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(1)

DIGITAL LOGIC CURRENT FLOW

By

Henry W. Ott

Henry Ott Consultants

(2)

INTRODUCTION

Many Engineers and Designers Are Confused About How

And Where Digital Logic Signal Return Currents Flow, and

What is the Source of the Digital Logic Currents.

Is it Better to Have a Digital Logic Trace Adjacent to a Ground

Plane or a Power Plane?

What About a Trace Between a Power and a Ground Plane?

Would it be Better to Have the Trace Between Two Ground

Planes, or Possibly Two Power Planes?

To Answer All These Questions, One Must Know Two Things:

(1) What is the Source of the Current, and

(2) What is the Path Taken by the Current

When Returning to the Source?

(3)

DIGITAL LOGIC RETURN CURRENT PATH

First, Let Me State, That the Logic Gate is Not the Source of the Current

The Logic Gate Only Acts as a Switch

The Source of the Current is:

—

The Decoupling Capacitor, or

—

The Trace and Load Capacitance

Only the Transient (Switching) Current is Important

The Transient Current Flow Does Not Depend Upon the Existence of a Load at

the End of the Line

The Output Trace Capacitance Exists (Mostly) Between the Trace and The

Closest Plane

What Then is the Return Current Path?

The Return Current Path is a Function of (1) the Trace Configuration (Microstrip

or Stripline), (2) What is the Adjacent Plane or Planes (Power or Ground), and (3)

What is the Logic Transition (Low-to-High or High-to-Low)?

(4)

GENERAL CASE–CMOS LOGIC GATE DRIVING A

LOAD

Vcc

Load Capacitance Load Capacitance Decoupling Capacitor

Vcc

Parasitic Capacitance Parasitic Capacitance

Ground Plane

Power Plane

Signal Trace

(5)

DIGITAL CIRCUIT CURRENT PATH

Trace Adjacent to a Ground Plane (Microstrip)

(Low-to-High Transition)

Vcc

Load Capacitance Load Capacitance Decoupling Capacitor

Vcc

Parasitic Capacitance

Ground Plane

Signal Trace

Source

Load

Source

Load

+

-Current Source: Decoupling Capacitor Return Current Path: Ground Plane

(6)

DIGITAL CIRCUIT CURRENT PATH

Trace Adjacent to a Ground Plane (Microstrip)

(High-to-Low Transition)

Vcc

Load Capacitance Load Capacitance Decoupling Capacitor

Vcc

Parasitic Capacitance

Ground Plane

Signal Trace Current Source: Parasitic/Load Capacitors Return Current Path: Ground Plane

Source

Load

+

+

+

+

+

(7)

-DIGITAL CIRCUIT CURRENT PATH

Trace Adjacent to a Power Plane (Microstrip)

(Low-to-High Transition)

Vcc

Load Capacitance Load Capacitance Decoupling Capacitor

Vcc

Parasitic Capacitance

Power Plane

Signal Trace

Source

Load

Current Source: Parasitic/Load Capacitors Return Current Path: Power Plane

+

+

+

+

+

-

-

-

-

-Load

Source

(8)

DIGITAL CIRCUIT CURRENT PATH

Trace Adjacent to a Power Plane (Microstrip)

(High-to-Low Transition)

Vcc

Load Capacitance Load Capacitance Decoupling Capacitor

Vcc

Parasitic Capacitance

Power Plane

Signal Trace

Current Source: Decoupling Capacitor Return Current Path: Power Plane

+

-Load

Source

(9)

DIGITAL CIRCUIT CURRENT PATH

Trace Between a Power & Ground Plane (Stripline)

(Low-to-High Transition)

Vcc

Load Capacitance Load Capacitance Decoupling Capacitor

Vcc

Parasitic Capacitance Parasitic Capacitance

Ground Plane

Power Plane

Signal Trace

Source

Load

+

-Current Source: Decoupling/Parasitic(power) Cap. Return Current Path: Ground/Power Planes

+

+

+

+

+

-

-

-

-

-Note: Current Flows In the Same Direction In the Power & Ground Planes

Load

Note: Green Current From Decoupling Cap. Red Current From Parasitic Cap.

(10)

DIGITAL CIRCUIT CURRENT PATH

Trace Between a Power & Ground Plane (Stripline)

(High-to-Low Transition)

Vcc

Load Capacitance Load Capacitance Decoupling Capacitor

Vcc

Parasitic Capacitance Parasitic Capacitance

Ground Plane

Power Plane

Signal Trace

+

-Current Source: Decoupling/Parasitic(ground) Cap.

+

+

+

+

+

-

-

-

-

-Note: Current Flows In the Same Direction In the Power & Ground Planes

Load

Note: Red Current From Decoupling Cap. Green Current

(11)

SUMMARY–DIGITAL CIRCUIT CURRENT PATH

Trace Between a Power & Ground Plane (Stripline)

Ground Plane Power Plane Signal Trace

Source

Load

Load

Source

Low-to-High Transition

Ground Plane Power Plane

Source

Load

Load

Source

High-to-Low Transition

(12)

DRIVER IC CURRENT

Low-to-High Transition

—

In all Cases, Current Enters the Driver Through the

Power Pin and Exits the Driver Via the Signal Pin

High-to-Low Transition

—

In all Cases, Current Enters the Driver Through the

(13)

SUMMARY

Power & Ground

Plane

Decoupling &

Parasitic (Ground)

Capacitance

High-to-Low

Power &

Ground

Stripline

Power & Ground

Plane

Decoupling &

Parasitic (Power)

Capacitance

Low-to-High

Power &

Ground

Stripline

Power Plane

Decoupling

Capacitor

High-to-Low

Power

Microstrip

Power Plane

Parasitic/Load

Capacitance

Low-to-High

Power

Microstrip

Ground Plane

Parasitic/Load

Capacitance

High-to-Low

Ground

Microstrip

Ground Plane

Decoupling

Capacitor

Low-to-High

Ground

Microstrip

Return Current

Path

Current Source

Transition

Reference

Plane

Configuration

Note: For Stripline Referenced to Two Ground Planes, See Microstrip Referenced to a Ground Plane, Except Each Plane Carries Only One-Half the Current. Similarly For Stripline Referenced to Two Power Planes, See Microstrip Referenced to a Power Plane.

(14)

CONCLUSIONS

(1) From the Previous Examples it Can Be Concluded That it Makes

No Difference Whatsoever to the Digital Logic Current if the

Reference Plane, or Planes, Are Ground or Power. In All Cases

the Current Returns Directly to the Source Through a Small Loop.

In None of The Cases Does The Current Have to Go Out of its Way,

or Flow Through a Large Loop in Order to Return to the Source.

(2) The Answer to All Questions On Slide 2 is, It Does Not Matter!

All Are Equally Acceptable Configurations.

(3) Stripline, However, Will Always Be a Better Configuration Than

Microstrip, Since Two Current Loops Exist. In One Loop the Current

Flows Clockwise, and in The Other it Flows Counterclockwise

References

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