CMOS vs HCMOS vs LVCMOS: Key Differences Explained

This article compares CMOS, HCMOS, and LVCMOS, highlighting their differences in voltage levels, advantages, and disadvantages.

Introduction: CMOS Technology

CMOS (Complementary Metal Oxide Semiconductor) circuits utilize both p-channel and n-channel FET (Field-Effect Transistor) devices. These are fabricated on the same substrate to create logic functions. CMOS technology primarily employs NAND and NOR gates as its fundamental building blocks.

CMOS | Complementary Metal Oxide Semiconductor

Here are the typical characteristics of the CMOS logic family:

• Basic gate used: NAND/NOR
• Fanout: >50
• Power per gate: 1 mW @ 1MHz
• Noise immunity: Excellent
• Noise margin: 0.3Vcc
• t PD (Propagation Delay): 1-200 ns
• Output drive current: Symmetric: Typ. 4mA, but AC family can drive 24 mA

In CMOS, binary one and zero are represented as follows:

• Binary Logic “0”: Represented by a voltage between 0V to 1V
• Binary Logic “1”: Represented by a voltage between 3.5V to 5V

Advantages of CMOS:

• Low power dissipation
• Excellent noise immunity
• Higher packing density
• Higher speed
• Highest fanout
• Wide supply voltage range

Disadvantages of CMOS:

• Average propagation delay is worse than TTL (Transistor-Transistor Logic) and ECL (Emitter-Coupled Logic) families.

Figure: Interfacing between CMOS and TTL.

HCMOS | High Speed CMOS

• HCMOS stands for High-Speed CMOS.
• It’s a higher-speed variant of the original CMOS.
• The specifications of HCMOS are defined by JEDEC (Joint Electron Device Engineering Council).
• Example: Philips semiconductor device 74HC/HCT/HCU

Advantages of HCMOS:

• Philips HCMOS devices offer low power in addition to the high speed and drive capability of LSTTL (Low Power Schottky TTL).

Figure: CMOS and LVCMOS voltage levels

LVCMOS | Low Voltage CMOS

• LVCMOS stands for Low Voltage CMOS.
• It’s defined by JEDEC.
• It represents a low-voltage class of CMOS.
• The figure above illustrates the voltage levels used by LVCMOS and CMOS logic families.

Applications of LVCMOS:

• LVCMOS output signals are suitable for low-powered imaging equipment, networking, communication, portable test and measurement equipment, industrial test equipment, etc.

Advantages of CMOS, HCMOS, and LVCMOS:

• All variants (TTL, CMOS, HCMOS, and LVCMOS) have a single-ended output in the range of 0.5 to 4.5V.
• The outputs have a square wave digital shape, which is suitable for lower frequency clocking below 250 MHz.
• They allow direct connection between the clock output and the input of the chip.
• They are used for high speed and low voltage requirements.

Disadvantages of CMOS, HCMOS, and LVCMOS:

• In some applications, a series resistor (of low value) is used to reduce signal feedback in order to maintain signal integrity.