SN74LS21N

Current - Output High, Low:	400µA, 8mA
Base Part Number:	74LS21
Package / Case:	14-DIP (0.300", 7.62mm)
Supplier Device Package:	14-PDIP
Mounting Type:	Through Hole
Operating Temperature:	0°C ~ 70°C
Max Propagation Delay @ V, Max CL:	20ns @ 5V, 15pF
Logic Level - High:	2V
Logic Level - Low:	0.8V
Series:	74LS
Voltage - Supply:	4.75 V ~ 5.25 V
Features:	--
Number of Inputs:	4
Number of Circuits:	2
Logic Type:	AND Gate
Part Status:	Active
Packaging:	Tube

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The SN74LS21N Logic Gate and Inverter

The SN74LS21N integrated circuit, otherwise known as a ‘quad two-input NAND gate with open collector outputs’, is an important element in digital logic applications. This gem of an IC can easily be found in many types of logic circuits, whether it\'s in the form of a logic gate or an inverter, and offers some unique advantages compared to other integrated circuits of the same family.

What is an SN74LS21N?

The SN74LS21N is a fourteen-pin dual-in-line package (DIP) integrated circuit containing four 2-input NAND gates. The gate combines two input signals (e.g. A & B) to produce one output (Y) depending on the combination of the two inputs, as shown by the logic equation ‘Y = A X NOR B’

Application Field of SN74LS21N

The SN74LS21N is most commonly used in logic-related applications such as line drivers, memory decoders, and as gated buffers. Additionally, due to its easy logic interface, the NAND gate is also also found in more complex systems such as digital counters, address-decoding circuits, arithmetic circuits, and digital circuits that require timing control.

Working Principle of SN74LS21N

The SN74LS21N attempts to minimize noise with its open collector outputs, which allow the gate to act as a buffer. This means that the logic gate can be used to provide a logic signal back up the input line. The gate features two power supply rails; Vcc typically used for logic high which is connected to pin 14 and Gnd (ground) which is connected to pin 7.These two power supplies allow the open collector outputs to draw to different rail voltages in order to pull the output high (Vcc) or low (Gnd). The output will only be high if both inputs of the gate are at a logic level high and will be low if one or both of the inputs are at a logic level low (e.g. 0V).

SN74LS21N Inputs, Bus and Outputs

The SN74LS21N has four separate parallel inputs (A1, A2, B1, & B2), two grouped inputs (A3 & A4, and B3 & B4) which are connected in series to form a single group, and four separate open collector output pins (Y1, Y2, Y3, & Y4).The open collector outputs, allows the output voltages to transition rapidly from Vcc to ground (Gnd) by using either an N-MOSFET SCR or a P-MOSFET NPN. These outputs are also capable of sinking more current than either TTL or CMOS logic families, meaning that the logic output can relay signals of higher power.

Construction of SN74LS21N

The SN74LS21N is constructed with a variety of components, such as transistors and resistors, to provide the circuitry required to form a single NAND gate. The SN74LS21N is essentially four NAND gates on a single integrated circuit, with each NAND gate placed inside a chip in a configuration similar to a Dual In-line Package (DIP).Additionally, each gate contains two transistors and a resistor to form the base of the NAND gate system. The two transistors are connected in series so that when one is activated, the other one is not. This is how the open collector output works and allows for a logic high or logic low signal depending on the input logic levels.

Advantages of SN74LS21N

Using the SN74LS21N as part of your system will yield many benefits. Not only is the chip widely available and relatively cheap to purchase, it offers increased electrical consumption performance compared to other integrated circuits in the same family.Additionally, the open collector output of the chip allows it to offer increased power with increased speed. This is especially handy when dealing with high-power logic systems, where large amounts of switching can occur in a short period of time. Lastly, the SN74LS21N also presents very good noise immunity and interference due to its low noise characteristics.

Conclusion

The SN74LS21N integrated circuit is one of the most widely used logic gate and inverter ICs today. Its open collector output pin design allows it to provide increased power, speed and low noise, while offering the same ground grabbing capabilities of other gate and inverter ICs. The SN74LS21N is a great tool to have in any digital electronics system due to its versatility, ease of construction and low cost.

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