4.4. IC Parameters

Digital ICs are evaluated and compared in terms of the following parameters which are to be seen on manufacturer's data sheet.

a. fan-out

b. fan-in

c. power dissipation

d. propagation delay

e. noise margin

f. current and voltage parameters

Part 4.4.1 Fan-Out

Specifies the number of standard loads that the output of a typical gate can drive without impairing its normal operation. A standard load is usually defined as the amount of current needed by an input of another similar gate of the same family.

Figure 1. Sample Logic Circuit with Ioh and Iih

Where:

I_OH = High-level output current

I_IH = High-level input current

Example no. 1

Given for NAND gate IC:

I_OH = 400 µA

I_IH = 40 µA

I_OL = 16mA

I_IL = 1.6mA

Find the fan-out number.

Solution no. 1

Therefore, the output of the gate can drive up to ten loads (inputs to other gates).

Note: Exceeding the specified maximum load may cause malfunction because the circuit has insufficient power to drive all the gates.

Part 4.4.2 Fan-In

Fan-in, in the context of digital logic gates, simply refers to the number of input connections that a gate is designed to accept. For example, a 2-input AND gate has a fan-in of two because it takes two distinct signals to produce its output, while a 4-input NOR gate can process four different input signals, thus having a fan-in of four. This characteristic is crucial in circuit design as it determines how many independent signals can directly feed into a single gate, impacting the overall complexity and potential propagation delay of the digital circuit.

Part 4.4.3 Power Dissipation

Power consumed by the gate that must be available from the power supply; expressed in mW (milliwatts)

Average power dissipation

Where:

I_cc =current drawn by the gate

V_cc = supplied voltage

Example no. 2

A standard NAND TTL gate uses a supply voltage of +5V and has current drains I_CCH = 1 mA and I_CCL = 3 mA.

Solution no. 2

Given:

I_CC(ave) = (1mA+3mA)/2

= 2mA

P_D = 2mA x 5v = 10mW/Gate

In the case of 7400 (quadruple two-input NAND gates):

P_D(total) = 4 x 10mW = 40mW

Part 4.4.4 Propagation Delay

Average transition delay time for the signal to propagate from input to output. The operating speed or frequency is inversely proportional to the propagation delay.

Figure 2. Measurement of propagation delay (Source: Digital Electronics by Moris Mano)

DC Noise margin

· Quantitative measure of noise immunity; noise immunity of a circuit refers to its ability to tolerate noise voltages on its inputs

· Minimum external noise voltage that causes an undesirable change in the circuit output.

· Maximum noise voltage added to the input signal of a digital circuit that does not cause an undesirable change in the circuit output.

Example no. 3
A TTL NAND gate has the following voltage specifications:

· Output HIGH Voltage (V_OH): 2.4 V

· Input HIGH Voltage (V_IH): 2 V

· Input LOW Voltage (V_IL): 0.4 V

· Output LOW Voltage (V_OL): 0.8 V

Calculate the high-state noise margin and the low-state noise margin for this TTL NAND gate. Explain the significance of these noise margin values in the context of digital circuit reliability.

Solution no. 3

For high-state noise margin = 2.4v – 2v = 0.4v

For low-state noise margin = 0.8v – 0.4v = 0.4v

Part 4.4.5 Current and Voltage Parameters

V_IH (min) - High-Level Input Voltage. The voltage level required for a logical 1 at an input. Any voltage below this level will not be accepted as a HIGH by the logic circuit.

V_IL (max) - Low-Level Input Voltage. The voltage level required for a logic 0 at an input. Any voltage above this level will not be accepted as a LOW by the logic circuit.

V_OH (min) - High-Level Output Voltage. The voltage level at a logic circuit output in the logical 1 state. The minimum value of Von is usually specified

V_OL (max) - Low-Level Output Voltage. The voltage level at a logic circuit output in the logical 0 state. The maximum value of V_OL is usually specified.

I_IH - High- Level Input Current. The current that flows into an input when a specified high-level voltage is applied to that input.

I_IL - Low-Level Input Current. The current that flows into an input when a specified high-level voltage is applied to that input.

I_OH - High-Level Output Current. The current that flows from an output in the logical state under specified load conditions. 1 state under specified load conditions.

I_OL - Low-Level Output Current. The current that flows from an output in the logical 0 state under specified load conditions.

With the use of a manufacturer's data sheet, try to find the different voltage and current parameters.

Shown in the table below is a summary and comparison of the various logic families.

Table 1. Summary of Different Logic IC Families (Source: Guidebook in Electronics Engineering by Villamor, HR Publishing, 2003)

Family	Propagation Delay per Gate (ns)	Power Dissipation per Gate	Advantages	Disadvantages
RTL	40	20 mW	Simple and inexpensive. Easy to interface	Sensitive to noise. Low fan-out ratios. Low packing density.
DTL	30	8 mW	Good noise rejection than RTL, easy to use and interconnect, inexpensive	Relatively low speed. Low packing density.
HDTL	40	15 mW	DTL especially made for industrial applications because of high noise rejection. Easy to use and interconnect.	Low speed, relatively high power dissipation than DTL, low packing density.
TTL	7	10 mW	Most popular logic family. Easy to interconnect, fast. MSI packages available, and inexpensive.	Generates noise spikes, relatively high power dissipation, modest packing density.
STTL SCTTL	3	20 mW	Faster than TTL	Dissipates more power than TTL
LPTTL	30	1 mW	Low power TTL, good for space and other portable applications.	Low speed.
ECL CML NSL	0.5	60 mW	Fastest, generates little internal noise.	Difficult to interface. Low packing density. Generates more heat.
PMOS	50	0.1 mW	Low power, good packing density, easy to manufacture, inexpensive.	Slow, delicate, and difficult to interface with other logic families.
NMOS	20	0.1 mW	Faster than PMOS, relatively low power, easy to manufacture, and good packing density.	Difficult to interface with other logic families.
CMOS	10	10 nW	Becomes popular because of its low power consumption. Average speed and packing density.	Sensitive to static electricity, delicate. Power consumption increases when switched at high speeds. Slower than TTL.

TTL and CMOS logic family are presented in the next sections. It is highly recommended that students do more in-depth reading and study of the different logic families to gain broader understanding of their similarities and differences.

RTL - Resistor Transistor Logic

· This family uses resistors and transistors as circuit elements. NOR gate is used as the standard gate.

DTL - Diode Transistor Logic

· Uses diodes and transistors as circuit elements and uses NAND gate as the standard gate

HLDTL - High Level Diode Transistor Logic

· Has higher power supply than DTL usually 25V instead of 5 V.

TTL - Transistor-Transistor Logic

· Uses multiple emitter transistors at the input; uses NAND gate as the standard gate and replaced DTL.

CMOS - Complementary Metal Oxide Semiconductor

· consists of two metal-oxide semiconductor field effect transistors (MOSFETs), one N-type and one P-type, integrated on a single silicon chip. Generally used for RAM and switching applications, these devices have very high speed and extremely low power consumption. They are, however, easily damaged by static electricity.

Part 4.4.6 Logic Levels

The actual voltage levels of Is for logic 0 and logic 1 depends on the logic family and indicated on specific manufacturer data sheet

Figure 3. Sample Logic Level for TTL family

Part 4.4.7 Sample Data Sheet

In a data sheet you will find the voltage and current parameters and other information vital in circuit design. Shown below is a typical manufacturer datasheet for SN54LS00 and SN74LS00.

Figure 4. Sample Data Sheet (Source: Texas Instrument)

A good understanding of the electrical aspects of digital circuit operation is needed for successful circuit design. Characteristics of today's logic families can be found in data

books published by the device manufacturers such as Texas Instruments and Motorola. For updated versions of their data books, you can visit www.ti.com and www.mot.com.