Issues that should be paid attention to when using CMOS circuits

tiankai001

Issues that should be paid attention to when using CMOS circuits [Copy link]

CMOS circuits are very susceptible to electrostatic charges due to their high input impedance. To prevent electrostatic breakdown, standard protection circuits are added to the input terminals when CMOS is produced. However, this does not guarantee absolute safety. Therefore, the following measures must be taken when using CMOS circuits. 1. Shield CMOS integrated circuits when storing them. They are usually placed in metal containers. You can also use metal foil to short-circuit the pins. 2. CMOS circuits can operate normally within a wide range of power supply voltage, but the upper limit operating voltage of the power supply (even the instantaneous voltage) shall not exceed the limit value allowed by the circuit; the lower limit voltage of the power supply (even the instantaneous voltage) shall not be lower than the minimum value of the power supply voltage required for the system speed, and shall not be lower than the ground voltage. 3. When testing the CMOS circuit, if the signal power supply and the circuit board use two sets of power supplies, the circuit board power supply should be turned on first and then the signal power supply. When shutting down, the signal power supply should be turned off first and then the circuit board power supply. That is, when the CMOS circuit itself is not powered on, no signal input is allowed. 4. When soldering CMOS circuits, generally use a 20W internal heating soldering iron, and the soldering iron must have a good grounding wire. You can also use the residual heat of the soldering iron after the power is turned off to quickly solder. Do not solder when the circuit is powered on.

5. To prevent the input protection diode from being damaged due to forward bias, the input voltage must be between VDD and VSS. 6. The redundant input terminals must not be left floating. Otherwise, it is not only easy to be interfered by the outside world, but also the input level is uncertain, which destroys the normal logical relationship and consumes a lot of power. Therefore, according to the logical function of the circuit, it needs to be handled according to the situation. For example: the redundant input terminals of AND gate and NAND gate should be connected to high level; the redundant input terminals of OR gate and NOR gate should be connected to low level; if the working speed of the circuit is not high and the power consumption does not need to be considered, the redundant input terminals can also be connected in parallel with the use terminals. The redundant input terminals mentioned above include all the input terminals of the CMOS circuit that are not used but have been turned on.

7. When the input terminal connection is relatively long, due to the influence of distributed capacitance and distributed inductance, it is easy to form an LC oscillator, and it may also damage the protection diode. Therefore, a 10~20K ohm resistor R must be connected in series at the input terminal, as shown in the figure below.

8. When a CMOS circuit is installed on a printed circuit board, there is always an input terminal on the printed circuit board. When the circuit is unplugged from the whole machine, the input terminal will inevitably be suspended, so a current limiting protection resistor should be connected to each input terminal, as shown in the figure below. If you want to install a CMOS integrated circuit on a printed circuit board, you must install the CMOS circuit after installing the components related to it to avoid the input terminal of the CMOS circuit being suspended.

9. When plugging or unplugging the circuit board power plug, be sure to cut off the power supply first to prevent the input protection diode of the CMOS circuit from burning out during the plugging or unplugging process. 10. CMOS circuits are used in parallel. When two or more identical devices are used in parallel on the same chip (AND gate, NOR gate, inverter, etc.), the output supply current and output absorption current can be increased. If the capacitive load does not increase much, it will increase the driving capability of the device and improve the speed. When in use, the output ends are connected in parallel and the input ends must also be connected in parallel. 11. Methods to prevent noise interference at the input end of CMOS circuit. The input end of CMOS circuit is often connected to mechanical nodes such as key switches and relay contacts, or components such as sensors. CMOS circuit has very high input impedance, and only a small current can drive CMOS circuit to work. When the output impedance of the circuit connected to the input end of CMOS circuit is high, the anti-interference ability is very poor, especially when the connection is long, it is more susceptible to interference. The method adopted is to reduce the output resistance of the input circuit. The specific method is: connect a Schmitt trigger shaping circuit between the connected circuit and the input end of the CMOS circuit, and change the output resistance through the hysteresis. You can also add a filter circuit to filter out noise. In order to prevent malfunctions caused by the jitter of the key switch and relay contacts, you can connect a capacitor in parallel to the node, or connect an RS trigger.

chunyang

The article is about the first generation of old CMOS circuits before 1980s. Since 1980s, CMOS circuits have not been required to be stored in metal containers, let alone short-circuited pins with metal foil. I have a few such old CMOS in my collection, the domestic c0xx series, produced in the late 1970s, with only a dozen to dozens of transistors in the small-scale circuit. The Western 4000 series products of the same period have no such requirements. As for the input current limiting resistor mentioned in the article, oh, it’s better not to repost such garbage articles, okay? It’s harmful and useless!