when using a USB interface with a minimum voltage of 4.35V to charge a lithium battery with a typical voltage requirement of 4.2V, there is only a small voltage margin, which makes the voltage drop of the charging circuit extremely important and a difficult point in the design.

Design of charging circuit based on PC USB interface

(1) Option 1

Figure 1 shows the internal circuit of some "USB charging cables" on the market. The main principle is to use a resistor to limit the maximum current, and use the voltage drop of a diode (about 0.7V) to reduce the output voltage of the USB interface of about 5V to about 4.2V and then charge the lithium battery. This circuit has the advantages of simplicity and low cost, but the disadvantages are also obvious: the charging current and charging voltage vary depending on the USB interface and the battery status, and are basically unpredictable. In many cases, the battery is very likely to be undercharged (the charging voltage does not reach 4.2V), and sometimes it may be overcharged and damaged. This circuit is acceptable for some lithium batteries with perfect internal protection circuits, but it is dangerous for ordinary lithium batteries.

(2) Option 2

Figure 2 is a practical USB lithium battery charging circuit composed of MAX1551/1555. MAX1551/1555 is a USB single-cell lithium battery charging chip designed and produced by MAXIM. It is a 5-pin thin SOT23 package that allows voltage input in the range of 3.7V to 7V from the USB interface or wall AC adapter. The chip also has a temperature limiting circuit inside.

For this circuit, the charge current is set to 100mA (maximum) when connected to the USB port but without a DC power source from a wall AC adapter. This allows charging from a powered or unpowered USB port without port communication. When the DC power source is connected, the charge current is set to 280mA (typical) and the input to the USB port is automatically cut off. If there is no voltage input to both input terminals, the circuit is automatically cut off. At this time, the reverse leakage current of the battery is less than 5uA, and no external diode is required to prevent battery leakage and loss.

When the battery voltage is lower than 3V, it will enter the pre-charge mode (trickle charge state) where the charging current is limited to 40mA. Only when the battery is charged to more than 3V will it enter the normal charging mode (100mA or 280mA). This mode protects the deeply discharged battery. Figures 3 and 4 are the relationship curves between the battery voltage and the charging current using the DC power supply and USB interface as input.

In addition, for the MAX1551, "/POK" is used to indicate whether the input power is connected (the output is low when the input voltage is 3.95V); and for the MAX1555, "/CHG" is used to indicate the charging status (the output is low when the charging current is greater than 50mA). For other characteristics, please refer to the MAX1551/1555 data sheet.

(3) Option 3

It should be said that Solution 2 is a safe, comprehensive and perfect USB lithium battery charging solution. However, the disadvantage of this solution is that the charging current is fixed at about 100mA or 280mA. Especially when the USB interface is powered, the charging current can only be 100mA. For batteries with larger capacity, it takes a long time to charge. Another disadvantage is that there is no charging timing function, and the end of charging cannot be controlled manually. Figure 5 is another practical USB interface charging circuit, which uses LTC4053 as the design core, which not only maintains the advantages of Solution 2 but also solves the above shortcomings.

LTC4053 is an independent timed termination constant current/constant voltage linear charger IC for lithium-ion batteries launched by Linear Corporation. It can be directly powered from a USB interface and is packaged in a 10-pin MSOP package. A single lithium battery can be charged directly from the USB interface, and the DC power supply of the wall AC adapter can also be used as the power supply. The required input voltage range is 4.25V to 6.5V. It includes an on-chip power MOSFET tube. The biggest feature of this chip is that it allows designers to set the charging current and charging time by themselves, and the charging termination voltage is preset to 4.2V.

In Figure 5, the charging current is set by the resistor R on the PROG (7) pin of the LTC4053. The calculation formula is: R = 1500V/ICHG. For example, if the charging current to be set is 500mA, then R = 1500V/015A = 3kΩ. The maximum charging current can be set to 1125A. Figure 6 is a curve diagram showing the relationship between the charging current and the battery voltage.

The capacitor C on the TIMER (4) pin of the LTC4053 is used to set the charging time, and the calculation formula is: T (hours) = C (uF) ・ 3/011uF. When C = 011uF, the charging time is 3 hours.

This circuit has a trickle charge mode similar to the pre-charge mode in Solution 2, and has a unique faulty battery detection function: when the battery voltage is lower than 2.48V, it is trickle charged at 10% of the full-scale current; if this state lasts for more than a quarter of the total charging time, the battery is considered damaged and the charging cycle is stopped.

LTC4053 itself has perfect temperature detection, compensation and adjustment functions, which can allow users to safely charge with a larger current and greatly reduce the charging time. Figure 7 is a curve of the relationship between charging current and ambient temperature. Similarly, when there is no voltage input to LTC4053, the circuit can be automatically cut off, and the reverse leakage current is less than 5uA. There is no need to add an external diode to prevent battery leakage, thus avoiding battery loss.

LTC4053 also has complete control, status indication and other functions. Please refer to the chip manual for details.

Conclusion

Undoubtedly, among these three solutions, the third solution has the most complete functions and the greatest flexibility in design and application, but the chip has more pins and the device cost may be higher than other solutions. In practice, we can choose different solutions to meet different requirements according to actual conditions such as purpose and cost. With the development of integrated circuit technology and the continuous introduction of new devices, we believe that there will be more and better USB charging solutions.