Design and production of 3-INPUT portable device power supply

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. It 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 are present, the external power supply is preferred. There are two external power supply interfaces, the power adapter and the 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, with strong practicality and good market prospects.

The power module package is shown in Figure 1.

Figure 1 Power module

The power supply system principle block diagram is shown in Figure 2. 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 output after being stabilized by a 3.3V voltage regulator. If the battery power is insufficient, the lithium battery is charged by the lithium battery charging circuit at the same time; when there is no external power supply, it is powered by a lithium battery and output after being stabilized by a 3.3V low-voltage difference linear regulator. The power supply selection circuit selects whether it is powered by an external power supply or a lithium battery according to whether there is an external power supply.

Figure 2 Principle block diagram

2. Circuit Design

1. Input selection circuit

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 must be between 4.5V and 6V. When both exist, the adapter has priority. The specific implementation method is shown in Figure 3, which is divided into the following three cases:

Figure 3 Input selection circuit

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

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

●When both are present at the same time, the PMOS tube is cut off, and the input voltage of the power adapter is stepped down by D1 to supply power to the subsequent circuit.

2. Lithium battery charging management circuit

The lithium battery charging circuit uses the CN3052 lithium battery charging chip. The CN3052 can charge a single lithium battery with constant current or constant voltage, requiring only a few peripheral components, and can program the charging current. The constant voltage charging voltage is 4.2V. It complies with the USB bus technical specifications and is very suitable for portable applications.

The application circuit is shown in Figure 4. It only requires a few external components, the output voltage is 4.2V, and the accuracy can reach 1%. CE is the chip enable terminal, and high level is valid.

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 the ground and the temperature detection function is not used. R4 is used to set the constant current charging current, and the calculation formula is:

I = 1800V/ R4;

In this design, R4 is 10KΩ and the charging current is 180mA.

Figure 4 Lithium battery charging management circuit

3. Battery output voltage regulator circuit

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

Figure 5 Battery voltage regulator circuit

4. External power supply voltage regulator circuit

When the battery is powered, the output current is limited after the LDO circuit stabilizes the voltage. When there is an external power supply, the SPX1117-3.3V regulator is used for voltage stabilization, and the output current can reach 800mA. The circuit diagram is shown in Figure 6.

Figure 6 External power supply voltage regulator circuit

5. Power supply selection and output circuit

As shown in Figure 7, there are three situations:

When there is only an external power supply and no lithium battery, the power input voltage is stabilized by the SPX1117-3.3V regulator and the output voltage is 3.3V.

When there is only a lithium battery and no external power supply, the battery output voltage is stabilized by TPS76333, and the output voltage is 3.3V, which is then stepped down by D2. D2 is a Schottky diode, and the final output voltage is about 3V.

When both are present at the same time, D2 is cut off, powered by an external power supply, and outputs a 3.3V voltage.

Figure 7 Power supply selection circuit diagram

6. System overall circuit

The overall circuit of the system is shown in Figure 8.

Figure 8 Overall circuit

3. Physical production

The PCB wiring diagram designed using single-sided wiring is shown in Figure 9.

The finished effect of single-sided board manufacturing is shown in Figure 10.

The soldered circuit board is shown in Figure 11.

Figure 9 PCB wiring diagram

Figure 10: Plate making effect diagram

Figure 11 Actual picture

4. Components and their parameters