The selection of the three discrete components shown in the above figure is simple and inexpensive, but there is also a disadvantage. When the battery voltage is gradually decreased to a value near the VT detection threshold, as long as there is a small glitch amplitude signal is superimposed on the supply voltage will cause the output signal RST between the high and low switch back and forth. As a result, the microcontroller cannot be reliably locked in the reset state.
In order to overcome the above disadvantages, it is necessary to add a hysteresis voltage Vhys (or hysteresis voltage or hysteresis voltage) to the undervoltage detection threshold VT. This is similar to the hysteresis characteristic of Schmitt triggers. Usually, the value of the hysteresis voltage Vhys is preferably selected to be about 0.2V. As a result, the detection threshold VT is decomposed into the upper threshold VTMAX (= VT + Vhys / 2) and the lower threshold VTMIN ( = VT - Vhys / 2). When the power supply voltage drops, the RST signal will only go high if it drops below VTMIN. When the power supply voltage rises, it only rises above VTMAX, and the RST signal will return to low. The setting of the hysteresis voltage Vhys can effectively avoid the undervoltage reset activity being frequently triggered 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 the dedicated chip, and about the hysteresis voltage. Detect the relationship between the threshold voltage, the regulator supply voltage and the nominal voltage range of the microcontroller. You can refer to the “Planning Reference Ruler†as shown in the figure below.
As can be seen from the following figure, from left to right, the nominal operating voltage range of the microcontroller, the supply voltage range of the regulated power supply, the upper/lower threshold range for the undervoltage detector circuit planning, and finally an underdetermination Upper/lower threshold voltage and hysteresis value of the pressure detector model. After observation and analysis, the following inspirations can be obtained: (1) Often a specific type of single-chip microcomputer, the voltage range suitable for operation specified in the product manual is relatively wide. For example, Philips' P87C552 is 2.7V to 5.5V; ATMEL's T89C51RD2-M is 3V to 5.5V. (2) A +5V regulated power supply built for the microcontroller application circuit, although its nominal value is 5V, it allows fluctuations in a relatively small range.
That is, with the weight of the downstream load, the temperature of the environment, the degree of self-heating, the change of the upstream input voltage, etc., the output voltage of the regulated power supply is shown in Table 1. The voltage value of the undervoltage detection threshold voltage is not absolutely constant. Instead, the value is allowed to change slowly around 5V. For example, a three-terminal regulator of the MC7805, the manufacturer allows the output voltage of its qualified products to vary within ±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 preliminarily planned for the undervoltage detection circuit should be lower than the lower limit value VOUTMIN of the regulator output in order to promptly 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 lower limit voltage VCCMIN of the MCU to maintain normal operation, and it is better to pull a little distance (that is, reserve a margin) Allow the MCU to perform on-site protection before exiting the operation for a certain period of time. (5) Under the premise of the above-mentioned preliminary planning of the detection threshold VT, select a specific model from the array of products available in the market. For a specific type of voltage detector product, the threshold voltage VT is often very accurate, the width of the hysteresis voltage is often narrow, and the Vhys value is generally set at about 0.2V or 0.3V. For example, a voltage detector of the type ADM810M has a threshold voltage of VT = 4.375V and a hysteresis voltage of Vhys = 0.25V.
A specific design example of the undervoltage detection threshold is shown in the following table. Here is an example of a specific type of single-chip microcomputer, a specific type of three-terminal voltage regulator and a specific type of three-legged voltage detector. This table can be compared with the following figure.
Female Header,Female Header Pitch 2.54Mm,Smt Female Header,In-Line Patch Female Connector
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