<strong>SRAM</strong> <strong>System</strong> <strong>Design</strong> <strong>Guidelines</strong>V DDOutput BufferQ1Q2Ref ARef BOutputThis is important in understanding what happens during theoperation of a capacitor. Consider Figure 3, which shows thebehavior of two ideal components, a capacitor and aninductor, which represent the reactive parts of the capacitorshown in Figure 2. Note that without any lead inductance orresistance, the resulting capacitive reactance approaches 0Ωwith increasing frequency. Note also that the inductivereactance of the ideal inductor, without any stray capacitance,approaches infinity.GNDFigure 1. Effects of Ground BounceThis equation allows us to calculate the capacitance requiredto prevent the voltage drops due to the switching outputdrivers. For example, suppose we have an <strong>SRAM</strong> witheighteen 3.3V output drivers driving a 50-ohm trace with arise and fall time of 3 ns. Let us also assume that a voltagedrop of 300 mV is the maximum amount of voltage dropallowable to the component.First, we need to calculate the output current during the riseof the clocks. Assuming the outputs reach 3V, each outputrequires 3V/50 ohm = 60 mA. Because the <strong>SRAM</strong> haseighteen such drivers, the total current required is 1080 mA.Solving for C in Equation 3 above yieldsdtC = I×------dV3nsC = 1080mA× ----------------- = 0.0108µF300mV. Eq. 4This suggests that this amount of total capacitance is requiredon the component to maintain less than 300 mV of voltageswing. Often, 0.1-µF capacitors are used for bypassing.Another way to view this situation is to say that a 0.1-µFcapacitor supplies 1080 mA of instantaneous current in 3nswith only 32.4 mV of voltage swing across the bypasscapacitor.This example assumed a worse-case scenario voltage swingon the outputs of 3V. Depending on the termination methodused, this swing could be less than this amount. However,since this is used only as a guide, it is best to overestimatethe value.Capacitor FilteringWe have seen that the main role of decoupling capacitors isto block unwanted noise going onto and coming from thepower plane. It should be noted, however, that decouplingcaps are much more than a capacitor. Since capacitorsalways have a finite, intrinsic resistance and inductance, theyare, in effect, a capacitor in series with an inductor and aresistor, as shown in Figure 2 below.Figure 2. Capacitor ModelFigure 3. Z vs. f for Parts of a Real CapacitorThe impedance curve of “Real” capacitors resembles thetraces marked 22 nF and 100 pF of Figure 4. The shape ofthese calculated curves match those found in a capacitormanufacturer’s data sheets. This means that, in a circuit, acapacitor acts as a low-impedance element only over alimited range of frequencies. To extend this frequency range,many references propose adding a second capacitor tobypass frequencies outside the limited range of the singlecapacitor. This approach expects a resulting impedancecurve like the solid line marked “Expected” in Figure 4. Thissolution, however, is not mathematically sound and has asignificant problem at “intermediate” frequencies.What actually happens when two capacitors of differentvalues are in parallel is shown in Figure 5. Notice the spike inimpedance just above 100 MHz. The goal of obtaining lowerimpedance across a wider frequency is not achieved. On thecontrary, the impedance has actually increased in someareas. Therefore, the practice of using two different valuedcapacitors to decouple a power supply bus is not recommended.But, there is a method that will help to lower the impedanceacross all frequencies. When two capacitors of the samevalue and package size are in parallel with each other, nopeak increase in impedance is realized. However, they dohave the effect of halving the inductance. This is a recommendedpractice, however it may be constrained by availableboard space.2
<strong>SRAM</strong> <strong>System</strong> <strong>Design</strong> <strong>Guidelines</strong>Figure 4. Expected Impedance of “Real” CapacitorsFigure 5. Real Z vs. f for Parallel 22-nF and 100-nF Capacitors3
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