1.5v Alkaline battery

　　A USB interface 1.5v Alkaline battery charging circuit design scheme

　　Personal computers (PC machines) have been widely used, and the USB interface has become one of the standard peripheral interfaces on PC machines. On the other hand, portable devices such as mobile phones, digital cameras, and MP3 players using lithium-ion batteries are emerging 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 USB interface applications.

　　1.5v Alkaline battery and USB interface overview

　　Lithium-ion (Li-ion) battery, referred to as lithium battery, is a new type of battery that has become increasingly 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. , but at the same time it also has some fatal flaws: the requirements for charging and discharging are relatively strict, and it cannot be overcharged or overdischarged, otherwise it will easily cause irreversible damage. In extreme cases such as short circuit, overcharge, etc., it may explode and produce Danger.

　　Generally, the nominal voltage of a single cell of lithium battery is 316~317V. When charging, it is generally required to use the voltage limiting and current limiting method. First, constant current charging, that is, the current is constant, and the charging current is 0.2C according to the low rate charging specified by the national standard (arbitration charging standard), 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, constant current charging should be changed to constant voltage and small current (about 011C) charging. This state is normal It is called the trickle charge state. The charging current gradually decreases as the charging process continues according to the saturation degree of the battery core. When it decreases to 0101C, the charging is considered terminated.

　　The USB interface is the abbreviation of English UniversalSerialBus, and its Chinese meaning is "Universal Serial Bus". It is a new interface technology applied 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, while the outer two pins provide power for external devices. According to the USB specification, each USB interface should be able to provide a current output of 500mA; for a USB interface provided by a USB host or a powered hub, the minimum available voltage at the connected peripheral end is 4.5V, while a hub driven by the USB bus can provide The minimum should be 4.35V. It is worth noting that although the USB specification defines that the upper limit of the current provided must not exceed 0.15A, in fact the current output by the USB port often exceeds several amps.

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

　　Design scheme of charging circuit based on PC USB interface

　　(1)Option 1

　　Figure 1 shows the internal circuits 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 the 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 due to different USB interfaces and battery status, and are basically impossible to grasp. In many cases, the battery may not be fully charged. (The charging voltage cannot reach 4.2V), and sometimes it may be overcharged and damage the battery. This circuit is acceptable for some lithium batteries with complete internal protection circuits, but it is dangerous for ordinary lithium batteries.

　　(2)Option 2

　　MAX1551/1555 is a USB single-cell lithium battery charging chip designed and produced by MAXIM Company. It is a 5-pin thin SOT23 package and allows a voltage input ranging from 3.7V to 7V from the USB interface or wall AC adapter. There is also a temperature limit circuit inside the chip.

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

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

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

　　(3)Option 3

　　It should be said that option two is a safe, comprehensive and complete USB lithium battery charging solution. However, the disadvantage of this solution is that the charging current is fixed at around 100mA or 280mA. Especially when powered by the USB interface, 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 manually controlled. Figure 5 is another practical USB interface charging circuit, using LTC4053 as the design core, which not only maintains the advantages of the second solution but also solves the above shortcomings.

　　LTC4053 is an independent time-terminated constant current/constant voltage linear charger IC for lithium-ion batteries that can be directly powered from a USB interface. It is a 10-pin MSOp package. The single-cell lithium battery can be charged directly through the USB interface, or the DC power supply of the wall AC adapter can be used as the power supply. The required input voltage range is 4.25V ~ 6.5V. Includes an on-chip power MOSFET. 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 has been preset to 4.2V.

　　In Figure 5, the charging current is set by the resistor R on the pROG(7) pin of LTC4053. The calculation formula is: R=1500V/ICHG. For example, 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 graph showing the relationship between charging current and battery voltage.

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

　　This circuit has a trickle charging mode similar to the precharge mode of Scheme 2, and has a unique fault battery detection function: when the battery voltage is lower than 2.48V, it will trickle charge at 10% of the full scale current; if this state continues for a long time If it exceeds a quarter of the total charging time, the battery is considered damaged and the charging cycle is stopped.

　　The LTC4053 itself has complete temperature detection, compensation, and adjustment functions, which allows users to safely charge with a larger current, thereby greatly reducing the charging time. Figure 7 is a graph showing the relationship between charging current and ambient temperature. Similarly, when the LTC4053 has no voltage input, the circuit can 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 and avoid battery loss.

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

　　Conclusion

　　There is no doubt that 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 the other solutions. . In practice, we can choose different solutions to meet different requirements based on actual conditions such as usage and cost. With the development of integrated circuit technology and the continuous introduction of new devices, we believe that more and better USB charging solutions will appear.

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