1. Electrical Characteristics of IC’s Part 2
Last Mod: January 2011
Paul R. Godin

2. Input/Output Current

3. Gate Currents
Digital Logic devices are constructed from analog components which include a variety of transistors, resistors, diodes and other semiconductors.
TTL devices, based on transistors, rely on current flow to sense the input logic. Current flow between the output of one device and the input of the other device is required to switch the transistors on or off. The action of the transistors is what determines the output logic state.

4. Source and Sink Current entering a gate is + (sink)
Every logic device will either source or sink current.
When the gate output is in a high state, it sources current. Sourcing = provides current
When the gate output is in a low state, it sinks current. Sinking = receives current
Gate inputs can either sink or source current, depending on the level of the output attached to it.
Current entering a gate is + (sink)
Current exiting a gate is - (source)

5. Source and Sink Output sinks current in a low state
Output sources current in a high state

6. Driving and Loading Driving Gate Loading Gates
Driving gate: A gate that provides a logic level to other gates.
Loading gate: A gate that receives a logic level from other gates.
Driving Gate
Loading Gates

7. Input and Output Current
IIL: Input Low Current. Current when input is in a low state.
IIH: Input High Current. Current when input is in a high state.
IOL: Output Low Current. Current when output is in a low state.
IOH: Output High Current. Current when output is in a high state.

8. Input and Output Current

9. Typical Current Values
7400:
IIL:-1.6mA
IIH: 40A
IOL:16mA
IOH: -0.4mA
74LS00:
IIL:-0.4mA
IIH: 20A
IOL:8mA
IOH: -0.4mA

10. Fanout How many loading inputs are present?
Fanout is the number of load inputs that a driving gate is capable of handling.
How many loading inputs are present?

11. Fanout IOH IIH IIH 1 mA 0.2 mA If:
the maximum output current of the driving gate is 1 mA, and
each loading input requires 0.2 mA
The driving gate can supply current to 5 inputs ( 5 x 0.2 mA = 1 mA )
0.2 mA
0.2 mA

12. Fanout IOL IIL IIL 0.7 mA 0.1 mA If:
the maximum current of the driving gate is 0.7 mA, and
each loading input requires 0.1 mA
The driving gate can sink current for 7 inputs ( 7 x 0.1 mA = 0.7 mA )
0.1 mA
0.1 mA

13. Calculating Fanout

14. Fanout Calculation Worst-case = 10
Must calculate for BOTH output high and output low. Select the worst-case condition. (Disregard the negative in the calculation)
Consider the 7404:
Worst-case = 10

15. Fanout Exercise
Use the Texas Instruments specification sheets to determine the following Fanout values:
SN74AS04 to SN74LS08
SN74S08 to SN74ALS04B

16. Power

17. Some Definitions
Quiescent: output logic that is not changing (also known as static)
Dynamic: output logic that changes (also known as switching)
VCC: TTL Supply Voltage
ICC: TTL Supply Current
ICCH: TTL Supply Current with all outputs high.
ICCL: TTL Supply Current with all outputs low.
VDD: CMOS Supply Voltage
VSS: CMOS Ground
IDD: CMOS Supply Current (static/quiescent)
IT: CMOS Supply Current (static and dynamic)
CPD: CMOS Internal Capacitance

18. Power (TTL)
Power = Voltage  Current = VCC  ICC
Vcc & Icc

19. Device Input Current (TTL)
With all the outputs = logic high
Input current = ICCH specification
ICCH 1
Make the outputs logic high by applying the appropriate input logic

20. Device Input Current (TTL)
With all the outputs = logic low
Input current = ICCL specification
ICCL
Make the outputs logic low by applying the appropriate input logic

21. TTL Power Calculation Power = Voltage  Current = VCC  ICC
If all gates are high:
Pd = VCC  ICCH
ICC = ICCH
If all gates are low:
Pd = VCC  ICCL
ICC = ICCL
Assume Vcc = 5V, unless otherwise specified

22. TTL Power Calculation where N=number of gates
However, if some of the gates are high and others are low:
where N=number of gates
This formula looks complicated...let’s approach it differently

23. Device input current (TTL)
With some inputs high and some low
Input current = ICC
ICC
In this example, ½ of the gates are logic high, ½ are logic low.
Pd= + ICCL)
Pd= Vcc((0.5ICCH) + (0.5ICCL)) 1 1

24. Device input current (TTL)
In this example, ¼ of the gates are logic high, ¾ are logic low.
Pd= + ICCL)
Pd=Vcc((0.25ICCH) + (0.75ICCL))
ICC 1

25. TTL Power Calculation
With switching outputs, Duty Cycle must be taken into account.
For a duty cycle of 50%:
For a duty cycle other than 50%:
Hint: Don’t study the equations...learn the process

26. Device input current (TTL)
ICC
DC=25%
In this example, ¼ of the gates are at 25% duty cycle, ¾ are logic low.
Pd=  ICCL) + ICCL))
Pd= Vcc((0.25(0.25ICCH+0.75ICCL) + (0.75ICCL))
Remember: A duty cycle of 25% means that the output is high for 25% of the time (using ICCH), and low for 75% of the time (using ICCL).

27. CMOS Power
CMOS uses very little power in the static state (in the order of W).
As switching increases (more dynamic), so does power consumption. This is primarily due to capacitance.
Power requirements also increase with ambient temperature. As temperature increases, so does power consumption.
Static power consumption for a B-Series gate  500W maximum
Other families of CMOS have lower power consumption.

28. CMOS Power Calculation
Most CMOS specification sheets provide the mathematical equation for calculating power consumption.
Care must be taken when utilizing the specification sheet.
Current is specified per gate or per IC package. Read carefully.
Formulas may vary (example: 4011B compared to the 4027B)
IDD is different from IT
Generally:

29. End
January 2011
Paul R. Godin
gmail.com