A new linear charging solution for lithium-ion batteries

With the development of modern electronic technology, electronic devices are becoming increasingly portable and multifunctional, which puts forward the requirements for their power supply batteries to be lightweight and efficient. Lithium-ion batteries are gradually replacing traditional nickel-cadmium, nickel-metal hydride and lead-acid batteries with their high energy density, excellent charge and discharge performance and no pollution, and are widely used in modern portable electronic products.

Compared with other types of batteries, lithium-ion batteries have excellent performance but also place higher requirements on chargers. These requirements are mainly reflected in the control of the charging process and the protection of lithium batteries, which are specifically manifested in larger charging currents, high-precision charging voltages, staged charging modes and complete protection circuits.

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

Chip Introduction

SE9018 is a constant current/constant voltage mode lithium-ion battery linear charging chip, which adopts internal PMOSFET architecture and integrates anti-reverse charging circuit, and does not require external isolation diode.

The preset full charge voltage of the chip is 4.2V with an accuracy of ±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 operating power, high ambient temperature or poor PCB heat dissipation performance, the internal thermal feedback circuit will automatically reduce the charging current to control the chip temperature within a safe range. In order to enable the chip to maintain an efficient working state, measures should be taken to minimize the chip operating power and chip temperature, such as connecting a small resistor in series at the input end (reducing the input voltage), increasing the area of ​​the PCB heat dissipation copper foil, and making full contact between the chip heat sink and the PCB copper foil.

Figure 1 SE9018 pinout

Figure 2 SE9018 schematic diagram

SE9018 has an internal integrated 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 over-high or under-temperature.

Battery temperature monitoring is achieved by determining the TEMP terminal voltage (VTEMP), which is provided by a resistor divider network including the battery's internal NTC thermistor.

When VTEMP is between 45%×VCC and 80%×VCC, the chip determines that the battery temperature is within the normal range; when VTEMP < 45%×VCC or VTEMP > 80%×VCC, the chip determines 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 indication output terminals CHRG and STDBY. When the circuit is in the charging state, the CHRG terminal is set to a 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 and the STDBY terminal is set to a low level. When the battery temperature monitoring function is used normally, if the chip is not connected to the battery or the battery temperature exceeds the normal range, the CHRG terminal and the STDBY terminal are both in a high impedance state; when the battery temperature monitoring function is disabled, if the chip is not connected to the battery, the STDBY terminal is low, and the CHRG terminal outputs a pulse signal.

Other features of SE9018 include manual shutdown, undervoltage lockout, automatic recharge, etc.

A typical lithium-ion battery charging circuit based on SE9018 is shown in Figure 3. When the CE terminal is high, SE9018 works normally.

Figure 3 SE9018 typical application circuit

1. Setting of charging current

The charging current Ibat during constant current charging is set by the resistor Rprog between the PORG terminal and the GND terminal. The relationship between Ibat and Rprog resistance 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 is mainly set 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 (the data of RTL and RTH can be found in the relevant battery manual or obtained through experiments), the resistance values ​​of R1 and R2 are:

Formula 2

Formula 3

In practical applications, if only high temperature protection is needed and low temperature protection is not needed, R2 can be removed. In this case, the resistance value of R1 is:

Formula 4

3. Manual stop setting

During the charging process, the SE9018 can be put into shutdown state at any time by setting the CE terminal to a low level or removing Rprog (PROG terminal floating). At this time, the battery leakage current drops below 2uA and the input current drops below 70uA.

4. Undervoltage lockout state

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, SE9018 is in undervoltage lockout state.

When the chip is in shutdown state or undervoltage lockout state, the CHRG terminal and STDBY terminal are both in high impedance state.

5. Normal charging working cycle

When the input terminals of SE9018 and the battery are in normal state, the charging circuit enters the normal charging cycle, which includes four basic working modes: trickle charging, constant current charging, constant voltage charging, charging end and recharging.

If the battery voltage Vbat is lower than 2.9V, the charging circuit enters the trickle charging mode, at which 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), and the trickle charging 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 large 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 a 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 influence of the battery internal resistance on the full charge voltage, so that the battery can be charged more fully.

When the charging current is reduced to 1/10 of the constant current charging current, the charging circuit stops charging the battery and enters a low-power standby state. In the standby state, the SE9018 will continue to monitor the battery voltage. If the battery voltage drops below 4.05V, the charging circuit will charge the battery again.

6. Indicator status

Table 1:

7. Circuit compatible with USB power supply and adapter power supply

At the same time, the SE9018 chip can be used to implement a charging circuit suitable for USB power supply and adapter power supply, and the circuit diagram is shown in Figure 4.

Figure 4 USB and adapter solution

When powered by USB, the gates of PMOS and NMOS are pulled down to a low potential, PMOS is turned on, USB power supplies SE9018, and the SCHOTTKY diode prevents leakage from the USB end to the adapter end. NMOS is turned off, Rp1 is disconnected, Rprog = 2.4kΩ, and the constant current charging current is 500mA.

When using a 5V adapter for power supply, the gates of PMOS and NMOS are at high potential, and PMOS is cut off to prevent leakage from the adapter end to the USB end. The adapter 5V voltage powers SE9018 through the SCHOTTKY diode. NMOS is turned on, and Rp1 is connected to the circuit. At this time, Rprog is Rp1 in parallel with a 2.4kΩ resistor. By setting Rp1, a constant current charging current greater than 500mA can be achieved.

Conclusion

In current portable products, the correct implementation of battery charging requires careful design considerations. This article discusses the intelligent high-current lithium-ion battery linear charging solution. The SE9018 chip used has the characteristics of fast charging speed, strong battery protection function, and a small number of peripheral components. In addition, the chip is also suitable for USB power supply and adapter power supply. It is a more practical intelligent high-current lithium-ion battery charging chip.