MEMORY DATABOOK - Al Kossow's Bitsavers

• APPLICATIONS •• -----------------------iII2. DECOUPLING CAPACITORSThe dynamic MOS RAM is featured by the- greatpower current at the active time in comparisonto that at the standby time.For example, the rated value (Icc!) of the meanpower current of the MSM3764 is 4SmA, whilethe standby current (lcc2) of the MSM3764-1S(150ns version) is SmA. The former is approximately10 times greater than the latter. The peak currentof the MSM3764 approaches 90mA in the worst case.It is approximately 20 times as great as the standbycurrent Icc2.Therefore,. the power circuit must be designed soas to prevent the above current variation fromcausing an erroneous operation of the memory.A by-pass capacitor must be inserted for thispurpose. There are two types of by-pass capacitors:high frequency capacitor and low frequencycapacitor.2.1 High Frequency CapacitorIn the Icc current waveform, the peak currentrises at a high speed such as 10ns, and a highfrequency noise represented by thl! followingexpression is caused to occur by the L componentof the current applied to the capacitor:To reduce the fluctuation AV, the value of Lmust be reduced.For this purpose, the capacitor must be placed asclose as possible to the power pin of the IC. Further,sufficient capacity for supplying the peakcurrent is required. The standard capacity for adouble sided circuit board (two layer circuitboard) is O.OS - O.lI'F or more. The capacitymay be less than this value for a multi layercircuit board since the L component is less thanthe former.When designing a board, mount one capacitorwith excellent high frequency characteristics forevery two or three MOS IC memory chips, nearthe power pins of these IC chips.2.2 Low Frequency CapacitorA low frequency capacitor is required for supthepower fluctuation due to a suddenpressingcurrent variation (for example, current variationcaused by a status change from the standbystatus to the continuous access status or concurrentrefreshment of the entire board) in a boardunit. The power fluctuation in this case is a slowvariation of several handred ns.For this reason, the low frequency capacitormust have a capacity larger than the high frequencycapacitor.Though the capacity requirement depends uponthe number of memories which operate simultaneously(bit width), SOI'F is enough for a 16-32 bit system in a practical use.As an example of capacitor which satisfies therequirements in both 2.1 and 2.2 above, asmall-sized tantalum capacitor with excellenthigh frequency characteristics is shown in thefollowing table. It is desirable to mount a lowfrequency capacitor near the power input pin inorder to suppress the fluctuation of power suppliedfrom outside, even if this capacitor ismounted.Manufacturer Model Capacity (I'F)Oki Ceramic Co.Model CAtantalum. capacitorModel CB0.1 - 20l'FThe frequency characteristics of the abovecapacitor and the power bus bar are illustratedin attached figure 1 .3. PRINTED CIRCUIT BOARD3.1 Number of LayarsConsidering the measures against power noise which wasdescribed in 2. above and the routing to be described in3.2, two layers are enough in principle.3.2 RoutingAn example of routing on a two-layer circuit board isshown in attached drawing 2. In designing the routing,note the following four points:(I) The MOS drive line based on the TTL must be asshort as possible to prevent ringing (reflection) andreduce crosstal k.(III Considerations are required to lower the impedanceof the power line (including the ground). (Forexample, make a solid or grid-formed power linepattern. It is desirable that the power line patternhas width of at least 1.27mm.)(1111 If a signal line is to be branched for multi drive,the line must be branched at the driving end.(See the following figure.) And, the memorymatrix must be designed in an integrated form, andperipheral drivers must be placed near the memorymatrix.Memory element446