Fairchild PCB Grounding System and FAN2001 - High Performance DC-DC Converters Data Manual

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AN-42036 PCB Grounding System and FAN2001/FAN2011 High Performance DC-DC Converters Introduction

Ground wiring can often limit system performance. The overall output voltage accuracy are affected by this noise. purpose of this application note is to help the system Another related issue of the ground system is its tendency to designer utilize the benefits of the FAN2011 and FAN2001 carry inductively induced currents. Since the ground system families of high performance step-down DC-DC regulators, can be spatially extended, it is possible to generate to avoid PCB layout related problems and maximize overall circulating currents in the ground caused by changing system performance. The basic concept of “star grounding” external magnetic fields. Because the ground system has is presented and ways of implementing star grounding for usually low resistance, the ground loop currents can become systems like a cell phone, are shown. quite large. This often happens in larger systems that magnetically couple to the 50 or 60 Hz power supply lines, Grounding Issues which are common in many areas. These circulating currents can then induce noise in the sensitive linear circuits through

Ground is defined as a reference potential for linear circuit

parasitic ground resistance, as shown in Figure 2.

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

Ground Loop Switching

converters. It is used as the return for power systems

Magnetic Field

elements such as switching regulators, power amplifiers, and digital circuit blocks. It is also used as a shield to prevent the VCC

propagation of electromagnetic noise in the circuit. Because LINEAR LINEAR CIRCUIT CIRCUIT

of these diverse uses, care must be taken in laying out the PCB ground system to avoid interference between different areas of the board. SYSTEM Parasitic Ground Resistance Magnetically Induced Ground

GROUND Current

The finite resistance of the grounding path may couple the noise generated by the power elements into the signal path of

Figure 2. Coupling of Magnetically Induced Ground Cur-

the precision circuit elements, as shown in Figure 1.

rent Noise

The Star Ground Technique

In all systems, the correct choice of the ground junction

LINEAR LINEAR POWER

point or single point ground reference, as it is sometimes

CIRCUIT CIRCUIT CIRCUIT

called, is important. The power management area in particular requires special care as it supplies the current for Parasitic Ground Resistance Power Ground Current SYSTEM the whole system. It is necessary to first evaluate the flow of

GROUND 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

Figure 1. Coupling of Ground Current Noise

offsets and noise into the signal return paths.

If a ground connection carries substantial current, a voltage

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

drop will develop in the ground because of the basic

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

resistance of the ground material. This voltage drop makes

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

local ground different than zero volts, depending on circuit

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

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

Any one of the ground lines radiating from the star ground

there are ground sensitive circuits connected to this current

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

path, the voltage drop across the parasitic ground resistance

Special care must be taken to separate the signal ground

will produce offsets in the voltage measurements of the

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

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

will flow through any signal line ground. Cell phone system

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

design is complex as it compounds various sub-circuits such

the system. Linear circuits are referenced to ground and

as analog baseband, digital baseband, audio amplifiers, LCD

some parameters of the converter such as load regulation and

REV. 1.00 4/05/05