Fast charging solutions for smartphones

Preface
In the early days, there was an Apple that created the earth's gravity. Now there is an Apple that has accelerated the arrival of the smart phone era. Since last year, major mainstream baseband manufacturers such as MTK, Qualcomm, Broadcom, Marvell, STE, etc. have launched smart phone solutions. In the 2G era, feature phones have become saturated, and competition is becoming increasingly fierce. Major mobile phone companies have bet on smart phones, hoping to seize the opportunity. Unfortunately, in the era of smart phones, no company can provide tunekey like MTK in the era of feature phones, and there is a serious shortage of software driver personnel. Smart phones have become an investment with high investment and a long payback period, so that only large companies such as Huawei and ZTE can gain something in smart phones. There are very few IDHs in China that focus on PCBA that have mass production of smart phones and are making money.

According to isuppli statistics, although the growth of smart phones in the past two years has been extremely amazing, more than 70% of users in the world still use feature phones, and in China, this proportion is about 85%. In addition to the many challenges that domestic IDHs have encountered in the development of smart phones, mobile phone chip giant MediaTek has also recently stated that they believe that most users still like relatively cheap feature phones, so the feature phone market still has broad room for development. Recently, MediaTek will launch a "quasi-smartphone" to allow feature phones to be superimposed with rich applications like smart phones. This quasi-smartphone will support user interface, Internet access, and large-screen full touch. Through the implementation of these functions, quasi-smartphones can download software through the Internet, allowing feature phones to continuously expand personalized applications like smart phones. Like

smart phones, as the functions of quasi-smartphones continue to become more powerful, battery life will become a dead end. In real life, we may often see friends around us carrying two mobile phones, one for making calls and the other for entertainment. Because no matter how big your mobile phone screen is or how high the performance configuration is, everything is nothing if the battery is out of power. Especially in third world countries such as India, there are still many places where charging is a problem. In 2007, executives of Micromax, an Indian pay phone company, noticed a new phenomenon. They found that in a village in eastern India, people lined up in front of a man with a bicycle behind him. The man's bicycle was tied with a car battery. After giving him a few rupees, they could plug their mobile phones into it and charge for half an hour. These villagers had no electricity at home.

In order to solve the problem of battery life, early feature phone manufacturers such as Changhong and Phillip developed large-capacity battery mobile phones, or dual-battery mobile phones using Awin's dual-battery single-chip solution AW3312. However, both applications have high requirements for the structure of mobile phones. In order to solve the battery problem, engineers in the mobile phone industry have come up with a roundabout way to solve the battery problem in the era of smartphones - "fast charging".

Three elements for mobile phone charging
To realize the fast charging function on a mobile phone, three elements must be met, and none of them can be missing. Charger, battery, charge IC. The charger needs to meet sufficient output current and output voltage, because the charger's wiring has a large parasitic resistance. If a larger charging current is to be achieved, the charger's load output voltage needs to be higher. Assuming the parasitic resistance of the charger is 1 ohm, the path resistance from the motherboard charger interface VCHG to VBAT is also about 1 ohm. If we simply calculate it based on the current of 1A fast charge, the output resistance of the charger needs to be around 6.5V.

Currently, the batteries used in mobile phones are all lithium-ion batteries. When the mobile phone is working, the battery is constantly discharged and the battery voltage is constantly decreasing. The voltage drop curve rate is different for different battery discharge currents. Generally speaking, 90% of the battery energy is concentrated between the battery voltage of 3.5V and 4.2V. The battery energy distribution is shown in the figure below.

Combined with the actual situation of lithium batteries, the working settings of general mobile phones are that when the lithium-ion battery is discharged to a certain extent, it will be set to be unable to make calls or forced to shut down through software. Deep discharge will cause irreversible damage to the life of lithium-ion batteries, so the shutdown voltage is generally set to about 3.5V. From the perspective of battery energy distribution and the actual situation of mobile phone use, the charging current in the constant current charging stage has the greatest impact on the charging time. The current charging current set by mainstream mobile phones is about 450mA. If you want to achieve fast charging, the most practical way is to increase the constant current charging time. The optimal charging rate of lithium batteries is 1C, which means that a 1000 mAh battery pack should be quickly charged at a current of 1000mA. Charging at this rate can achieve the shortest charging time without reducing the performance of the battery pack or shortening its service life. For batteries with increasing capacity, it is inevitable to increase the charging current value in order to achieve this satisfactory charging rate.

This article takes the charging of MT6235 as an example. Mobile phone charging is divided into trickle charging, constant current charging, and constant voltage charging. When the VBAT voltage is less than 3.3V, it is trickle charging. When the VBAT voltage is between 3.3 and 4.1V, it is constant current charging. When it is greater than 4.1V and charging is completed, it is constant voltage charging.

The AW3208 launched by Shanghai Awin Electronics specifically for the MT6235/36 platform not only adds the OVP function, but also provides the intelligent charging function to meet the fast charging function. As shown in Figure 2, the AW3208 has a built-in proprietary K-ChargeTM technology that can intelligently adjust the output current according to the chip temperature to ensure the safety of the entire charging system during charging. When the mobile phone is fast charged, it will not be unable to charge normally because the chip enters over-temperature protection due to high ambient temperature or excessive charging current. Figure 3 is a typical application diagram on MT6235.

In addition to the integrated K-Charge technology, the AW3208 uses a DFN2X2-8L package, and there is a large heat sink at the bottom of the chip. Compared with the early BJT+NMOS solution of MT6236, the heat dissipation and maximum power consumption performance are greatly improved. The lower left picture is the AW3208 test demo, with flying wires to the MT6235 machine for actual charging test. At ACIN=5.25, VBAT=3.3V (using Agilent power supply), the mobile phone charging is set to 800mA, the charging time lasts for 4 hours, and the board temperature meets the standards of a large IDH in Shanghai (the temperature does not exceed 15 degrees from room temperature).

Note 1: Connect a 1uF capacitor to GATDRV to activate the over-discharged battery.
Note 2: 1KΩ ensures that the charging software does not need to be adjusted.

USB solution for fast charging
Now almost all mobile phones support USB charging. The current of the USB interface is 500mA and the voltage is +5V. If you still use the fast charging mode of the original charger when plugging in USB for charging, the charging may fail because the USB interface cannot provide such a large current for fast charging. For this kind of fast charging application, the current recommended solution is to modify the charging software process. When the software detects the VCHG voltage, do not turn on the charging module first, and execute the detection whether it is USB charging. If it is charging current, execute the normal charging mode, otherwise support the fast charging mode.

Precautions for AW3208 fast charging solution in layout
AW3208 can not only support domestic chargers to achieve fast charging, but also support overseas Nokia chargers with high output voltage because AW3208 integrates the function of step-down OVP. High voltage will inevitably bring high power consumption. In order to ensure that AW3208 can achieve the maximum and most reliable charging current, please make sure that the heat sink of AW3208 is connected to the main ground of PCB, and it is best to place AW3208 in the middle of the board to facilitate chip heat dissipation.

Summary
Whether it is the era of quasi-smartphones or smart phones, battery life has become the biggest bottleneck restricting the development of mobile phones from ordinary call and text messaging feature phones to large screen, game and other integrated smart phones. The AW3208 fast charging solution launched based on quasi-smartphones can only be regarded as a compromise solution. How to solve the battery life has become a research topic for the entire mobile phone industry chain. Battery manufacturers respond to the national green environmental protection concept and have also begun to launch solar mobile phone batteries. In order to reduce IC power consumption, mobile phone IC manufacturers are also constantly upgrading process manufacturing and design capabilities to launch ultra-low power consumption products.