About Li-ion battery linear charging solution

　　This article discusses the use of high-current lithium-ion battery charging chip SE9018 to design a linear charging scheme for lithium-ion batteries.

　　With the development of modern electronic technology, electronic devices are becoming more portable and multifunctional, so their power supply batteries are also required to be portable and efficient. Lithium-ion batteries are gradually replacing traditional nickel-cadmium, nickel-hydrogen batteries, and lead-acid batteries with their high energy density, excellent charge and discharge performance, and pollution-free characteristics, and are widely used in modern portable electronic products. Compared with other types of batteries, lithium-ion batteries have excellent performance and higher requirements for chargers. These requirements are mainly reflected in the control of the charging process and the protection of lithium batteries. The specific performance is large charging current and high charging current. Accurate charging voltage, staged charging mode and complete protection circuit, etc.　　 This article discusses the use of high-current lithium-ion battery charging chip SE9018 to design a linear charging scheme for lithium-ion batteries.　　Chip introduction　　SE9018 is a linear charging chip for lithium-ion batteries in constant current/constant voltage mode. It uses an internal PMOSFET architecture and integrates an anti-reverse charging circuit without external isolation diodes.　　The preset charging voltage of the chip is 4.2V, the accuracy is ±1.5%, the charging current can be set by an external resistor, and the maximum continuous charging current can reach 1A. When the junction temperature of the chip is higher than 140°C due to high working power, high ambient temperature or poor PCB heat dissipation, the internal thermal feedback circuit will automatically reduce the charging current to keep the chip temperature within a safe range. In order to keep the chip working efficiently, measures should be taken to reduce the chip's working power and chip temperature as much as possible, such as connecting a small resistor in series at the input (reducing the input voltage), increasing the area of the PCB heat dissipation copper foil, and making the chip heat sink and PCB copper foil sufficient Contact etc.

　　Figure 1SE9018 pin map

　　Figure 2SE9018 schematic

　　SE9018 integrates a battery temperature monitoring circuit. When the battery temperature exceeds the normal range (too high or too low), the chip automatically stops the charging process to prevent the battery from being damaged due to the high or low temperature.　　Battery temperature monitoring is achieved by judging the TEMP terminal voltage (VTEMP). VTEMP is provided by a resistor divider network including the internal NTC thermistor of the battery. When VTEMP is between 45%×VCC and 80%×VCC, the chip judges that the battery temperature is within the normal range; when VTEMP<45%×VCC or VTEMP>80%×VCC, the chip judges that the battery temperature is too high or too low ; When the TEMP terminal is grounded, the battery temperature monitoring function is disabled. SE9018 contains two open-drain status indicating output terminals CHRG and STDBY. When the circuit is in the charging state, the CHRG terminal is set to low level, and the STDBY terminal is in a high impedance state; when the battery is fully charged, the CHRG terminal becomes a high impedance state , STDBY terminal is set to low level. When the battery temperature monitoring function is used normally, if the chip is not connected to the battery or the battery temperature is out of the normal range, the CHRG terminal and STDBY terminal are both high impedance; when the battery temperature monitoring function is disabled, if the chip is not connected to the battery, the STDBY terminal is Low level, the CHRG terminal outputs a pulse signal.　　 SE9018's other functions include manual shutdown, undervoltage lockout, automatic recharging, etc.　　 A typical SE9018-based lithium-ion battery charging circuit is shown in Figure 3. When the CE terminal is high, the SE9018 works normally.　　Smart high-current lithium-ion battery linear charging scheme

　　Figure 3SE9018 typical application circuit　　1. Setting of charging current 　　 The charging current Ibat during constant current charging is set by the resistance Rprog between PORG and GND. The relationship between Ibat and Rprog is:

　　Formula 1　　For example, if you want to get a constant charging current of 1A, according to formula 1, you can get Rprog=1200Ω. 2. Battery temperature monitoring circuit settings The battery temperature monitoring circuit settings are mainly to set R1 and R2, assuming that the resistance of the NTC thermistor at the lowest operating temperature is RTL, and the resistance at the highest operating temperature is RTH (RTL and RTH The data can be found in the relevant battery manual or obtained through experiments), then the resistance values of R1 and R2 are:

　　formula 2

　　formula 3

　　In practical applications, if only high temperature protection is needed, but low temperature protection is not required, R2 can be removed. At this time, the resistance of R1 is:

　　Formula 4　　3. Manual shutdown setting During the charging process, SE9018 can be placed in shutdown state by setting CE terminal to low level or removing Rprog (PROG terminal floating) at any time. At this time, battery leakage current drops below 2uA, input current Reduced to below 70uA.　　4. Undervoltage lockout status 　　If the input voltage VCC is lower than the undervoltage lockout threshold or the difference between VCC and the battery voltage Vbat is less than 120mV, the SE9018 is in the undervoltage lockout state.　　 When the chip is in shutdown state or under-voltage lockout state, both CHRG and STDBY are in high impedance state. 5. Normal charging cycle. When the input terminals of SE9018 and the battery are in the normal state, the charging circuit enters the normal charging cycle. This cycle includes four basic working modes: trickle charging, constant current charging, constant voltage charging, and charging end And recharge. If the battery voltage Vbat is lower than 2.9V, the charging circuit enters trickle charging mode, at this time the charging current is one-tenth of the constant current charging current (if the constant current charging current is set to 1A, the trickle charging current is 100mA) , The trickle charge state will be maintained until the battery voltage Vbat reaches 2.9V. The trickle charging mode is mainly to avoid the damage to the internal structure of the battery caused by the high current impact when the battery voltage is too low.　　 When the battery voltage is higher than 2.9V but lower than the preset full charge voltage of 4.2V, the charging circuit is in constant current charging mode. As mentioned above, the charging current is determined by Rprog.　　 When the battery voltage reaches 4.2V, the charging circuit enters the constant voltage charging mode. At this time, the BAT terminal voltage is maintained at 4.2V, and the charging current gradually decreases. The main function of this process is to reduce the impact of the battery's internal resistance on the full-charge voltage and make the battery more fully charged.