An anti-interference power supply based on low-power single-chip microcomputer

In recent years, various low-power microcontrollers have been widely used in various instruments, especially the 89C51/2 microcontroller, which has won a large number of users with its excellent performance, low price, standard power consumption reduction characteristics, and fast erasability of on-chip memory. However, various test instruments often require convenient portability, so the instrument power supply often uses a dedicated battery. The general dedicated battery voltage is 12V (or 12V in series), and the application system composed of 89C51/2 microcontrollers requires the power supply voltage Vcc to be within the range of 5×(1±0.1)V. Some instruments are often used in harsh environments with many interference factors. Therefore, in order to make the microcontroller system work reliably, a power supply circuit with strong anti-interference ability is very important.

1 Design and analysis of anti-interference power supply circuit

The anti-interference power supply circuit consists of two parts, as shown in Figure 1. The DC step-down circuit is composed of MAX638 as the core, which is required to convert the 12V DC voltage into 5V; the anti-interference circuit is composed of TL7705AC as the core, which is required to make the microcontroller enter the power-down mode when the power supply has transient undervoltage, instantaneous pulse interference or power failure.

Figure 1 Schematic diagram of anti-interference power supply circuit

1.1 DC step-down circuit

MAX638 is a step-down product in the MAX series of monolithic switching DC-DC voltage conversion circuits produced by MAXIM, USA. Its own working current is only 0.135mA, its output voltage is 5V, and the input voltage can be 5V~16.5V. Since MAX638 contains a MOS field effect tube with a peak switching current of 375mA, a +1.31V reference voltage source, a 65kHz self-excited oscillator and an error comparator, when used, only an external energy storage inductor L (220μH is selected in this system) and an output capacitor C (100μF is selected in this system) and a freewheeling diode can form a step-down DC power converter.

MAX638 also has a low power supply voltage detection circuit, which provides a detection voltage after dividing the input power supply through resistors R1 and R2. When the input voltage is as low as Vmin, the second pin of MAX638 outputs a low-level undervoltage alarm signal. Change the resistance value of R1 to change the size of Vmin. Since the acceptable input voltage range is wide, when the input voltage is not less than 5V due to interference, the microcontroller application system can still work normally.

1. 2 Anti-interference circuit

The 89C51/2 microcontroller has two power-saving operation modes: standby operation mode and power-down operation mode. When the 89C51/2 microcontroller enters the power-down mode, the on-chip oscillator stops working, various activities stop immediately, and only the on-chip RAM retains the original data. Using this feature, it is convenient to realize power supply anti-interference. As shown in Figure 1, diodes D2, D3 and battery B realize the switching of backup power when the main power is off. When the main power is normal, D2 is turned on, D3 is turned off, and Vout is powered by D2; when the main power is off, D3 is turned on, D2 is turned off, and the microcontroller is powered by battery B through D3. TL7705AC is a voltage monitor, and its detection standard value is 4.55V. When the main power supply drops to 4.55V for some reason, the 5th pin of TL7705AC outputs a low level, requests an interrupt to the microcontroller, and the microcontroller can enter the power-down mode through the interrupt service program. In order to ensure that the microcontroller has enough time to deal with power supply interference, the detection voltage of TL7705AC should be slightly greater than 4.55 V. For this purpose, a 1.5 kΩ potentiometer P can be connected in series between the voltage detection terminal of TL7705AC and the main power supply. According to the length of the interrupt service program, the appropriate resistance value can be determined to obtain the corresponding detection voltage. The detection sensitivity of TL7705AC is high, and it can detect abnormal voltage drop of the main power supply within 500ns. Therefore, it can also capture microsecond interference pulses or undervoltage, which can fully meet the requirements of the anti-interference power supply of the microcontroller.

2. Processing of power supply interference

Power supply interference can easily cause confusion in the single-chip microcomputer system. Therefore, when power supply interference occurs, the best solution is to respond to the interrupt request and switch the computer system to power-down mode. After the interference is eliminated, restore the scene and switch to normal mode. There are the following solutions to exit the power-down mode, namely hardware reset, power-on reset or manual reset. In Figure 1, the reset circuit is composed of R3, R4, C2 and button Ks. The specific solution steps are shown in Figures 2 and 3. Figure 2 is a flowchart for processing potential interference, and Figure 3 is a flowchart for the reset program.

Figure 2: Power supply interference processing

Figure 3 Reset program flowchart

3 Conclusion

This power supply was designed by the author for students' field practice in the practical teaching link. Repeated experiments have shown that the power supply
has achieved the ideal design requirements and has the following characteristics:

(1) Reliable operation, small size, simple structure, stable output voltage (its accuracy can reach ±0.001V), and high conversion efficiency.

(2) Strong anti-interference ability, can effectively suppress small transient undervoltage and transient pulse interference; for larger interference, can make the microcontroller enter power-down mode to operate.

(3) It has a certain degree of versatility. Since the performance of 87C51, 80C51 and 89C51/2 is similar, this power supply can also be suitable for low-power microcontrollers such as 87C51 and 80C51.