AN-42036

PCB Grounding System and FAN2001/FAN2011

High Performance DC-DC Converters

www.fairchildsemi.com

REV. 1.00 4/05/05

Introduction

Ground wiring can often limit system performance. The

purpose of this application note is to help the system

designer utilize the benefits of the FAN2011 and FAN2001

families of high performance step-down DC-DC regulators,

to avoid PCB layout related problems and maximize overall

system performance. The basic concept of “star grounding”

is presented and ways of implementing star grounding for

systems like a cell phone, are shown.

Grounding Issues

Ground is defined as a reference potential for linear circuit

elements such as amplifiers, voltage references, and A/D

converters. It is used as the return for power systems

elements such as switching regulators, power amplifiers, and

digital circuit blocks. It is also used as a shield to prevent the

propagation of electromagnetic noise in the circuit. Because

of these diverse uses, care must be taken in laying out the

PCB ground system to avoid interference between different

areas of the board.

The finite resistance of the grounding path may couple the

noise generated by the power elements into the signal path of

the precision circuit elements, as shown in Figure 1.

Figure 1. Coupling of Ground Current Noise

If a ground connection carries substantial current, a voltage

drop will develop in the ground because of the basic

resistance of the ground material. This voltage drop makes

local ground different than zero volts, depending on circuit

topology and element location. If the current is high and if

there are ground sensitive circuits connected to this current

path, the voltage drop across the parasitic ground resistance

will produce offsets in the voltage measurements of the

sensitive linear circuit. In the case of a switching regulator,

where the power current is time varying, noise is induced in

the system. Linear circuits are referenced to ground and

some parameters of the converter such as load regulation and

overall output voltage accuracy are affected by this noise.

Another related issue of the ground system is its tendency to

carry inductively induced currents. Since the ground system

can be spatially extended, it is possible to generate

circulating currents in the ground caused by changing

external magnetic fields. Because the ground system has

usually low resistance, the ground loop currents can become

quite large. This often happens in larger systems that

magnetically couple to the 50 or 60 Hz power supply lines,

which are common in many areas. These circulating currents

can then induce noise in the sensitive linear circuits through

parasitic ground resistance, as shown in Figure 2.

Figure 2. Coupling of Magnetically Induced Ground Cur-

rent Noise

The Star Ground Technique

In all systems, the correct choice of the ground junction

point or single point ground reference, as it is sometimes

called, is important. The power management area in

particular requires special care as it supplies the current for

the whole system. It is necessary to first evaluate the flow of

the power supply and signal currents, and then configure the

path of the return currents and the location of the junction

point. This is necessary to minimize the introduction of DC

offsets and noise into the signal return paths.

In star grounding, a common point is chosen to which to

connect all ground lines. A physical point is chosen on the

circuit board to serve as the star ground point and all grounds

radiate from that point to the separate parts of the system.

Any one of the ground lines radiating from the star ground

may star again, forming a local star ground for a sub-circuit.

Special care must be taken to separate the signal ground

from the power ground at a star, such that no power current

will flow through any signal line ground. Cell phone system

design is complex as it compounds various sub-circuits such

as analog baseband, digital baseband, audio amplifiers, LCD

LINEAR

CIRCUIT

LINEAR

CIRCUIT

POWER

CIRCUIT

Parasitic Ground Resistance

Power Ground Current

SYSTEM

GROUND

VCC

LINEAR

CIRCUIT

LINEAR

CIRCUIT

Parasitic Ground Resistance

Magnetically Induced Ground

Current

SYSTEM

GROUND

VCC

Ground Loop

Switching

Magnetic Field