Design and implementation of a monolithic linear lithium-ion battery charger IC

Lithium-ion and lithium polymer batteries have the advantages of high operating voltage, no memory effect, wide operating temperature range, low self-discharge rate and high specific energy. This enables them to better meet the requirements of portable devices for power supply miniaturization, light weight, long working time and long cycle life, and environmentally friendly. At the same time, with the increase in lithium-ion battery production and the reduction in cost, lithium-ion batteries have achieved a dominant position in portable device power supply with their excellent cost-effective advantages, which has also led to huge development and a broad market for lithium-ion battery chargers. This article designs a linear charger IC for single-cell lithium batteries. The IC uses a three-stage charging method of trickle current-constant current-constant voltage to control the charging process.

1 Overall structural design of linear lithium-ion battery charger

Figure 1 shows the overall functional module diagram of the lithium-ion battery charger in this article. These submodules include reference voltage source, reference current source, undervoltage lockout module, constant current charging amplifier, constant voltage charging amplifier, intelligent thermal adjustment amplifier, clamping amplifier, oscillator, counter, battery temperature protection module, power tube substrate protection module, logic module and multiple comparator modules.

Considering the practical application of the chip, the lithium-ion battery charger designed in this article has the following characteristics:

(1) Temperature protection of the chip During the charging process, when the battery voltage reaches the trickle charge transition voltage threshold and enters the constant current stage, the constant current stage is a high current charging. Since the power tube in this article is a PMOS, there is only this power tube between the load battery and the power supply. At this time, the battery voltage is low and the chip power dissipation reaches the maximum. Its power dissipation is:

P = (Vcc-VBAT) Icc (1)

High power dissipation will cause the temperature of the chip to rise sharply, so an intelligent thermal feedback loop is set. When the chip temperature rises to the thermal feedback temperature point of 105°C, the thermal feedback loop is started to maintain the chip temperature at 105°C. When the battery voltage increases further, it can be seen from formula (1) that the power dissipation gradually decreases. Under smaller power dissipation, the temperature of the chip will gradually decrease. At this time, exit the intelligent thermal adjustment working mode and enter the constant current charging mode, use a large current Icc to charge the battery, or directly enter the constant voltage charging stage. The use of this thermal feedback loop maximizes the charging rate, and users do not need to worry about the chip temperature being too high.

(2) Cost: The chip introduced in this article is designed using CMOS technology, which is low-cost and easy to implement.

(3) Interactive management with users. The chip provides multiple external user programming pins to facilitate user management and use of the chip. In terms of charging current control, users can program the charging current by connecting a resistor to a pin of the chip; in terms of final charging voltage control, users can set the final charging voltage to 4.1 V or 4.z V by connecting a pin of the chip to a high level or a low level to accommodate charging of lithium-ion batteries using different negative electrode materials; in terms of charging time control, users can program the charging time by connecting a capacitor to a pin of the chip to meet different charging time requirements of users. The characteristics and parameters expected to be achieved by the chip design are shown in Table 1.

The external connection of the chip pins is shown in Figure 2. In Figure 2, the three pins CHRG, FAULT, and ACPR are connected to a 1 kΩ resistor and a light-emitting diode respectively to indicate the charging status of the chip; the 4.7 μF capacitor is the bypass capacitor of the power supply Vcc, and a 1 μF bypass capacitor with an ESR of 1 Ω is connected to the battery BAT pin to keep the ripple voltage at a low level when there is no battery. At the NTC pin, a 10 kΩ negative temperature coefficient resistor RNTC is connected in series with a 4 kΩ resistor, and the voltage division on RNTC is used as the input of the NTC pin.

2 Overall simulation results of linear lithium-ion battery charger

In the simulation, in order to shorten the simulation time, the battery is equivalent to a large capacitor CBAT, and its equivalent series resistance is RESR. 2 is the result obtained after simulating the preset charger chip characteristic parameter table.

2.1 Electrical appliance charging process waveform

Figures 3 to 5 are the results of the simulation of the charger's charging process under different conditions. To shorten the simulation time, the battery voltage is preset to 2.3 V so that the charging process can quickly transition from trickle charging mode to constant current charging mode.

In the simulation, the value of RPROG is set to 3 kΩ, the trickle charge current is 50 mA, and the constant current charge current is 500 mA; the SEL pin is grounded, and the final charge voltage of the battery is 4.1 V. As shown in Figures 3 to 5, the charger can work normally under various working conditions. In the curve of the relationship between the charging process and temperature in Figure 4, when the temperature is 125°C, the charging current is zero. This is because the intelligent thermal adjustment temperature Tc in the chip is 105°C. The normal operation of the intelligent thermal adjustment circuit causes the chip's charging current to drop to zero at 125°C, and the battery voltage is maintained at 2.3 V.

2.2 Charger charging current and intelligent thermal adjustment waveform

The charger charging current and intelligent thermal adjustment waveforms are shown in Figure 6. As shown in Figure 7, when the chip temperature reaches around 105°C, the intelligent thermal adjustment circuit automatically starts to reduce the charging current to reduce the power consumption of the chip.

2.3 Simulation data of battery final charging voltage under the worst case

To ensure that the final charging voltage of the lithium-ion battery meets the requirements even in the worst case, cross-simulation of all corners RES_TT, RES_FF, RES_SS of the resistor and all corners TT, FF, FS, SF, SS of the MOSFET is performed on the whole circuit, and the typical and worst cases of the final charging voltage of the battery are obtained after simulation as shown in Table 2 and Table 3. Among them, Table 3 is the simulation result obtained when the reference voltage is 2.485 V without fine-tuning, and Table 4 is the simulation result obtained after fine-tuning the reference voltage 2.485 V.

From the simulation results in Table 2 and Table 3, it can be seen that the simulation results of the final charging voltage of the battery meet the design requirements and their accuracy is as follows:

(1) After VREF fine-tuning, when SEL = 0 or Vcc, VBAT can be controlled at 4.1 V or 4.2 (1 ± 0.4%) V.

(2) When VREF is not fine-tuned, SEL = 0 or VCC, VBAT can be controlled at 4.1 V or 4.2 (1 ± 0.8%) V.

3 Conclusion

This chip has the characteristics of fast charging speed, strong battery protection function and low cost. It is a more practical intelligent lithium battery charger chip.