Clean Power for Every IC, Part 1:=20 Understanding Bypass Capacitors
Articles That Provide Supporting Information
Capacitors, Capacitors Everywhere
It is not inconceivable that a dedicated, successful engineering = student=20 would graduate from college knowing almost nothing about one of the most = pervasive and important components found in real circuits: the bypass = capacitor.=20 Even experienced engineers may not fully understand why they include 0.1 = =C2=B5F=20 ceramic capacitors next to every power pin of every IC in every circuit = board=20 they design. This article provides information that will help you to = understand=20 why bypass capacitors are necessary and how they improve circuit = performance,=20 and a follow-up article will focus on details related to choosing bypass = capacitors and the PCB layout techniques that maximize their = efficacy.
The Perils of Transient Current
Any component in which outputs transition rapidly from one state to = another=20 will generate transient currents. When these transient currents are = drawn=20 directly from the power supply, transient voltages are created as a = result of=20 the power supply=E2=80=99s source impedance as well as parasitic = inductance associated=20 with wires and PCB traces. This effect is increasingly problematic when = a=20 component must drive a low-resistance or high-capacitance load: = low-resistance=20 loads create higher-magnitude transients, and high-capacitance loads can = lead to=20 ringing or even severe oscillations in the power line. The end result = can be=20 anything from suboptimal circuit performance to system failure.
Let=E2=80=99s briefly explore this issue of transient current using a = very simple=20 simulation.
The circuit is the famous CMOS inverter, as confirmed by the = relationship=20 between the input and output voltage. Though the eminently clever design = of this=20 inverter requires no steady-state current, we need to remember that = significant=20 transient current flows as the input voltage passes through the region = in which=20 both transistors are conducting. This current creates a disturbance on = the=20 inverter=E2=80=99s voltage supply corresponding to the voltage drop = across the source=20 resistance (this simulation uses 2 =E2=84=A6, which is about how much = internal=20 resistance you would expect from a 9 V battery):
It is true that the magnitude of this disturbance is very small, but = remember=20 that an integrated circuit could contain hundreds or thousands or = millions of=20 inverters. Without proper bypassing, the cumulative effect of all these=20 transient currents would be a seriously noisy=E2=80=94if not = catastrophically=20 unstable=E2=80=94voltage supply. Experiments performed by engineers at = Texas Instruments=20 demonstrated that an improperly bypassed line driver IC switching at 33 = MHz=20 resulted in ringing amplitude as high as 2 V peak-to-peak=E2=80=94on a 5 = V power=20 rail!
The following plot shows the supply voltage when the simulation = circuit is=20 expanded to include a mere 8 inverters along with 1 nH of parasitic = inductance=20 in series with the source resistance:
The magnitude of the transients has increased to almost 0.5 mV, and = both=20 disturbances exhibit some oscillatory behavior:
Digital circuits certainly have a special aptitude for degrading = power=20 quality, but analog ICs also need bypassing to compensate for rapid = output=20 transitions and to protect them from power supply noise generated by = other=20 devices. For example, an op-amp=E2=80=99s power supply rejection ratio = decreases as the=20 power supply noise increases in frequency; this means that an improperly = bypassed op-amp could create high-frequency power-line disturbances that = would=20 propagate to the op-amp=E2=80=99s own output signal.
The Solution
It is convenient that such a serious problem can be effectively = resolved with=20 a simple, widely available component. But why the capacitor? A = straightforward=20 explanation is the following: A capacitor stores charge that can be = supplied to=20 the IC with very low series resistance and very low series inductance. = Thus,=20 transient currents can be supplied from the bypass capacitor (through = minimal=20 resistance and inductance) instead of from the power line (through = comparatively=20 large resistance and inductance). To better understand this, we need to = review=20 some basic concepts related to how a capacitor affects a circuit.
First, though, a brief note about terminology: The components = discussed in=20 this article are regularly referred to as both =E2=80=9Cbypass = capacitors=E2=80=9D and=20 =E2=80=9Cdecoupling capacitors.=E2=80=9D There is a subtle distinction = here=E2=80=94=E2=80=9Cdecoupling=E2=80=9D refers=20 to reducing the degree to which one part of a circuit influences = another, and=20 =E2=80=9Cbypass=E2=80=9D refers to providing a low-impedance path that = allows noise to =E2=80=9Cpass by=E2=80=9D=20 an IC on its way to the ground node. Both terms can be correctly used = because a=20 bypass/decoupling capacitor accomplishes both tasks. In this article, = however,=20 =E2=80=9Cbypass capacitor=E2=80=9D is favored in order to avoid = confusion with a series=20 decoupling capacitor used to block the DC component of a signal.
Charging and Discharging
The fundamental action of a capacitor is storing charge and releasing = charge=20 in such a way that it opposes changes in voltage: If the voltage = suddenly=20 decreases, the capacitor supplies current from its charged plates in an = attempt=20 to maintain the previous voltage. If the voltage suddenly increases, the = capacitor=E2=80=99s plates store charge from the current generated by = the increased=20 voltage. The following simple simulation can help you to visualize = this:
Note that the current is positive (i.e., flowing from the source = through=20 R1 to C1) when the capacitor is charging and = negative=20 (i.e., flowing from C1 through R1 to the = source)=20 when the capacitor is discharging.
This fundamental charging and discharging behavior does not change = depending=20 on whether the capacitor is exposed to low-frequency or high-frequency = signals.=20 However, in a discussion of power supply bypassing, it is helpful to = analyze a=20 capacitor=E2=80=99s influence in two different ways=E2=80=94one for = low-frequency situations and=20 one for high-frequency situations. In a low-frequency or DC context, a = bypass=20 capacitor opposes changes in the voltage line by charging or = discharging. The=20 capacitor functions like a low-impedance battery that can supply small = amounts=20 of transient current. In a high-frequency context, the capacitor is a=20 low-impedance path to ground that protects the IC from high-frequency = noise on=20 the power line.
A Standard Approach
The foregoing analysis helps us to understand a classic bypassing = scheme: a=20 10 =C2=B5F capacitor within an inch or two of the IC, and a 0.1 =C2=B5F = ceramic capacitor=20 as close to the power pin as possible:
The larger capacitor smooths out lower-frequency variations in the = supply=20 voltage, and the smaller capacitor more effectively filters out = high-frequency=20 noise on the power line.
If we incorporate these bypass capacitors into the 8-inverter = simulation=20 discussed above, the ringing is eliminated and the magnitude of the = voltage=20 disturbance is reduced from 1 mV to 20 =C2=B5V:
Ideal vs. Reality
At this point you may be wondering why we need a 0.1 =C2=B5F = capacitor in addition=20 to a 10 =C2=B5F capacitor. What is the difference between 10 =C2=B5F and = 10.1 =C2=B5F? This is=20 where the bypass-cap discussion becomes more complicated. The efficacy = of a=20 particular bypassing scheme is closely related to two of the chosen = capacitor=E2=80=99s=20 nonideal characteristics: equivalent series resistance (ESR) = and=20 equivalent series inductance (ESL). In the simulation just mentioned, = the=20 parallel 10 =C2=B5F and 0.1 =C2=B5F ideal capacitors become nothing more = than a 10.1 =C2=B5F=20 ideal capacitor. To make the simulation anything approaching realistic, = we need=20 to include reasonable values of ESR and ESL. With this modification, we = have the=20 following:
Though still an improvement compared to the case with no bypass = capacitors,=20 these results are significantly worse than what we saw with the ideal = caps.
This simple simulation cannot possibly account for all the parasitic=20 impedances and other subtle influences present with real integrated = circuits on=20 a real PCB (especially one that includes high-speed digital signals). = The point=20 here is to demonstrate that designing a bypass network involves careful=20 consideration of a capacitor=E2=80=99s ESR and ESL. Equally important = are proper=20 component placement and PCB layout techniques. We will explore all these = details=20 in the next article.
Next Article in Series: Clean=20 Power for Every IC, Part 2: Choosing and Using Your Bypass=20 Capacitors
Comments
3 CommentsLogin
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<= B=20 itemprop=3D"name">HighVoltage!
Great Article! Are there formulas for choosing what capacitor sizes = are=20 needed (Ex: AC Filter design)?
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hariharanm
Good Work! I=E2=80=99m waiting for the next part :D
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Thanks for this article as just this past wednesday I was showing a = friend=20 a circuit of reverse voltage he thought was untrue.I would like to = know the=20 reason for utilizing a ceramic 10uf cap and the reason for not using a = 10uf=20 electrolytic or axial lead one. I know of a young fellow whom lives = out in bfe=20 and the parts are not available for months. He gets discouraged.Maybe = delve=20 into this for a sentence or two.