EDN_Ground_bounce

round bounce” is the amount that a ground return rises or falls relative to the system’s 0V reference, and, in a dc/dc-switching converter, ground bounce can be many volts, often because of changing magnetic flux. Magnetic flux is propor-tional to a magnetic field that passes through a loop area. Fig-ure 1illustrates magnetic flux in a simple circuit loop. A volt-age source pushes current through a resistor and around a loop of wire. Imagine that you are grabbing the wire with your right hand. Pointing your thumb in the direction of current flow, your fingers wrap around the wire in the direction of the magnetic-field lines. As those field lines pass through the loop, they estab-lish magnetic flux. If you change either the magnetic-field strength or the loop area, the flux will change, inducing a volt-age in the wire. Figure 2shows the same circuit with an added switch. When the switch opens, current stops flowing, so the mag-netic flux collapses, inducing a voltage everywhere along the wire.Generally, pc-board-ground-plane resistance is a less important source of ground bounce

than magnetic-flux

change. (The sheet resistance of 1-oz copper is about 500??/Ⅺ, so a 1A change in current produces 500 ?V/Ⅺof bounce—a prob-lem for thin, long,or daisy-chained grounds or precision

electronics.) Para-sitic capacitance is a path for large tran-sient currents to a

ground return. The change in magnetic flux from those current spikes induces ground bounce. Therefore, the best way to reduce ground bounce in a switching dc/dc converter is to control changes in magnetic flux.In a basic circuit, output current remains constant, but the loop area changes (Figure 3). In Figure 3a , ideal wires connect

an ideal voltage source to an ideal current source. Current flows in a loop that includes ground return. In Figure 3b , the switch changes position. The current source is still dc, but the loop area changes and generates a magnetic-flux change, inducing a ground-bounce voltage (see sidebar “Five pc-board-layout con-figurations affect ground bounce”).BUCK-CONVERTER GROUND BOUNCE The buck converter in Figure 4is similar to the simple cir-cuit in Figure 3. At high frequencies, a large capacitor, such as a buck input capacitor, looks like a dc voltage source. Similar-ly, the large output buck inductor looks like a dc current source.Magnetic flux changes as the switch moves between the posi-BY JEFF BARROW ?ANALOG DEVICES

Reducing ground bounce in dc/dc-converter applications

ELECTRICAL GROUND, WHICH LOOKS SIMPLE ON A SCHEMAT-IC, CAN BECOME COMPLEX DEPENDING ON HOW YOU LAY OUT THE PC BOARD. UNFORTUNATEL Y, GROUND-NODE ANAL Y-SIS IS DIFFICUL T. HOWEVER, UNDERSTANDING THE PHYSICS OF GROUND NOISE HELPS TO REDUCE THE PROBLEM.

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“G

BUCK put current roughly constant. Similarly, C VIN maintains a more or less constant voltage across the parasitic-input inductance,so the input current is also approximately constant. Although the input and output currents are roughly dc, as the switch moves from Position 1 to Position 2, the total loop area rapid-ly changes in the circuit’s middle. This change means that mag-netic flux is changing, which in turn induces ground bounce along the return wire.

Buck converters comprise semiconductor switches (Figure 6).But, as the complexity increases, the analysis of ground bounce

that changing magnetic flux induces remains simple and intu-itive. Knowing that a change in magnetic flux induces voltage everywhere along a ground return brings up an interesting ques-tion: Where is true ground? After all, ground bounce means a ground-return trace is bouncing with respect to ground, and you must identify that point.

In the case of power-regulation circuits, true ground needs to be at the point of load. A dc/dc converter delivers quality voltage and current to the load. All other points returning current are not grounds but just return lines to ground. Figure 7shows how care-ful placement of C VIN reduces ground bounce. Capacitor C VIN bypasses the top of the high-side switch to the bottom of the low-side switch, shrinking the changing-loop area. Additionally, the changing-loop area is isolated from the ground return. From the

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provides an example in

Fig-, the ground plane is solid

op-Array

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ucts Group at Analog Devices (Tucson, AZ), where he is respon-sible for the development of multichannel power ICs and analog-sig-nal conditioners for LCD TVs and monitors. He received a bache-lor’s degree in electrical engineering from the University of Arizona (Tucson), and his personal interests include physics and electronics.

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