How to select undervoltage detection threshold

　　The selection using the three discrete circuits shown in the figure above is simple and cheap, but there is also a disadvantage. When the battery voltage slowly drops to the vicinity of the detection threshold value VT, as long as there is a small amplitude glitch interference signal superimposed on the power supply voltage, it will cause the output signal RST to switch back and forth between high and low levels. As a result, the microcontroller cannot be reliably locked in the reset state.
　　
　　In order to overcome the above disadvantages, it is very necessary to add a hysteresis voltage Vhys (or hysteresis voltage or hysteresis voltage) to the undervoltage detection threshold value VT. This is similar to the hysteresis characteristics of the Schmitt trigger. Usually, the value of the hysteresis voltage Vhys is selected to be about 0.2V. In this way, the detection threshold VT is decomposed into the upper threshold value VTMAX (=VT+Vhys/2) and the lower threshold value VTMIN (=VT-Vhys/2). When the power supply voltage drops, the RST signal will only become high if it drops below VTMIN; and when the power supply voltage rises, the RST signal will only return to low if it rises above VTMAX. The setting of hysteresis voltage Vhys can effectively avoid the frequent triggering of undervoltage reset activity when the power supply fluctuates near the VT value.
　　
　　How to set the upper/lower threshold voltage, how to set the hysteresis voltage, how to select a dedicated chip, and about the hysteresis voltage. The relationship between the detection threshold voltage, the voltage regulator supply voltage and the nominal voltage range of the microcontroller. You can refer to the "Planning Reference Scale" shown in the figure below. As can be
　　
　　seen from the figure below, from left to right are the nominal operating voltage range of the microcontroller, the power supply voltage range of the voltage regulator, the upper/lower threshold range planned for the undervoltage detector circuit, and finally determine the upper/lower threshold voltage and hysteresis value of a type of undervoltage detector. After observation and analysis, the following inspirations can be obtained: (1) Often, for a specific type of microcontroller, the voltage range suitable for operation specified in its product manual is relatively wide. For example, Philips' P87C552 is 2.7V~5.5V; ATMEL's T89C51RD2-M is 3V~5.5V, etc. (2) A +5V regulated power supply built for the microcontroller application circuit, although its nominal value is 5V, is allowed to fluctuate within a relatively small range.
　　
　　That is, with the weight of the downstream load , the high and low ambient temperature, the degree of self-heating, the change of the upstream input voltage and other factors, the output voltage of the regulated power supply is not absolutely constant, but the value is allowed to change slowly around 5V. For example, for the three-terminal regulator model MC7805, the manufacturer allows the output voltage value of its qualified products to vary within the range of ±5%. That is, the fluctuation range of the output voltage value is specified to be between 4.75V and 5.25V. (3) The upper limit value VTMAX of the detection threshold VT initially planned for the undervoltage detection circuit should be lower than the lower limit value VOUTMIN of the regulator output, so as to timely warn the precursor of the power supply voltage drop. (4) The lower limit value VTMIN of the detection threshold VT preliminarily planned for the undervoltage detection circuit should be higher than the voltage lower limit VCC MIN for the microcontroller to maintain normal operation , and it is better to keep a little distance (i.e., keep a little margin) so as to allow a certain amount of time for the microcontroller to perform on-site protection before exiting operation. (5) After the above preliminary planning of the detection threshold VT, select a specific model from the product array available in the market. For a specific model of voltage detector product, its threshold voltage VT is often very precise, the width of the hysteresis voltage is often relatively narrow, and the Vhys value is generally set at about 0.2V or 0.3V. For example, a voltage detector model ADM810M has a threshold voltage VT=4.375V and a hysteresis voltage Vhys=0.25V. An
　　
　　example of the design of a specific undervoltage detection threshold value is shown in the following table. Here, a specific model of microcontroller, a specific model of three-terminal regulator and a specific model of three-pin voltage detector are used as examples for illustration. You can compare this table with the figure below.