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Low voltage CMOS.


Introduction to Level

CMOS: Complementary Metal Oxide Semiconductor PMOS+NMOS

1 The logic level voltage is close to the power supply voltage, and the 0 logic level is close to 0V. And it has a wide noise tolerance.

Vcc: 5V; VOH>=4.45V; VOL<=0.5V; VIH>=3.5V; VIL<=1.5V.

Compared with TTL, it has a larger noise margin, and the input impedance is much larger than TTL input impedance. Corresponding to 3.3V LVTTL, LVCMOS appears, which can directly drive each other with 3.3V LVTTL.


Vcc: 3.3V; VOH>=3.2V; VOL<=0.1V; VIH>=2.0V; VIL<=0.7V.


Vcc: 2.5V; VOH>=2V; VOL<=0.1V; VIH>=1.7V; VIL<=0.7V.

CMOS use note: There is a thyristor structure inside the CMOS structure. When the input or input pin is higher than a certain value of VCC (for example, some chips are 0.7V), if the current is large enough, it may cause a latch-up effect and cause the chip to burn .

Lock-in effect

Due to the input of too much current in the CMOS circuit, the internal current increases sharply, unless the power supply is cut off, the current has been increasing. This effect is the lock-in effect. When the lock-in effect occurs, the internal current of CMOS can reach more than 40mA, which can easily burn the chip.

Defense measures:

1) Add a clamp circuit at the input and output ends to make the input and output not exceed the specified voltage.

2) A decoupling circuit is added to the power input terminal of the chip to prevent instantaneous high voltage on the VDD terminal.

3) Add a current-limiting resistor between VDD and the external power supply to prevent it from entering even if there is a large current.

4) When the system is powered by several power sources, the switches should be in the following order: when turning on, first turn on the power of the CMOS circuit, then turn on the power of the input signal and load; when turning off, first turn off the power of the input signal and load, then Turn off the power to the CMOS circuit.

Circuit comparison

Comparison of TTL circuit and CMOS circuit

1) TTL circuit is a current control device, while CMOS circuit is a voltage control device.

2) The speed of TTL circuit is fast, the transmission delay time is short (5-10ns), but the power consumption is large.

The CMOS circuit has slow speed, long transmission delay time (25-50ns), but low power consumption.

The power consumption of the CMOS circuit itself is related to the pulse frequency of the input signal. The higher the frequency, the hotter the chipset, which is normal.


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