Intelligent Monolithic Linear Li-Ion Battery Charger IC Design

Abstract: Lithium-ion batteries have been widely used in portable devices due to their small size, light weight, high energy density and long cycle life. Since the service life of lithium-ion batteries is closely related to the charging method of lithium-ion battery chargers, chargers must be safe, fast and efficient. Considering the cost of IC, a monolithic linear tantalum-ion battery charger IC with intelligent thermal adjustment function is designed using CMOS technology. The linear lithium-ion battery charger IC designed here adds trickle charging mode and intelligent thermal adjustment mode on the basis of constant current/constant voltage charging mode.
Keywords: linear charger; lithium-ion battery; constant current charging; constant voltage charging; intelligent thermal adjustment

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. It can better meet the requirements of portable devices for miniaturization, light weight, long working time and long cycle life, and harmlessness to the environment. 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 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 trickle-constant current-constant voltage three-stage charging method 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 actual application of the chip, the lithium-ion battery charger designed in this paper has the following characteristics:
(1) Chip temperature protection During the charging process, when the battery voltage reaches the trickle charge jump voltage threshold and enters the constant current stage, the constant current stage is a large current charging. Since the power tube in this paper is 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)
Large 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℃, the thermal feedback loop is started to maintain the chip temperature at 105℃. When the battery voltage rises further, it can be seen from formula (1) that the power dissipation gradually decreases. Under a 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 the user does not need to worry about the chip temperature being too high.
(2) Cost. The chip introduced in this article is designed with 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 4.2 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.

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 charger charging process simulated under different conditions. In order to shorten the simulation time, the battery preset voltage is 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 charging 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 charging process and temperature relationship curve 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 always maintained at 2.3 V.

2.3 Simulation data of final battery charging voltage under the worst case
To ensure that the final charging voltage of the lithium-ion battery meets the requirements even under 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. After simulation, the typical and worst case final battery charging voltages are obtained 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 to 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 is fine-tuned, 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, when 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.