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LP2951 Application Note: One Button Switch Circuit

  • Contents

Hi there! In this blog, we will introduce you a simple and stable one-button switch circuit based on LP2951.

In daily life, products such as regular electronic equipment and smart instruments are usually powered by batteries, and single-button switch circuits are commonly used forms of power supply, with various implementation methods. Generally speaking, we can use programmable devices such as flip-flops, 555 integrated circuits and single-chip microcomputers. However, in actual operation, these implementations will increase the complexity of the entire circuit, making it difficult to achieve concise and practical effects.

The simple and stable one-button switch circuit based on LP2951 can achieve the effect of introduction and use. So, let's get started with a basic Introduction to LP2951.

Catalog

1 Introduction to LP2951

2 Design of power supply circuit

3 Working principle

3.1 Working principle of booting

3.2 Working principle of shutdown 

4 Conclusion

Component Datasheet

FAQ

1 Introduction to LP2951

LP2950/LP2951 are low-power regulators with the following characteristics:

Its quiescent current is small, and the input and output voltage difference is low, so it is suitable for battery-powered systems. In addition, when the battery voltage drops for a long time, the quiescent current increases very little.

LP2950 is a 3-pin TO-92 package. Figure 1 below shows the LP2951 pinout and Figure 2 shows the internal structure of the LP2951.

LP2950 Pinout

Figure 1. LP2951  pinout

LP2951 Internal Structure

Figure 2. LP2951 internal structure 

Among them, LP2951 has the following characteristics related to power supply:

  1. Logic controls the electronic switch, the low level is effective, and the power chip is turned off when the high level is connected.
  2. The output voltage is adjustable within 1.24~30V. When there is no regulating circuit, the default output voltage is 5V.
  3. The maximum output current is 100mA.
  4. It has a shutdown function when the battery voltage is too low. In normal working state, Pin5ERROR terminal outputs high level, when the external power supply voltage is lower than the working voltage, ERROR terminal outputs low level.

 

2 Design of power supply circuit

Since smart instruments have multiple functions, they are often implemented by multiple keys. Now we set two buttons as the power-on button; set one button as the power-off button.

The design power circuit is shown as in Fig. 3.

Hardware circuit diagram of the power supply circuit

Figure 3. Hardware circuit diagram of the power supply circuit

In Figure 3:

  • Button S1 is the shutdown button;
  • S2 and S3 are the power-on buttons;
  • VDD is the DC regulated output voltage of the power chip LP2950, +5V;
  • VFH is the battery voltage;
  • U2 is a low-voltage shutdown power supply circuit with an output voltage of VCC;
  • U3 and its peripheral circuit are composed of a power supply circuit with adjustable output voltage, and the output voltage is VCC1.

Among them, VREF is the internal reference voltage of LP2951 1.235V; IFB is the feedback terminal current -20nA.

3 Working principle

3.1 Working principle of booting

Press the function key S2 or S3 arbitrarily to make the power chip work. Both work on the same principle, and the specific working process is as follows.

In the shutdown state, the U1-1A and U2-1A input terminals of the NAND gate CD74HC132 are high level, and the corresponding output terminal is low level.

When S2 or S3 is pressed, according to the capacitance characteristics, a low level is formed at the input end of U1-1A, and the output end of U1-2B, U1-3C, that is, Pin8 is a high level. A high level appears at the U2-2B input terminal through C4, R7 and diode CR1, so that the 6 pin of U2 is low. In this way, make the power chip LP2951 work, and make its ERROE output high level.

After powering on, press S2 or S3 several times to realize the function. Because ERROE outputs a high level, the U2-2B input end maintains a high level, so that the SHUTDOWN end is guaranteed to be low level and the power chip is kept working.

3.2 Working principle of shutdown 

In the power-on state, press the shutdown key S1 to make the 3 terminals of U2 and the 8 terminals of U1 become "0" and the 6 terminals of U2 to "1". That is, SHUTDOWN of LP2951 is high, turn off the power chip, so that the ERROR terminal outputs low level;

When the button is lifted, the 3 terminal of U2 and the 8 terminal of U1 become "1", but the ERROR terminal is "0", so it is always in the shutdown state. When the external battery voltage is too low, the ERROR terminal of LP2951 outputs a low level, thus shutting down.

4 Conclusion

The power supply circuit was once used in the signal source of cable detection equipment, and the working effect was good. The signal source of the cable detection equipment is shown in Figure 4. It can be seen from the circuit structure and principle that the circuit has a simple structure, easy to use, reliable, and versatile.

The actual signal source of the cable detection equipment

Figure 4. The actual signal source of the cable detection equipment


Component Datasheet

LP2951 Datasheet


FAQ

  • What is the Output Voltage of LP2951? 

5 V

 

  • What is the Max Output Current of LP2951? 

100 mA

 

  • What is the Max Input Voltage of LP2951?  

30 V

 

  • What is the Output Type of LP2951? 

Fixed

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