Analysis of key points and mainstream solutions for mobile phone charging management design

Although there are many brands of mobile phones on the market, their charging circuits are roughly the same, all using PMOS-based charging circuits. When using PMOS, it is easy to control its switch at a lower voltage, and the voltage on the mobile phone motherboard is mostly around 3V, so switch control can be easily achieved. In addition, MOS tubes are voltage-type control devices, and their power consumption is much lower than that of triodes.

Common mobile phone charging circuits can be divided into the following three types:

1. PMOS + Schottky diode: MTK and Spreadtrum platforms use this method. PMOS controls the charging switch and the size of the charging current. The Schottky diode prevents the battery from backflowing current through the internal parasitic diode of the PMOS, and its forward voltage drop is smaller than that of ordinary diodes when forward conducting.

Recommended products: BF9024SPD-M (8Pin), BF9024SPD-MS (6Pin): Please refer to Table 1 for main parameters.

Table 1. Main parameters of BF9024SPD series

Figure 1. PMOS+diode

Figure 2. Separation of PMOS and diode

BF9024SPD-M (8Pin) and BF9024SPD-MS (6Pin) are power devices for this application, as shown in Figure 1. In order to reduce the heat generation of MOS during long-term charging, BYD Microelectronics exposes the bottom heat sink on the package for better heat dissipation. Don't underestimate this heat sink, it not only improves the heat dissipation efficiency during charging, helps improve product reliability, but also enables the battery to be charged more fully. Because the internal resistance of MOS is a positive temperature coefficient, that is, the higher the temperature, the greater its internal resistance. At 50°C, its internal resistance will increase by about 5% for every 10°C increase in temperature. This heat sink can accelerate the heat dissipation of the device and control its internal resistance increase to less than 3%. When charging, the battery charging voltage can be about 50mV higher than similar products.

We recorded more than 2,000 data points when the battery voltage was charged from 2.8V to a stable 4.185V when BF9024SPD-M was applied on the MTK6223 platform. During the whole process from the pre-charge current of about 60mA to the constant current charging current of 550mA, and then to the pulse current of about 50mA when fully charged, the monitored battery voltage finally stabilized between 4.185 and 4.195, please refer to Figure 3; when charging with a USB port, the battery voltage was stabilized at 4.188V when fully charged, please refer to Figure 4. During the whole charging process, the battery full charge voltage consistency is very good, and the entire charging curve consistency is also very good.

Figure 3. Charging curve of BF9024SPD-M applied to MTK6223 platform - adapter charging

Figure 4. Charging curve of BF9024SPD-M applied to MTK6223 platform - USB port charging

Since the heat sink at the bottom of BF9024SPD-M and BF9024SPD-MS is connected to the drain (D) of PMOS and the cathode (K) of Schottky diode, it cannot be used as GND or connected to other signal lines during design. When designing the PCB layout, please pay attention to the fact that there should be no ground PAD or other signal lines at the bottom. For the corresponding layout, please refer to Figures 5 and 6 below.

These two products have been certified by Spreadtrum platform, you can refer to Spreadtrum's original reference design BOM.

Figure 5. BF9024SPD-M Layout Reference

Figure 6. BF9024SPD-MS Layout Reference

In low-cost applications, separate MOS tubes and Schottky diodes can be used to replace integrated devices, as shown in Figure 2. The small package of BF92301P can meet your design requirements. Please refer to Table 2 for its main parameters.

2. PMOS+PMOS: This method is adopted in TI and some MTK platforms and dual-battery platforms. This application is an improvement on the application of PMOS+Schottky diode. In the charging circuit of PMOS+Schottky diode application, the Schottky diode will occupy a voltage drop of more than 0.4V due to continuous conduction. After replacing the Schottky diode with a PMOS tube, the voltage drop can be greatly reduced because the conduction internal resistance of MOS is very small, thereby ensuring that the USB port or external 5V reference voltage can still be higher than the voltage required for charging a single lithium battery after the loss of the charging circuit.

Recommended product: BF9024DPD-MS. For main parameters, please refer to Table 2.

As mobile phone motherboards become smaller and smaller, and mobile phone functions become more and more, people hope that mobile phones or digital products can be used for a longer time after a single charge. Under this demand, the application of dual batteries has emerged. Dual PMOS can make good use of its extremely low on-state voltage drop and easy control of current single flow in dual battery applications.

3. PNP tube + PMOS: Qualcomm platforms almost all use this method. The PNP tube is used to control the charging switch and the size of the charging current, and the PMOS is used as a switching element to connect and disconnect the charging circuit.

Recommended product: BF92301P. For main parameters, please refer to Table 2.

Table 2. Main parameters of BF92301P and BF9024DPD-MS

With the continuous updating of products from various platform suppliers, the external circuit of PMOS+Schottky diode has always been one of the most concise and reliable choices in mobile phone charging management applications. From Spreadtrum's 6600L to 6600L6, 6600L7, and 6610K, this method has been used as a charging design. As for the new platform MTK6253 that MediaTek is currently promoting, in addition to the power management part it contains, it also concentrates overvoltage protection (OVP), constant current and other functions in its external circuit to form secondary protection. Of course, this approach has also appeared in MediaTek's early designs, that is, using PMIC (Power Management IC) to specifically handle the power supply part. However, with the maturity of application technology, the output interface of mobile phone adapters is unified, and its output voltage (5V±5%) can be completely consistent with the standard USB interface. Some design companies with R&D capabilities have applied PMOS+Schottky diodes to the MTK6253 platform, but they add a voltage regulator diode to the external power input part, thereby greatly saving the cost of the power management part.

Conclusion

In common charging circuits, PMOS is a key component in the application, and its quality and performance directly affect the charging heat and charging time. After years of market experience, BYD Microelectronics' MOS products have been greatly improved in terms of product quality, price, delivery time and product technical support.