# Understand and reduce DC/DC switching-converter ground noise

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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, C _{L}, and into the parasitic ground-path inductors, L_{p1} and L_{p2} , causing ground noise*.

*(Click*

**here**for enlarged image.)

Voltage
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, C_{L}, and the buck-filter capacitor, C_{buck}, to appear across the parasitic ground inductors, L_{p1} and L_{p2}.

Initially, no charge flows, but in the next moment, current builds in all of those components until the energy stored in the parasitic buck inductor capacitor,

E_{CL} = ½ C_{L}V_{LX}^{2},

transfers to the wiring’s parasitic magnetic field,

E_{Lp} = ½ L_{p}i^{2}_{changing_max}

(where L_{p}
= 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.

Both
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, C_{L}, and additional parasitic trace capacitance (not shown). A large C_{L}
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 C_{L}.

An inductor’s self-resonate frequency is:

f_{self_resonates }= 1/[2π√(L_{buck}C_{L})].

Notice that a doubling of the self-resonate frequency reduces the parasitic inductor capacitance, and therefore the ground-noise energy, by a factor of four!

In the case where performance takes priority over cost, maintain the same value of inductance by replacing the single L_{buck} inductor in Figure 5 with two series-connected inductors, each having ½L_{buck} (**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.

When
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 L_{buck} inductors, total inductance will be L_{buck_new} and total parasitic capacitance will drop by a factor of four to one-quarter C_{L}.

This reduction is parasitic inductor capacitance will, in turn, reduce ground bounce, **Figure 6**.

*Figure
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.)*

By 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 and re-spins.

** **

**About the author**

** Jeff Barrow**
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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