Solve the problem of abnormal MCU startup

灞波儿奔

Solve the problem of abnormal MCU startup [Copy link]

When the product is tested for rapid power on and off, if the power on and off requirements of the MCU are not met, the MCU will often fail to start or even lock up. For MCUs powered by a single power supply, the circuit does not need to be modified. This article recommends you an LDO, which can solve the abnormal startup problem of the MCU.
　　For products that require power-off preservation or power-off alarm functions, using the energy storage function of large-capacity capacitors to provide time for saving data and shutting down the system is often the choice of many engineers. In systems that do not require power-off to save data, in order to suppress power ripple, power interference and load changes, a capacitor of appropriate capacity is also connected in parallel at the power supply end.
　　However, the larger the capacitor in the circuit, the better. Due to the energy storage function of the capacitor, a large-capacity capacitor may extend the system power-on and power-off time. The extension of the power-on and power-off time is likely to cause the MCU to fail to start or enter the latch state. Therefore, it is particularly important to shorten the power-on and discharge time of the MCU power supply. For single-power supply systems, ZLG has launched an LDO with EN control and built-in fast discharge function: ZL6205 to help your system.
　

　　Figure 1 ZL6205
　　1. Use EN to shorten the power-on time
　　As we all know, meeting the power-on timing of the MCU is one of the most basic and important requirements of system design, so it is very necessary to carefully study the power-on and power-off timing of the chip. The following Figure 2 shows the power-on time requirements of a series of MCUs.
　　

　　Figure 2 Power-on requirements
　　From the above figure, we can see that the main requirements of the MCU for power-on are:
　　the power-on time tr cannot exceed 500ms;
　　the voltage VI before power-on needs to be lower than 200mV for at least 12us.
　　This requires that the power-on time be shortened as much as possible, especially when there are large capacitors or supercapacitors in the circuit. Too long power-on time can easily cause the system to fail to start or the device to latch.
　　A simple way to shorten the power-on time is to control the EN pin of the power chip. The clever use of the voltage divider resistor of the EN pin can effectively shorten the power-on time of the system. When many people use power chips, they generally enable them by default with external pull-ups. If the enable threshold is reached too early, the output will follow the input, that is, the slower the input, the slower the output, and the jitter at the input end during power-on will also be transmitted to the output. The following Figure 3 shows the output curve diagram when setting EN to directly pull up and using a voltage divider circuit.

　　　　Figure 3 Output curves when EN is pulled up to input and voltage divider circuit is used
　　Curve ①, enable is pulled up to input, at this time, the output rise time is long and will be affected by input fluctuations;
　　Curve ②, reasonable use of voltage divider resistors, when VIN rises to 70%~80%, then the EN voltage reaches the enable threshold, at this time the output rising edge is steep, the output is stable, the unstable stage of the input power supply is eliminated, and the impact of input voltage fluctuations is reduced. At the same time, a 20%~30% margin is reserved to avoid power supply fluctuations causing the output to shut down. At this time, power-on is clean and neat for the MCU!
　　Speaking of this, we have to talk about our company's independent chip ZL6205, which uses SOT-23 package and has EN enable pin, which can flexibly control the output power supply and provide a clean, fast and stable power supply for the subsequent circuit. The following Figure 4 shows a typical application diagram of ZL6205.
　　

　　Figure 4 ZL6205 typical application circuit
　　It is not enough to solve the power-on problem? There is also the power-off problem? Don't worry, ZL6205 also has a built-in fast discharge circuit to improve the system power-off speed.
　　2. ZL6205 has a built-in discharge circuit to help fast power-
　　off We solved the problem of slow power-on earlier, but it does not mean that the system can start stably. As shown in Figure 2, it is also necessary to meet the input voltage of MCU at power-on is lower than 200mV for at least 12us. This shows that when the power is turned on and off quickly, whether the system power is off "cleanly" is also closely related to the system startup.
　　　Figure 5 Schematic diagram of slow power-off
　　As shown in Figure 5, when the system power-off load cannot discharge energy quickly, the MCU and other digital devices will power off slowly. If the power is turned on again at this time, since the chip cannot be "reset" in time, it is an uncertain state for digital devices such as MCU. At this time, if the system is powered on again, it is easy to cause MCU logic confusion, resulting in device latching and the system cannot start.
　　Therefore, after the power is turned off, the MCU power supply is quickly reduced to nearly 0V, so that the system reaches a certain state in a short time. It is also a key factor for the system to start normally when the power is quickly restarted.
　　The problem of slow power-off is easily overlooked during the design process. It is often too late to find the problem in the product debugging stage. It is time-consuming and labor-intensive to add a fast discharge circuit to the system again. However, if the system is equipped with our ZL6205, the power-off problem can be easily solved!
　

　　Figure 6 ZL6205 internal block diagram
　　As shown in Figure 6 above, this is the internal block diagram of ZL6205. When the input voltage is powered off, if the EN voltage is lower than the enable threshold, the internal fast discharge circuit will be started, and a 240Ω discharge resistor will be loaded at the output end to make the output voltage power off quickly. At this time, the output voltage of the LDO, which is the input power supply of the MCU, can quickly "return to zero" to avoid system startup failure when it is powered on quickly again.
　　3. Solution recommendation
　　When encountering the problem of system startup failure, please use an oscilloscope to check whether the power supply pin of the device is slow to power on and incomplete power off. When encountering this situation, you can choose to use the LDO independently developed by Guangzhou Zhiyuan Microelectronics Co., Ltd. in the circuit: ZL6205. ZL6205 is a 500mA low-dropout linear regulator designed by our company, which can respond quickly when the load current and power supply voltage change.
　　The main features are:
　　500mA maximum output current;
　　low voltage drop (typical value is 240mV@IO=500mA);
　　external 10nF bypass capacitor when necessary for low noise;
　　fast startup;
　　with fast discharge function;
　　typical quiescent current value of 50μA;
　　initial voltage accuracy of ±1.0%;
　　undervoltage protection;
　　overcurrent protection;
　　short circuit protection;
　　overtemperature protection;
　　the selection table is shown in the following table.

　　Table 1 ZL6205 selection table
　　How to quickly solve the problem of abnormal MCU startup
　　Note: Other output voltages can be customized.
　　4. Conclusion
　　The components in the system have strict requirements for the power on and off of the power supply. In the design of the product, attention should be paid to the power on and off requirements of the core components, including the timing and slope of the power on and off. Unreasonable design often causes abnormal situations such as the system being unable to start after power on.