AN-42036 Datasheet, PDF (1/3 Pages) Fairchild Semiconductor – PCB Grounding System and High Performance DC-DC Converters

www.fairchildsemi.com
AN-42036
PCB Grounding System and FAN2001/FAN2011
High Performance DC-DC Converters
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.
VCC
LINEAR LINEAR
CIRCUIT CIRCUIT
POWER
CIRCUIT
SYSTEM
GROUND
Parasitic Ground Resistance
Power Ground Current
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.
Ground Loop
Switching
Magnetic Field
VCC
LINEAR LINEAR
CIRCUIT CIRCUIT
SYSTEM
GROUND
Parasitic Ground Resistance
Magnetically Induced Ground
Current
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
REV. 1.00 4/05/05