Portable device charging power circuit design

　　This article introduces the design and production of a portable power system with strong versatility and low cost. The system has two power supply modes, which can be powered by an external power supply or a built-in lithium battery. The final output voltage of the system is 3V. When both exist at the same time, the external power supply is preferred. It has two external power interfaces, power adapter and USB interface. When both are used at the same time, the power adapter has priority. This system can be widely used in various portable devices and has strong practicability and good market prospects.

　　Input selection circuit module

　　The input selection circuit is used to select the external power supply. This design uses the current mainstream USB power supply and power adapter power supply to adapt to different power supply environments. The power supply voltage of the external power supply needs to be between 4.5V and 6V. , when both coexist, the adapter has priority. The specific implementation method is shown in Figure 1, which is divided into the following three situations:

　　Figure 1 Input selection circuit

　　●Only the power adapter supplies power, and the PMOS tube is cut off. After the input voltage is stepped down by D1, it supplies power to the subsequent circuit. D1 uses a Schottky diode, and the conduction voltage drop is about 0.3V;

　　●Only USB supplies power, the PMOS tube is turned on, and D1 is used to prevent the USB interface from consuming power through resistor R2;

　　●Both exist at the same time, the PMOS tube is turned off, and the input voltage of the power adapter is stepped down by D1 to supply power to the subsequent circuit.

　　Lithium battery charge management circuit module

　　The lithium battery charging circuit uses the CN3052 lithium battery charging chip. CN3052 can charge a single lithium battery with constant current or constant voltage. It only requires very few peripheral components. The charging current can be set programmably. The constant voltage charging voltage is 4.2V. And it complies with the USB bus technical specifications and is very suitable for portable applications. The application circuit as shown in Figure 4 requires only a few external components, the output voltage is 4.2V, and the accuracy can reach 1%. CE is the chip enable terminal, which is active at high level. The green LED is used to indicate whether the battery is in a fault state, and the red LED is used to indicate whether it is in a charging state. In this design, the TEMP pin is connected to ground and the temperature detection function is not used. R4 is used to set the constant current charging current. In the design, R4 is 10KΩ and the charging current is 180mA.

　　Figure 2 Lithium battery charging management circuit

　　Battery output voltage stabilizing circuit module

　　Because the output voltage of the lithium battery can vary between about 3.5~4.3V when the power of the lithium battery is different, a low dropout linear regulator (LDO) is used to stabilize the battery output voltage. After stabilization, a constant voltage of 3.3V is output. This The design uses the TPS76333 voltage stabilizing chip, which requires very few external components and is easy to use. This voltage stabilizing chip can output a maximum current of 150mA. The circuit diagram is shown in Figure 5.

　　Figure 3 Battery voltage stabilizing circuit

　　External power supply voltage stabilizing circuit module

　　Because when the battery is powered, the output current is limited after being stabilized by the LDO circuit. When there is an external power supply, the SPX1117-3.3V voltage regulator is used to stabilize the voltage, and the output current can reach 800mA. Alternating current can be converted into direct current after rectification, but its voltage is unstable: changes in the supply voltage or power current can cause fluctuations in the power supply voltage. To obtain a stable DC power supply, a voltage stabilizing circuit must be added. The circuit diagram is shown in Figure 6.

　　Figure 4 External power supply voltage stabilizing circuit

　　System overall circuit module

　　The overall circuit of the system is shown in the figure. The input selection circuit selects the power supply mode of the external power supply. The voltage value of the power input is 4.5~6 volts. When there is an external power supply, it is directly regulated by a 3.3V voltage regulator and then output. If the battery power is insufficient, it is charged by a lithium battery at the same time. The circuit charges the lithium battery; when there is no external power supply, it is powered by the lithium battery, which is stabilized by a 3.3V low-voltage dropout linear regulator and outputs. The power supply selection circuit selects whether it is powered by an external power supply or a lithium battery based on whether there is an external power supply.

　　Figure 5 Overall circuit

　　Review analysis:

　　The system implements a fully integrated variable bandwidth intermediate frequency wideband low-pass filter. The structure, design and specific implementation of the transconductance amplifier-capacitor (OTA-C) continuous time filter are discussed and analyzed. External programmable circuits are used to control all the filters. Design the filter bandwidth for control, and use ADS software for circuit design and simulation verification. The simulation results show that the filter bandwidth has an adjustable range of 1~26 MHz, the stopband rejection rate is greater than 35 dB, and the in-band ripple is less than 0.5 dB. It uses a 1.8 V power supply and TSMC 0.18μm CMOS process library simulation, and the power consumption is less than 21 mW, the frequency response curve is close to the ideal state.