the radiated emissions are going to be until
the system is tested. And radiated emissions
testing cannot be formally performed until the
design is essentially complete.
Filters are often used to reduce EMI by attenuating
the strength at a certain frequency or over
a range of frequencies. A portion of this energy
that travels through space (radiated) is attenuated
by adding metallic and magnetic shields.
The part that rides on PCB traces (conducted)
is tamed by adding ferrite beads and other
filters. EMI cannot be eliminated but can be
attenuated to a level that is acceptable to other
communication, signal processing and digital
components. Moreover, several regulatory bodies
enforce standards to ensure compliance in
both industrial and automotive systems.
Modern input filter components in surface
mount technology have better performance
than through-hole parts. However, this improvement
is outpaced by the increased demands
created by today's high frequency switching
regulators. The low minimum on and off times
required at higher operating frequencies result
in higher harmonic content due to the faster
switch transitions, thereby increasing radiated
noise. However, these high switch edge rates
are needed to get higher conversion efficiencies.
A switched capacitor charge pump does
not exhibit this behavior since it operates at
Figure 1. Simplified charge pump block diagram
of a voltage inverter
much lower switching frequencies and most
importantly can tolerate slower switching transitions
without degradation in efficiency.
Experienced PCB designers will make the ‘hot’
loops small, and use shielding ground layers as
close to the active layer as possible. Nevertheless,
device pin-outs, package construction,
thermal design requirements and package sizes
needed for adequate energy storage in decoupling
components dictate a minimum hot-loop
size. To further complicate matters, in typical
planar printed circuit boards, the magnetic
or transformer-style coupling between traces
above 30 MHz will outpace all filter efforts since
the higher the harmonic frequencies are, the
more effective unwanted magnetic coupling
SWITCHED CAPACITOR CHARGE PUMPS
Charge pumps have been around for decades,
and they provide DC/DC voltage conversion,
using a switch network to charge and
discharge two or more capacitors. The basic
charge pump switch network toggles between
charge and discharge states of the capacitors.
As shown in Figure 1, C1 the “flying capacitor”
shuttles charge, and C2 the “reservoir capacitor”
holds charge and filters the output voltage.
Additional “flying capacitors” and switch arrays
enable multiple gains.
When switches S1 and S3 are on, or closed,
and switches S2 and S4 are off, or open, the
input power supply charges C1. During the next
cycle, S1 and S3 are off, S2 and S4 are on, and
charge transfers to C2, generating VOUT = -
However, until recently, charge pumps have
had limited input and output voltage ranges,
which has limited their use in industrial and
automotive applications where inputs up to 40V
or greater are commonplace.
The conclusion of this article describes a recently
introduced highly-integrated, low-noise,
step-down dual voltage regulator that uses
switched-capacitor conversion. Click the panel
below for full pdf.
Find Voltage Regulators
25 DesignNews NOVEMBER 2017 www.eedesignnewseurope.com