Learn about the physics of current in a wire and why you should be concerned with small amounts of inductance.
PCB designers often collect information from a variety of sources, including “streets and data sheets.” To make good decisions, you must understand the underlying physics and mechanical construction of its components. This article discusses inductance as it applies to ground bounce.
How does current travel through a wire? A review of electromagnetic physics
A wire with charges that flow randomly back and forth in short sections of its length will have no measurable current or magnetic field. However, if there is a potential difference between two points on that wire, charges in places with high electrical energy potential will begin to migrate to places with lower electrical energy potential, creating a current.
As the current increases from zero, a certain amount of energy will be transferred and stored in a magnetic field surrounding the wire. As time increases, the changes in the electric field and the magnetic field will propagate along and out of the wire. These changes in electric and magnetic fields occur at a rapid but finite rate that is determined by the permittivity and permeability of the wire and its surroundings.
In the image below, a wire (green / red cylinder) is shown with the accompanying magnetic fields, shown in circular cross sections. The green / red color of the wire represents 0 V and 1 V of potential.
When the left end of the cable transits from 0 V to 1 V, the charges begin to move to the right, generating a magnetic field that surrounds the cable. When the wire returns to 0, the magnetic field dissipates.
This visual aid shows that transitions to electromagnetic fields take time to propagate. This is an artistic interpretation rather than a strict mathematical model by Mark Hughes.
As long as the current is constant, the magnetic field remains constant. If the current decreases, the magnetic field will decrease, but not before it puts up a fight.
The energy previously stored in the magnetic field is rapidly converted into electrical energy in the form of a potential difference that induces a current to run along the conductor. Over a period of time, the energy of the magnetic field and the potential difference are reduced to zero.
The role of inductors
Inductors have the ability to create large instantaneous voltages, a property often exploited in boost voltage converters. If the amplitude of the current can be made to change in a very short period of time, as is often the case during switching states, the potential difference generated will be significant, even if the inductance of the circuit is very low.
$$ triangle text {V} = – L frac {text {dI}} {text {dt}} $$
Modern ICs happen to be designed with very low rise and fall times (<10-9s), they have multiple output pins sharing the same Vss and Gnd lines (which increases current during simultaneous switching ), and have very small noise margins. That means we have to worry about very small amounts of inductance.
During switching, induced voltages can make an IC’s Gnd and Vss potentials significantly different than the Gnd and Vss potential of the rest of the circuit.
Package inductance
Inside an IC package there is a small die. The matrix pads are connected to the outer pins of the pack with thin bonding cables. Those cables have a small, but not trivial inductance associated with them. The short switching intervals, combined with the inductance of the packet (mainly attributed to the junction wires), can cause significant voltages to be generated between the pins of the packets and the semiconductors in the IC die.
Most PCB designers have no control over package construction, but can sometimes select smaller packages or reverse chip packages in their designs; it is almost certain that a QFN package will have a lower inductance than a DIP package containing the same die.
Inside each IC pack is a knockout that is physically connected to the pins of the IC pack, usually with tie wires.
Trace inductance
Unlike package inductance, you can manipulate your PCB layout to decrease or increase the inductance of your circuit. Take steps to decrease inductance whenever possible. Do this by providing immediate and uninterrupted return paths for all of your signal lines, provide return paths to ground close to your signal paths, and provide uninterrupted ground planes on adjacent layers. For more information, see this TI application note.
This 3D cross section and top view of a dummy circuit shows the current path in the vertical and horizontal orientation through an arbitrary component. All changing signal lines will find a return path to the source.
Circuits store recoverable energy in electromagnetic fields that surround their tracks. When that field energy is converted back to a potential difference within your circuit, it can disrupt logic states and cause your circuit to behave unpredictably. Take a moment to read this article on bouncing off the ground to learn more.
