The second, major ground-noise problem, shown in Figure 5, is a result of parasitic-inductor capacitance.
Figure 5: Changing LX node voltage pumps charge through the parasitic buck-inductor capacitance, CL, and into the parasitic ground-path inductors, Lp1 and Lp2 , causing ground noise. (Click here for enlarged image.)
cannot change instantaneously across a capacitor, nor can current
instantaneously change through an inductor. So, voltage changes on the
LX node couple directly across both the parasitic buck-inductor
capacitance, CL, and the buck-filter capacitor, Cbuck, to appear across the parasitic ground inductors, Lp1 and Lp2.
no charge flows, but in the next moment, current builds in all of those
components until the energy stored in the parasitic buck inductor
ECL = ½ CLVLX2,
transfers to the wiring’s parasitic magnetic field,
ELp = ½ Lpi2changing_max
= the sum of all parasitic loop inductors). Then like a swing, that
unwanted energy passes back-and-forth from the electric to the magnetic
field until it radiates or dissipates in resistive elements not modeled
in Figure 5.
the peak voltage and the duration of a ground-noise oscillation are a
problem. The peak voltage, measured at node Vgb, is a function of the LX
node’s voltage change, the parasitic buck inductor capacitance, CL, and additional parasitic trace capacitance (not shown). A large CL
stores more energy, so smaller is better. After selecting the buck
inductor’s inductance and current rating, choose an inductor with the
highest self-resonate frequency to limit the capacity of CL.
An inductor’s self-resonate frequency is:
fself_resonates = 1/[2π√(LbuckCL)].
that a doubling of the self-resonate frequency reduces the parasitic
inductor capacitance, and therefore the ground-noise energy, by a factor
In the case where performance takes priority over cost, maintain the same value of inductance by replacing the single Lbuck inductor in Figure 5 with two series-connected inductors, each having ½Lbuck (Figure 6).
For a manufacturer’s series of inductors, the parasitic capacitance is
typically proportional to the rated inductance, so one-half the
inductance results in one-half the parasitic capacitance.
inductors are series connected, their values add to increase
inductance, but parasitic capacitors add as the inverse sum of inverse
values, to decrease total parasitic capacitance. In the case of two
series-connected one-half Lbuck inductors, total inductance will be Lbuck_new and total parasitic capacitance will drop by a factor of four to one-quarter CL.
This reduction is parasitic inductor capacitance will, in turn, reduce ground bounce, Figure 6.
6: Two series-connected inductors have the same inductance but with
one-quarter the parasitic capacitance; charge-pumping is reduced and,
therefore, so is ground bounce. (Click here for enlarged image.)
exploring the models and understanding the two sources and mechanisms
of ground noise as induced by the ubiquitous DC/DC switching converter,
engineers can minimize the effects in the early stages of design,
component selection, and layout, and the subsequent product headaches
About the author
is a Senior Director of Analog IC Design at Integrated Device
Technology, Inc. in Tucson, AZ. He works on the development and
usability of power integrated circuits and is an active analog-IC
designer. He received a bachelor's degree in electrical engineering from
the University of Arizona (Tucson), and his personal interests include
geology, astronomy, physics, and electronics.
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