A novel and simple lithium battery charger solution

As mobile phones, MP3, PMP, DC/DV and other handheld electronic products become more and more popular, lithium metal (Li) and lithium ion (Li+) batteries are becoming more and more common. Most of the chargers used, whether they are 15 yuan mobile phone chargers on the market or high-end DC/DV compatible chargers, use charging voltage detection and control circuits composed of LM324. These circuits cannot meet the requirements of lithium battery charging characteristics in terms of control accuracy and function, which directly leads to insufficient battery charging, shortened battery life, and more and more battery damage and explosion cases. If foreign dedicated ICs are used for design, the extremely high cost is really unacceptable.

In view of this, the author introduces two lithium battery chargers composed of new domestic ICs, which provide performance far exceeding that of the LM324 solution at roughly the same overall cost, and have extremely high novelty and market prospects.

PT7M7433T is one of the latest high-precision voltage detectors designed by Shanghai Bailitong Company. Its detection accuracy is less than 1mV in the range of 0-Vcc (5.5V), and the detection value deviation of its batch IC is <±2.5%, which fully guarantees the performance consistency and extremely high overall performance of batch products. We can use it with a small amount of external circuits to form a very simple lithium battery charging circuit.

The internal block diagram and brief introduction of the IC are as follows: (Figure 1)

The IC contains a high-precision 615mV reference voltage source, two comparators, an RS trigger and some other logic circuits. Its general function is: VCC voltage or other voltage to be detected is connected to the LTHIN/HTHIN detection pin of the IC through the voltage divider network composed of R1-R3. When the voltage to be measured drops and causes the LTHIN pin to be lower than 615mV, the output pin LBO outputs a low level. If the voltage to be measured rises and causes the HTHIN pin to be higher than 615 mV, after internal logic judgment and simple delay, the output pin LBO outputs a high level.

The charger circuit assembled using this IC is as follows (Figure 2)

 The working process is roughly as follows: when the Li+ battery and power supply are connected, the IC detects the battery voltage through the network composed of R1/R2/R3. If the battery voltage is lower than 3.3V (determined by the resistance value of R1-R3), or the button SW1 is pressed, the voltage of IC3 pin is lower than 615mV, then IC4 pin outputs a low level, and pulls down the gate of Q1 through R5/D2 to make it conductive, thereby charging the battery with a large current through Q1/R9/D1. When the battery voltage rises all the way until it exceeds 4.20V, the voltage of IC1 pin is higher than 615mV. After a simple judgment and delay inside the IC, the 4th pin outputs a high level, thereby closing the high-current charging channel. However, the high level of the 4th pin also powers the charging circuit of R7/C2, causing the gate voltage of Q3 to slowly increase, and providing a path for R8 to cause Q2 to turn on, and providing a weaker conduction current to Q1 through R10, causing it to turn on slightly, thereby providing a small supplementary charging current for the battery. According to the component parameters shown in the figure, after about ten minutes, due to the continuous charging of C2, its terminal voltage, i.e. the gate voltage of Q3, continues to rise until Q3 is turned on, thereby turning off Q2, and the entire charging process is completed.

The above solution still has some shortcomings. For example, the detection accuracy is determined by the accuracy of the external resistors R1/R2/R3. There is no small current pre-charging process for over-discharged batteries (battery terminal voltage is lower than 2.8V). In addition, the 10-minute recharge time of this solution is not enough for larger capacity batteries. Therefore, in view of the above shortcomings, we also provide a solution with a slightly higher cost and better performance: (Figure 3)

At first glance, this solution uses three ICs, but U1 and U2 are packaged in SOT-23 or TO-92, just like ordinary transistors, and their price is the price of 2-3 transistors, and U3 (PT8A2513NE) is also packaged in TO-94, and its appearance is very similar to that of transistors, and its price is also quite cheap. However, this circuit can intelligently determine whether the battery is over-discharged and decide whether to use a small current pre-charge at the beginning of charging. In addition, the use of U3 will also extend the supplementary charging process in the later stage of charging to about one hour! First, let us introduce the functions of several ICs in this circuit:

(Figure 4) is the internal block diagram of U1/U2. The only difference between these two ICs is the value of the internal resistors R1/R2. Their functions are also very simple: when Vcc is lower than the set value of the IC (depending on the IC number, the internal R3/R4 resistance value is also different, resulting in different detection voltage values, for PT7M6128 it is 2.80V), the RST pin outputs a low level. Conversely, when Vcc is higher than 1.05 times the nominal value (for example, for PT7M6140, this value is 1.05x4.0 =4.20V), the RST pin outputs a high level.

Another IC (PT8A25 13) in the above circuit is a very simple but very stable delay IC, whose delay time depends only on the frequency of the OSC pin. In fact, this IC controls the output after dividing the OSC oscillation frequency by 32768 times. The reason why this circuit is used instead of general ICs such as LM555 or CD4060 is that it has relatively higher timing accuracy (other ICs do not divide the frequency) and longer timing time (up to several hours). In addition, the circuit is simpler and uses TO-94 or SOT-23-4 packaging, just like a triode. The price is similar to that of CD4060, but the effect is much better.

At this point, everyone must have a general understanding of the working process of Figure 3: When the battery and charger are connected, if the battery voltage is lower than 2.8V, U1 output is low (U2 output is also low), so that the circuits of R3 and R4 are blocked, and only the R2 circuit is connected, providing weak conduction to Q1, so that it outputs a current of about several milliamperes to pre-charge the battery. When the battery voltage rises to more than 2.94V (1.05X2.8) or the battery voltage exceeds 2.94V just after being connected, U1 outputs a high level and U2 continues to output a low level (the battery voltage has not reached 4.2VJ). At this time, the R2 circuit is cut off. The R3 path is turned on (because Q4 is turned on and its source level is low - because U1 output is pulled low), so that the smaller resistance R3 makes Q1 fully turned on, providing a large current constant current charging of hundreds of milliamperes. When this constant current charging process slowly makes the voltage of the charged Li+ battery terminal rise to 4.2V, U2 also outputs a high level to turn off Q4, but it also provides power to U3, causing U3 to start working. In this way, U3 triggers Q3 to turn on R4 to provide a very small supplementary charging current until U3 reaches the timing time and turns off Q3. At this time, the entire charging process is completely over.

In comparison, the second solution adds intelligent judgment of battery status and automatic pre-charging process in the early stage, and extends the time of the supplementary charging process at the end. The charging current in each stage is adjustable and the supplementary charging time is adjustable (from several minutes to several hours). Therefore, this solution has better accuracy and safety for charging lithium batteries. In addition, it is cheap and has more complete functions. It is believed that it will soon replace the mid- and low-end chargers on the market, especially those cheap mobile phone battery chargers.