Mobile phone to achieve three elements of charging
To achieve fast charging on mobile phones, three elements must be met, and the three are indispensable. Charger, battery, chargeIC. The charger needs to meet enough output current and output voltage because the charger's trace has a large parasitic resistance. If a larger charging current is to be achieved, the charger's on-load output voltage needs to be higher. Estimated by the parasitic resistance of the charger is 1 ohm. The path resistance of the motherboard charger interface VCHG to VBAT is about 1 ohm. If the current is fast calculated by 1A fast charge, the output resistance of the charger needs to be around 6.5V. .
At present, the batteries used in mobile phones are all lithium-ion batteries. When the mobile phone is working, the battery is continuously discharged, the battery voltage is continuously decreased, the current discharged by the battery is different, and the curve rate of the voltage drop is also different. Generally, the battery voltage is 3.5V to 4.2. 90% of the battery energy is concentrated between V. The battery energy distribution is shown in the figure below.
In combination with the actual situation of the lithium battery, the working setting of the general mobile phone is set to be unable to talk or forcibly shut down when the lithium ion battery is discharged to a certain degree. 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 energy distribution of the battery and the actual situation of the use of the mobile phone, the charging current that affects the charging time is the charging current in the constant current charging phase. The current charging current of the mainstream mobile phone is about 450 mA. If you want to achieve fast charging, the most practical way is to increase the charging time of constant current. The best charging rate for lithium batteries is 1C, which means that a 1000mAh battery pack needs to be quickly charged at 1000mA. Charging at this rate can achieve the shortest charging time without reducing the performance and shortening the battery pack. life. For a battery with an increasing capacity, to achieve such a satisfactory charging rate, it is inevitable to increase the charging current value.
This article takes the charging of MT6235 as an example. The mobile phone charging is divided into turbulent flow, constant current, and constant voltage charging. When the VBAT voltage is less than 3.3V for trickle charging, the VBAT voltage is charged at 3.3~4.1V for constant current charging, and is greater than 4.1V until the charging is completed for constant voltage charging.
Shanghai Aiwei Electronics' AW3208, specially launched for the MT6235/36 platform, provides an intelligent charging function to meet the function of fast charging while increasing the OVP function. As shown in Figure 2, the AW3208 has built-in proprietary K-ChargeTM technology that intelligently adjusts the output current based on the chip temperature to ensure the safety of the entire charging system during charging. This makes the mobile phone not achieve normal charging due to high ambient temperature or excessive charging current, which causes the chip to enter over-temperature protection. Figure 3 is a typical application diagram on the MT6235.
In addition to the integrated K-Charge technology, the AW3208 uses a DFN2X2-8L package. The AW3208 has a large heat sink at the bottom of the chip. Compared to the MT6236's early BJT+NMOS solution, the heat dissipation and maximum power consumption performance. There is a big improvement. The bottom left picture shows the AW3208 test demo, flying to the MT6235 machine to do the actual charging test, in ACIN=5.25, VBAT=3.3V (using Agilent power), the phone charging is set to 800mA, charging time lasts 4 hours, the board temperature meets Shanghai Large IDH standard (temperature does not exceed 15 degrees Celsius).
Note 1: External 1uF capacitor on GATDRV ensures that the over discharge battery is activated.
Note 2: 1KΩ ensures no adjustment to the charging software
USB in the fast charging program
Now almost all mobile phones support USB charging. The current of the USB interface is 500mA, and the voltage is +5V. If the USB charging is still in the fast charging mode of the original charger, it may not be as fast as the USB interface. The current causes the charging to fail. For this fast charging application, the current proposed solution is to modify the charging software flow. When the software detects the VCHG voltage, do not open the charging module first, perform detection to detect whether USB charging, if it is charging current, perform normal charging. Mode, otherwise support fast charge mode.
AW3208 fast charge program in the layout of the precautions
AW3208 can not only support domestic chargers for fast charging, but also integrates the function of step-down OVP with AW3208, and can also support fast charging and charging when the output voltage of overseas Nokia charger is high. The high voltage brings high power consumption. In order to ensure the AW3208 can achieve the largest and most reliable charging current, please ensure that the AW3208's heat sink is connected to the PCB's main ground, and it is better to place the AW3208 in the middle of the board. The chip is cooled.
to sum up
Whether it is a smart phone or a smart phone era, the battery life has become the biggest bottleneck restricting the development of mobile phones from ordinary calls, SMS functions to large screens, games and other functions. The AW3208 fast charging solution based on the smart phone is only a compromise solution. How to solve the battery life has become a topic of research in the entire mobile phone industry chain. Battery manufacturers responded to the concept of national green environmental protection, and also began to introduce solar cell phone batteries. In order to reduce IC power consumption, manufacturers of mobile phone ICs are also constantly upgrading process layers and design capabilities to introduce ultra-low power products.
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