Application charger IC for single lithium ion battery

There are many options for single Li ion (Li-Ion) battery chargers. With the continuous development of handheld devices business, the demand for battery charger is also increasing. To select the right integrated circuit (IC) for completing this work, we must weigh several factors. Before starting design, we must consider factors such as solution size, USB standard, charging rate and cost. These factors must be arranged in order of importance, and then select the corresponding charger IC. In this article, we will introduce different charging topology and study some characteristics of battery charger IC. In addition, we will explore an application and existing solutions.
Charge cycle for lithium ion batteries
Lithium ion batteries require special charging cycles to achieve safe charging and maximize battery life. Battery charging can be divided into two stages: constant current (CC) and constant voltage (CV). When the battery is below full full voltage, the current enters the battery through a regulated voltage. Under the CC mode, the current reaches one of the two values through a regulated voltage. If the battery voltage is very low, the charging current will be reduced to the pre charge level to suit the battery and prevent battery damage. The threshold is different from the chemical properties of the battery, and generally depends on the battery manufacturer. Once the battery voltage rises above the precharge threshold, the charge rises to the fast charging current level. The maximum recommended fast charge current for a typical battery is 1C (the current required for battery consumption within C=1 hours), but the current depends on the battery manufacturer. The typical charging current is ~0.8C, aiming at maximizing battery life. When the battery is charged, the voltage rises. Once the battery voltage rises to a regulated voltage (usually 4.2V), the charging current gradually decreases, while the battery voltage is stabilized to prevent overcharging. In this mode, the current gradually decreases while the battery impedance decreases. If the current falls to a predetermined level (usually 10% of the fast charge current), the charge is terminated. We generally do not charge the battery floating, because this will shorten the battery life. Figure 1 illustrates the typical charging cycle graphically.

Figure 1 Typical lithium ion charging cycle
Comparison between linear solution and switch mode solution
There are two ways to turn the adapter voltage into battery voltage and control the topology of different charging stages: linear regulator and inductive switch. These two topologies have their own advantages and disadvantages in terms of volume, efficiency, solution cost and electromagnetic interference (EMI) radiation. We will introduce the advantages and some compromise methods of these two topologies.
Generally speaking, inductive switches are the best choice for achieving maximum efficiency. The charging current is detected at the output end by means of resistors and other detection components. When the charger is in CC mode, the current feedback circuit controls the duty cycle. The battery voltage detection feedback circuit controls the duty cycle in the CV mode. Depending on the characteristic set, other control loops may appear. We will discuss these loops in detail later. The inductance switch circuit requires switch components, rectifiers, inductors and input and output capacitors. For many applications, the size of the solution can be reduced by choosing a device that embeds switch components and rectifiers into IC. According to different loads, the typical efficiency of these circuits is 80% to 96%. Switching converters usually require more space and are more expensive because of their inductance size. The switching converter also causes inductive EMI radiation and the output noise caused by the switch.
The linear charger reduces the DC voltage by reducing the input voltage of the bypass component. The advantage of this is that the solution only requires three components: bypass components and input / output capacitors. Compared with inductive switches, linear voltage regulator (LDO) is usually a low-cost solution with lower noise. The current of the battery is restricted by the resistance of the voltage bypass component to control the charging current. Current feedback generally comes from the input of the charger IC. The battery voltage is detected to provide CV feedback. Changing the resistance of bypass components to maintain a constant current or constant battery voltage into the IC input. The input current of the device is equal to the load current. That is to say, the efficiency of the solution is equal to the ratio of output voltage to input voltage. The drawback of the LDO solution is that the efficiency of the high input and output voltage ratio is low. All power is consumed by bypass components, which means that LDO is not ideal for high charging current applications with large input and output differences. These high power applications require heat dissipation, thereby increasing the size of the solution.
Calculation of power and temperature rise
Among them, the ETA is the efficiency of the charger, and POUT = VOUT * IOUT. Using thermal resistance, the temperature rise caused by power consumption can be calculated. The thermal resistance of each application varies, depending on the specific parameters such as layout, airflow and encapsulation. We should model thermal resistance for terminal PCB. Keep in mind that the definition of "JA" in the product specification is not an appropriate representation of thermal resistance in this application.

What topology should be used?
The first parameter you need to study is the charging current. For some small applications, for example, a Bluetooth TM headset with a charge current between 25Ma and 150mA, the best solution is almost linear charger. These applications generally have very small volume and can not provide extra space for more components of the switch. In addition, because of its non