This article mainly talks about the difference between COMS and TTL circuits, and hopes to help you learn.
First, the difference between CMOS and TTL circuits
1. CMOS is a FET (monopolar circuit) and TTL is a bipolar transistor (bipolar circuit)
2, COMS logic level range is relatively large (5 ~ 15V), TTL can only work at 5V
3. The difference between high and low levels of CMOS is relatively large, anti-interference is strong, TTL is small, and anti-interference ability is poor.
4, CMOS power consumption is small, TTL power consumption is large (1 ~ 5mA / door)
5, CMOS operating frequency is slightly lower than TTL, but high-speed CMOS speed is almost equivalent to TTL
6, CMOS noise margin is greater than TTL noise tolerance
7, usually think that the speed of the TTL gate is higher than the "CMOS gate circuit. The main factor affecting the working speed of the TTL gate circuit is the switching characteristics of the internal tube of the circuit, the circuit structure and the internal resistance resistance values. The larger the resistance value, the higher the working speed. Low. The longer the switching time of the tube, the lower the working speed of the door. The speed of the door is mainly reflected in the “transmission delay†tpd of the output waveform relative to the input waveform. The product of tpd and no-load power consumption P is called “ Speed-power product is an important indicator of device performance. The smaller the value, the better the performance of the device (generally about tens of picoliters (10-12) joules). Unlike TTL gates, The main factor affecting the operating speed of the CMOS circuit is the external part of the circuit, that is, the load capacitance CL. CL is the main reason that affects the operating speed of the device. The transmission delay affecting the CMOS gate determined by CL is about several tens of nanoseconds.
8. The TTL circuit is a current control device, and the coms circuit is a voltage control device.
Second, CMOS use precautions
1) The voltage control device of the COMS circuit has a large input total resistance and a strong ability to capture interference signals. Therefore, do not use the unused pins. Connect a pull-up resistor or a pull-down resistor to give it a constant level.
2) When inputting the signal source of the low internal group, connect the current limiting resistor between the input terminal and the signal source to limit the input current to 1mA.
3) When the long signal transmission line is connected, the matching resistor is connected at the COMS circuit.
4) When the input terminal is connected to a large capacitor, the resistor should be indirectly protected at the input and capacitor. The resistance value is R=V0/1mA. V0 is the voltage on the external capacitor.
5) If the input current of COMS exceeds 1 mA, it is possible to burn out COMS.
Third, what is the totem pole, what is the difference between it and the open drain circuit?
In TTL integrated circuits, the output with the pull-up transistor is called the totem pole output, and the other is called the OC gate. Because TTL is a three-level switch, the totem pole is also connected by two three-stage push-pull. So push-pull is a totem. General totem output, high level 400UA, low level 8MA
What is the locking effect of CMOS circuits?
Due to the input of too much current in the COMS circuit, the internal current increases sharply, and the current continues to increase unless the power supply is turned off. This effect is the locking effect. When the lock-in effect occurs, the internal current of the COMS can reach 40 mA or more, and it is easy to burn the chip.
Defensive measures:
1) Add a clamp circuit at the input and output terminals so that the input and output do not exceed the specified voltage.
2) Add a decoupling circuit to the power input terminal of the chip to prevent instantaneous high voltage on the VDD terminal.
3) Add a line resistance between VDD and the external power supply, even if there is a large current, it will not be allowed to enter.
4) When the system is powered by several power sources separately, the switch should be in the following order: when it is turned on, first turn on the power of the COMS circuit, then turn on the power of the input signal and the load; when it is off, first turn off the power of the input signal and the load, then Turn off the power to the COMS circuit.