IEEE Solid-States Circuits Magazine - Summer 2023 - 11

flow off chip and over to the board
power supply to complete this loop,
it's going to cause all kinds of trouble,
such as making the supply rails
sag and/or ring.
Bypass capacitors (also referred to
-
VDD
+ External_Supply
Load
as decoupling capacitors; see " Bypass
or Decoupling? " ) are supposed to provide
a local path for the return currents,
as shown in Figure 2(b). But if
you don't think carefully about the
current path, you can put the capacitors
in the wrong place.
We usually treat bypass capacitors
as small batteries. Conceptually, they
provide quick surges of current to
meet local demand. That's not wrong,
but unless you focus on exactly what
currents are flowing and where, you
can set yourself up for disaster.
Figure 3 is an example of what
can go wrong. Figure 3(a) shows the
normal,
if
somewhat uninformed,
practice of just putting caps across
the supply. While this is a big improvement
over doing nothing, it can still
fail to suppress supply noise. Notice
that the current must flow down one
finger, onto the bus, and then back
down another finger causing a substantial
divot in the supply voltage
if the di/dt is large. (There is also
another path through the ground
bus, which is not highlighted with
the dashed line.) Of course, long
thin fingers of supply routing like
those in Figure 3 are a very bad idea,
but it happens.
[By the way, an inverter is shown
as the driver in Figures 2 and 3 to
emphasize that this is not exclusively
an analog problem. This does not
mean that you need to fuss over every
single logic gate. The return current
path becomes a concern when you
have large currents (for which low
path resistance is crucial), large di/dt
(for which low path inductance is crucial),
or long distance (where the path
is likely to be complex). The threshold
for special treatment also depends on
the sensitivity of the other circuitry
sharing the supply and ground lines
in question. If all of the circuitry is
digital, you can safely ignore many
of the transients. If the circuitry contains
precision analog, you cannot.]
+
(a)
VDD
Cbypass
Load
(b)
FIGURE 2: The principle is simple: keep the transient return path short. (a) Wrong. (b) Right.
(The dashed line is the path used by transient current.)
BYPASS OR DECOUPLING?
What do you call those capacitors that we sprinkle around to keep the supply voltage from
moving? Out of habit, most of us call them decoupling capacitors. They keep one block
from disturbing another block through the supply, hence " decoupling. " This must have gotten
started in the old discrete op-amp days when people would have cascades of op-amps
daisy-chained on a single supply wire. As you might expect, this would often lead to an
ugly low-frequency oscillation known as " motorboating " because of what it sounded like in
audio channels. Big capacitors were used to fix the problem, thus " decoupling " one op-amp
from another.
You will also see the term " bypass " used for these supply capacitors, often with a more highfrequency
nuance. For this column, the label " bypass capacitor " is preferred because it emphasizes
the need to give current transients a shortcut back to their source, to " bypass " the rest of
the supply network. " Decoupling " sounds more like a filtering action, which is not what you
want to do with supply networks. (Filtering implies impedance, at least at some frequencies,
and that is the last thing you need in your supplies.) No, the object of these capacitors is not to
filter out ugly noise; it is to prevent the noise from happening in the first place.
+ External_Supply
(a)
(b)
FIGURE 3: Complex in practice: simply putting capacitors across the supplies doesn't always
work. (a) Conventional practice. (b) The right way.
IEEE SOLID-STATE CIRCUITS MAGAZINE
SUMMER 2023
11
VDD
VDD
GND
GND

IEEE Solid-States Circuits Magazine - Summer 2023

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