round bounce” is the amount that aground return rises or falls relative to thesystem’s 0V reference, and, in a dc/dc-switching converter, ground bounce canbe many volts, often because of changingmagnetic flux. Magnetic flux is propor-

tional to a magnetic field that passes through a loop area. Fig-ure 1 illustrates magnetic flux in a simple circuit loop. A volt-age source pushes current through a resistor and around a loopof wire. Imagine that you are grabbing the wire with your righthand. Pointing your thumb in the direction of current flow, yourfingers 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-fieldstrength or the loop area, the flux will change, inducing a volt-age in the wire. Figure 2 shows the same circuit with an addedswitch. 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 lessimportant source of ground bouncethan magnetic-fluxchange. (The sheetresistance of 1-ozcopper is about 500��/�, so a 1Achange in currentproduces 500 �V/�of bounce—a prob-lem for thin, long,or daisy-chainedgrounds or precisionelectronics.) Para-sitic capacitance is apath for large tran-sient currents to a

ground return. The change in magnetic flux from those currentspikes induces ground bounce. Therefore, the best way to reduceground bounce in a switching dc/dc converter is to controlchanges in magnetic flux.

In a basic circuit, output current remains constant, but theloop area changes (Figure 3). In Figure 3a, ideal wires connectan ideal voltage source to an ideal current source. Current flowsin a loop that includes ground return. In Figure 3b, the switchchanges position. The current source is still dc, but the loop areachanges and generates a magnetic-flux change, inducing aground-bounce voltage (see sidebar “Five pc-board-layout con-figurations affect ground bounce”).

BUCK-CONVERTER GROUND BOUNCEThe buck converter in Figure 4 is similar to the simple cir-

cuit in Figure 3. At high frequencies, a large capacitor, such asa 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 bouncein dc/dc-converter applicationsELECTRICAL GROUND, WHICH LOOKS SIMPLE ON A SCHEMAT-IC, CAN BECOME COMPLEX DEPENDING ON HOW YOU LAYOUT THE PC BOARD. UNFORTUNATELY, GROUND-NODE ANALY-SIS IS DIFFICULT. HOWEVER, UNDERSTANDING THE PHYSICSOF GROUND NOISE HELPS TO REDUCE THE PROBLEM.

JULY 6, 2006 | EDN 73

Figure 2 When the formerly closed switch inthis simple circuit opens, the magnetic flux goesto zero, inducing a voltage along the circuit.

_

_

_

+

+

+INDUCED VOLTAGE =GROUND BOUNCE

VDC

Figure 1 Magnetic flux is proportional to the magnetic fieldpassing through a loop that a circuit forms.

VDC

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X X X X X

X X X X X

X

LOOPAREA

MAGNETICFLUX IS

MAGNETICFIELD

PASSINGTHROUGH

A LOOP AREA

MAGNETIC FIELDINTO THE PAGE

_+

“G

tions (Figure 5). The large buck inductor, LBUCK, holds the out-put current roughly constant. Similarly, CVIN maintains a moreor less constant voltage across the parasitic-input inductance,so the input current is also approximately constant. Althoughthe input and output currents are roughly dc, as the switchmoves 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 bouncealong 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 voltageeverywhere along a ground return brings up an interesting ques-tion: Where is true ground? After all, ground bounce means aground-return trace is bouncing with respect to ground, and youmust identify that point.

In the case of power-regulation circuits, true ground needs tobe at the point of load. A dc/dc converter delivers quality voltageand current to the load. All other points returning current are notgrounds but just return lines to ground. Figure 7 shows how care-ful placement of CVIN reduces ground bounce. Capacitor CVINbypasses the top of the high-side switch to the bottom of the low-side switch, shrinking the changing-loop area. Additionally, thechanging-loop area is isolated from the ground return. From the

74 EDN | JULY 6, 2006

Figure 4 A high-frequency switch sees a large CVIN as a voltagesource and a large buck inductor as a current source.

_+ +

VIN

_+

VDCIDC

LEAD INDUCTANCE LBUCK

CBUCKCVIN

SWITCH

SWITCH

LOAD VOUT

�

Figure 5 In this simple buck converter, the red current loops indi-cate the switch in Position 1, and the blue loops indicate theswitch in Position 2.

_+

_+ GROUND BOUNCE

VIN

LEAD INDUCTANCE LBUCK

CVIN

1

2

SWITCH

LOAD

VOUT

CBUCK

Figure 6 A switching power semiconductor replaces the simpli-fied switch of Figure 5.

_

_

+

+

LEADINDUCTANCE

LOADCHANGINGLOOP AREA

LOW-SIDESWITCH

HIGH-SIDESWITCH

GROUNDBOUNCE

CBUCK

LBUCK

VIN CVIN

CONTROL

Figure 7 CVIN bypasses the top of the high-side switch to the bot-tom of the low-side switch, shrinking the changing-loop area andgreatly reducing ground bounce.

_+

LEADINDUCTANCE

LOADCHANGINGLOOP AREA

NO GROUND BOUNCE

VOUT

CBUCK

LBUCK

VIN

CVIN

CONTROL

HIGH-SIDESWITCH

Figure 3 The area of the loop changes based on whether theswitch is in position 1 or 2 (a). The collapse of the magnetic fieldas I1 goes to zero induces a voltage (b).

_+

RETURN

RETURN

DCLOOP AREACONTAINING

MAGNETIC FLUX

_

_+

+

_

_

_

+

+

+

MAGNETIC-FIELD LINESCOLLAPSE

GROUND BOUNCE

NEWLOOPAREA

VDC

VDC

DC

I

I1 I2

TP1�0V. TP2=0V.

TP3=0V.

(a)

(b)

1

2

76 EDN | JULY 6, 2006

ground of VIN to the true ground ofthe load, no loop area or switch-current changes flow from one caseto the next. Consequently, groundreturn does not bounce.

Figure 8 shows an inferior butperhaps typical pc-board layout ofthe buck schematic in Figure 6. InFigure 8, the high-side switch ison, and dc current flow follows theouter red loop. The low-sideswitch is on, and dc current flownow follows the blue loop. Notethe changing loop area and,hence, the changing magneticflux. So, this arrangement induces

PC-BOARD-LAYOUT CONFIGURATIONS AFFECTGROUND BOUNCEConductors that cross atright angles do not interactmagnetically: The magneticfield from the vertical traceinduces positive and nega-tive voltages that cancel inthe horizontal trace (FigureA).

Magnetic-field linesaround parallel wires withequal currents canceleverywhere between thewires, so the total storedenergy is less than whatyou would find for the indi-vidual wires. Wide pc-board traces have lessinductance than narrowtraces (Figure B).

Magnetic-field linesaround parallel conductorswith opposite current flowcancel everywhere outsidethe conductors and addeverywhere inside. If youmake the inside loop areasmall, then the total mag-netic flux and, therefore,the inductance will also besmall (Figure C). Thisbehavior is the reason thatthe ac ground-plane returncurrent always flows underthe top-trace conductor.

Corners have moreinductance because boththe vertical and horizontaltraces see a magnetic fieldfrom themselves as well asfrom the perpendiculartrace (Figure D). A currentflows into a top trace,down a via, into a groundplane, and back up a via tothe bottom of the source(Figure E). The return cur-rent flows, with dc currenttaking the path of leastresistance and ac currenttaking the path of leastimpedance. Because top-trace corners and ground-plane cuts increase imped-ance, you can expectground bounce. Thechange in magnetic flux atthose points induces thebounce.

The upper trace in FigureF shows good layout prac-tice; the capacitor is in linewith the current flow, creat-ing a minimal loop size.The bottom trace, with thecapacitor at right angles tothe current flow, creates anunnecessarily large loop,resulting in ground bounce.

CANCELS

CURRENTFLOW

CURRENTFLOW

ADDS

ADDS

Figure B Wide pc-board traces have less inductance than narrowtraces.

CURRENT FLOW

CURRENT FLOW

MAGNETIC FIELDINTO THE PC BOARD FROM

THE VERTICAL TRACEINDUCES POSITIVE

AND NEGATIVEVOLTAGES THATCANCEL IN THE

HORIZONTAL TRACE.

Figure A The magnetic field from the vertical trace induces pos-itive and negative voltages that cancel in the horizontal trace.

Figure 8 A bad layout causes a large change in current-loop area.

CONTROL

HIGH-SIDESWITCH

VOUTVIN

CHANGINGLOOP AREA

GROUND

LOAD

L

CHANGE IN MAGNETIC FLUX INDUCES

GROUND BOUNCE

� �

CVIN

CBUCKLOW-SIDESWITCH

LBUCK

voltage, and the ground bounces.Figure 9 provides an example inwhich a solid ground plane maybe a poor choice. In this case,designers constructed a two-layer pc board so that a bypasscapacitor attaches at a right angleto a top-layer supply line. In Fig-ure 9a, the ground plane is solidand uncut, a common but some-times poor layout practice. Top-trace current flows through thecapacitor, down the via, and outthe ground plane.

Because ac current always

JULY 6, 2006 | EDN 77

C

CURRENT IN

CURRENT IN

CURRENT OUT

CURRENT OUT

VIAVIA

(a)

(b)

CURRENT-LOOP AREA

GROUND PLANE

VG

RO

UN

DB

OU

NC

E

+

�

C

VIA

SIGNALCURRENT

FLOWS OUTON TOPTRACE

CORNERINCREASESCURRENT-

LOOP-AREAGROUNDBOUNCE

CUT IN GROUNDPLANE INCREASES

CURRENT-LOOP-AREAGROUND BOUNCE

RETURN CURRENTTAKES THE PATH OFLEAST IMPEDENCE.

GROUND PLANE

GROUND BOUNCE

DC-RETURN-CURRENT PATH

AC RETURN

GROUNDBOUNCE

GROUNDBOUNCE

VIA VIAAC AND DCCURRENTSOURCE

Figure F In good layout practice, the capacitor is in line with thecurrent flow, creating a minimal loop size (a). Having the capacitorat right angles creates a large loop, causing ground bounce (b).

THE VERTICAL TRACE SEES AMAGNETIC FIELD FROM ITSELFAND FROM THE HORIZONTAL

TRACE. THE HORIZONTALTRACE SEES A MAGNETIC

FIELD FROM ITSELF AND FROMTHE VERTICAL TRACE.

CORNERS HAVE MORE INDUCTANCE.

AC-CURRENT FLOW

Figure D Both the vertical and horizontal traces see a magneticfield from themselves as well as from the perpendicular trace.

Figure E A current flows into a top trace, down a via, into aground plane, and back up a via to the bottom of the source.

CANCELS

CURRENTFLOW

CURRENTFLOW

CANCELS

MAGNETIC FIELD

ADDS, SO IFSEPARATION=0,

THEN INDUCTANCE=0

Figure C If the inside loop area is small, then the total magneticflux and, therefore, the inductance will also be small.

Figure 9 The ground plane is solid and uncut, and the top-trace current flows through the capaci-tor, down the via, and out the ground plane (a). A careful cut in the ground plane constrains thereturn current to a minimum-loop area and fixes the bounce (b).

NO GROUND BOUNCE

VIA

VIA

VGROUND BOUNCECURRENT-LOOP AREA GIVES RISE

TO GROUND BOUNCE

GROUND PLANEON BOTTOM LAYER

CUT IN GROUND

PLANECONSTRAINS

CURRENT- LOOP AREA

SURGICAL CUTTO GROUND

PLANE ELIMINATES GROUND BOUNCE

C

C

(a)

+

–

(b)

takes the path of least impedance, ground-return current cuts the corner on its way backto the source. But the current has a magneticfield and draws out a loop area that changeswith a change in current magnitude or fre-quency. Magnetic flux is in that loop andchanges if either the current magnitude or thefrequency changes. That change means that asheet ground plane—even if it’s supercon-ducting—can bounce. However, a careful cutin the ground plane constrains the return cur-rent to a minimum loop area and fixes thebounce. This approach also isolates the cutreturn line’s bounce voltage from the generalground plane.

The pc-board layout in Figure 10 uses thesame principle as the one in Figure 9 toreduce ground bounce. Designers built a two-layer pc board’s input capacitor and bothswitches over an island in the ground plane.This layout is not necessarily the best, but itworks well and illustrates the key principles.Note the large loop areas traced out by the redand blue current paths in Figure 10. But thetwo loops differ only slightly. The smallchange in loop area means a small change inmagnetic flux and, hence, a small groundbounce. Additionally, in the ground-return island in whichmagnetic fields and loop area do change, the cut contains anyground-return bounce. Also, the input capacitor, CVIN (Figure10), may look as if it resides in a different area from the high-

and low-side switches from CVIN in Figure 7, but it is actuallyelectrically nearby. Physical proximity can be good, but theelectrical proximity that you achieve by minimizing the looparea is more important.

BOOST-CONVERTER GROUND BOUNCEA boost converter is the inverse of a buck converter, so you

must place the output capacitor so that it goes from the top ofthe high-side switch to the bottom of the low-side switch to min-imize the change in loop area (Figure 11). Ground bounce resultsprimarily from a change in magnetic flux, which induces aground-bounce voltage. In a dc/dc-switching power supply, the fluxchanges because high-speed switchesdirect current to different current-loopareas. However, careful placement ofthe buck/boost-input/output capacitorand a surgical cut to a ground planecan isolate bounce. Be careful whencutting a ground plane, because doingso can increase the loop area for someother return current in the circuit.EEDDNN

AU T H O R ’ S B I O G R A P H YJeff Barrow is a design manager for the Display Technology Prod-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.

78 EDN | JULY 6, 2006

Figure 11 Unlike in a bad design (a), you must place the boostconverter’s output capacitor from the top of the high-side switchto the bottom of the low-side switch to minimize the change inloop area (b).

_+

_

+CHANGINGLOOP AREA

VOUT

LBOOST SWITCH

VIN CVOUT

LOAD2

1

SWITCH1

_+

_

+VOUT

LBOOST

VIN CVOUT

LOAD2

(a)

(b)

Figure 10 Designers built a two-layer pc board so that the input capacitor and bothswitches are over an island in the ground plane. Although the loop area by the redand blue currents is large, the difference between the two loops is small. The smallchange in loop area means a small change in magnetic flux and a correspondinglysmall ground bounce.

CONTROL

LENGTH OFCHANGING LOOP

HIGH-SIDESWITCH

GROUND PLANEON BOTTOM LAYER

TOP LAYERVIN

LOADVOUTL

LOW-SIDE

SWITCHCVIN

CBUCK

VIA VIAVIA VIA

VIA VIA

OVERLAPPINGLOOP AREAS

FOR BOTH CASES

DC GROUND-PLANERETURN CURRENTS

CHANGE ISLOOP=LENGTH�

BOARD THICKNESSCUT IN

GROUNDPLANE

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