Nanxin SC8551—the first high-voltage charge pump charging IC in China, the charging efficiency is as high as 98%
Nanxin DCDCIC
1 Introduction

With the popularization of 4G network and the advent of the 5G era, the battery capacity of mobile phones is getting larger and larger to meet the high energy consumption requirements of various applications such as mobile Internet. Using traditional charging technology, the charging time is getting longer and longer, which cannot meet daily needs. The fast charging technology came into being, which allows people to use the fragmented time to quickly restore the power, and has gradually become the standard for more and more mobile phones.

There are two different technical routes for fast charging: high-voltage low-current fast charging and low-voltage high-current fast charging. The representative of high-voltage fast charging is Qualcomm. QC2.0, which Qualcomm first launched around 2014 , increased the charging voltage from the traditional 5V to 9V/ 12V and the charging power to 18W. However, high-voltage fast charging has an insurmountable disadvantage - the charging IC generates serious heat. After the 9V/12V voltage output by the adapter enters the mobile phone, it will be stepped down twice by the buck charger inside the mobile phone to charge the battery. The traditional buck converter has low conversion efficiency under high-voltage input conditions, causing the chip to heat up very seriously, and the power cannot be used. further improvement.

The technical route of low-voltage direct charging is represented by OPPO and HUAWEI. OPPO launched VOOC flash charging 22.5W (5V/4.5A) low-voltage fast charging technology in 2014. Since low-voltage fast charging does not require intermediate-level voltage conversion, the real adapter can "directly charge" the battery, so the problem of heating of the charging IC is well solved. However, in order to solve the problems of the small current capacity of the MicroB interface and the large resistance of the traditional charging wire, low-voltage fast charging needs to customize the charging interface and charging cable, and the cost is higher than the high-voltage fast charging solution.

2. Charge pump fast charging

A charge pump is a non-inductive DC-DC converter that uses a capacitor as an energy storage element to convert voltage and current. The half-voltage charge pump (2:1 charge pump) can achieve half the output voltage and double the output current, and the conversion efficiency can reach more than 97%, which is much higher than that of ordinary charging ICs, thus solving the problem of charging during high-voltage fast charging. IC heating problem. In addition, since the charge pump can naturally achieve the effect that the input current is half of the output current, the cost of wires and interfaces can be greatly reduced compared to the low-voltage direct charging solution.

It can be seen that the charge pump fast charging can perfectly resolve the contradiction between high voltage and high current, breaking the limit of mobile phone fast charging.

Figure 1. Typical application diagram of SC8551

3. SC8551: Compatible with charge pump fast charging and low voltage direct charging

SC8551 is Nanxin's latest charge pump fast charging IC for the mobile phone fast charging market. As the first high-voltage charge pump fast-charging IC in China, SC8551 also pioneered the dual-mode charging function of high-voltage fast-charging and low-voltage direct-charging. SC8551 adopts 56pin CSP package, and the chip size is 3.32mm*3.35mm. Figure 1 is a typical application diagram of SC8551. During the charging process, SC8551 acts as a slave charging IC in the master-slave charging architecture, and starts to work after the mobile phone enters the fast charging stage. Its main features are as follows:

1. Dual mode: with charge pump 2:1 step-down charging and bypass charging two modes

2. High efficiency: under the condition of charging current above 6A, the efficiency is increased by 0.4% compared with similar foreign products

3. Perfect protection mechanism: 26 layers of protection ensure safe and reliable charging

a) Dual Mode

SC8551 supports charge pump 2:1 step-down charging mode and low-voltage direct charging mode at the same time. As shown in Figure 2(a), when working in the charge pump charging mode, the internal switch tubes Q1-Q8 of the chip are always in the state of alternate switching, and the peripheral flying capacitor CFLY1/2 is in the state of alternate charging and discharging, and the input adapter's The energy is moved into the battery. During steady-state operation, the input voltage is slightly higher than twice the battery voltage, and the charging current can support up to 8A.

When SC8551 works in bypass mode, as shown in Figure 2(b), the internal switches Q1/Q2/Q5/Q6 are always on, and the switches Q3/Q4/Q7/Q8 are always off. At this time, the adapter directly charges the battery through several switches in series and parallel. As long as the equivalent resistance from VBUS to VOUT is small enough, the temperature rise of the chip can be controlled to be very low. The maximum charging current supported by the SC8551 in bypass mode is 6A.

(a) High-voltage charge pump fast charge mode

(b) Low-voltage direct charging mode Figure 2. Schematic diagram of SC8551 in different working modes

b) high efficiency

Considering that the application of charge pump fast charging IC is mainly in the case of high current, SC8551 focuses on optimizing the efficiency of charging current above 6A. Compared with similar foreign products, the efficiency under heavy load is increased by about 0.4%. Figure 3 shows the efficiency curves of SC8551 at different frequencies. It can be seen from the figure that even if the charging current reaches 8A, the charging efficiency of SC8551 is still above 96%.
Figure 3. Efficiency curve of SC8551 operating in charge pump mode

Figure 4 shows the equivalent on-resistance curve of SC8551 from VBUS to VOUT in bypass mode. As can be seen from the figure, the equivalent resistance is about 18mohm. As the charging current increases, the equivalent resistance increases slightly.
Figure 4. On-resistance curve of SC8551 operating in bypass mode

Figure 5 is a schematic diagram of the temperature rise of SC8551 working in different modes. Figure 5(a) is a schematic diagram of the temperature rise when the charging current is 8A in the charge pump mode. As can be seen from the figure, the temperature rise of the chip surface is about 59.6-25=34.6°C (the ambient temperature is about 25°C). Figure 5(b) is a schematic diagram of the temperature rise when the charging current is 6A in the low-voltage direct charging mode. The temperature rise of the chip surface is about 44.5-25=19.5℃.

(a) Charge pump mode, VBAT=4.4V, IBAT=8A

(b) Low voltage direct charging mode, VBAT=4.4V, IBAT=6A

Figure 5. Schematic diagram of temperature rise of SC8551 in different modes

c) Perfect protection mechanism

SC8551 integrates 26 different protection mechanisms to ensure the safe and reliable charging process. The protection mechanisms can be divided into three categories: 12-level system-level protection, 7-level charge pump-related protection, and 7-level system-level alarms. Among them, after system-level protection and charge pump related protection are triggered, SC8551 will stop charging and output INT interrupt at the same time. After the system-level alarm is triggered, an INT interrupt will also occur, but the charging will not stop.

4 Conclusion

SC8551 is the first high-voltage charge pump fast charging IC in China, which is compatible with charge pump fast charging and low-voltage direct charging functions. Its maximum charging current in charge pump fast charging mode reaches 8A, and the current in low-voltage direct charging mode reaches 6A. With excellent heavy-load efficiency performance and comprehensive protection mechanism, SC8551 is very suitable for application in mobile phone high-power charging solutions.

5. About Nanxin

Shanghai Nanxin Semiconductor Technology Co., Ltd. was established at the end of 2015 in Zhangjiang Hi-Tech Park, Pudong, Shanghai. The start-up team of Nanxin is from Texas Instruments, with more than 10 years of industry experience, and has been deeply involved in the field of power supply. With the development of the company, Nanxin continues to introduce outstanding talents and build a high-quality R&D team. At present, it has R&D teams from well-known semiconductor companies such as Texas Instruments, Linear Technology, Li?, ADI, Rohm, O2, etc., and adheres to continuous innovative products. Culture, is committed to providing the industry with high performance, high quality and high cost-effective IC solutions.

Nanxin has been leading domestic counterparts in the research and development of fast charging and power management chips. It has launched the industry's first buck-boost buck-boost battery charging management IC, which can realize bidirectional intelligent charging and discharging up to 100W in the range of 2.7~40V. , Efficiently supports USB PD high-power charging and discharging applications, significantly reducing the time required for charging, and can be widely used in consumer electronics and charging peripheral products such as smartphones, PADs, TVs, notebooks, and automobiles. Nanxin has launched the first domestic charging IC compatible with charge pump fast charging and low-voltage direct charging. With its excellent performance, Nanxin's IC products have appeared frequently in the products of well-known domestic and foreign brands such as Huawei, Samsung, Xiaomi, and Anker, and sales continued to grow rapidly.

In January 2018, Nanxin Semiconductor announced the completion of tens of millions of RMB in Series A financing. In April 2019, Nanxin Semiconductor once again announced the completion of the B round of financing of nearly 100 million yuan, led by the Shanghai Integrated Circuit Industry Fund. Nanxin constantly explores and develops high-end chips, and has a strong first-mover advantage. It is a pioneer in the wave of replacement of domestic analog chips. With its solid technology and experience accumulation, it has established a leading technology, excellent quality and cost-effective products. The leading brand image of "High-end China Chip".