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PCS2I99448G-32-LT Datasheet(PDF) 7 Page - PulseCore Semiconductor |
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PCS2I99448G-32-LT Datasheet(HTML) 7 Page - PulseCore Semiconductor |
7 / 15 page September 2006 PCS2I99448 rev 0.4 3.3V/2.5V LVCMOS 1:12 Clock Fanout Buffer 7 of 15 Notice: The information in this document is subject to change without notice. APPLICATIONS INFORMATION Timing Diagram Figure 1. Output Clock Stop (CLK_STOP) Driving Transmission Lines The PCS2I99448 clock driver was designed to drive high speed signals in a terminated transmission line environment. To provide the optimum flexibility to the user, the output drivers were designed to exhibit the lowest impedance possible. With an output impedance of 17Ω (VCC=3.3V), the outputs can drive either parallel or series terminated transmission lines. In most high performance clock networks, point–to–point distribution of signals is the method of choice. In a point–to–point scheme, either series terminated or parallel terminated transmission lines can be used. The parallel technique terminates the signal at the end of the line with a 50Ω resistance to VCC÷2. Figure 2. Single versus Dual Transmission Lines This technique draws a fairly high level of DC current and thus only a single terminated line can be driven by each output of the PCS2I99448 clock driver. For the series terminated case, however, there is no DC current draw; thus, the outputs can drive multiple series terminated lines. Figure 2 “Single versus Dual Transmission Lines” illustrates an output driving a single series terminated line versus two series terminated lines in parallel. When taken to its extreme, the fanout of the PCS2I99448 clock driver is effectively doubled due to its capability to drive multiple lines at VCC=3.3V. The waveform plots in Figure 3 “Single versus Dual Line Termination Waveforms” show the simulation results of an output driving a single line versus two lines. In both Figure 3 . Single versus Dual Line Termination Waveforms cases, the drive capability of the PCS2I99448 output buffer is more than sufficient to drive 50Ω transmission lines on the incident edge. Note from the delay measurements in the simulations a delta of only 43pS exists between the two differently loaded outputs. This suggests that the dual line driving need not be used exclusively to maintain the tight output–to–output skew of the PCS2I99448. The output waveform in Figure 3 “Single versus Dual Line Termination Waveforms” shows a step in the waveform; this step is caused by the impedance mismatch seen looking into the driver. The parallel combination of the 33Ω series resistor plus the output impedance does not match the parallel combination of the line impedances. The voltage wave launched down the two lines will equal: VL = VS ( Z0 ÷ (RS+R0 +Z0)) Z0 = 50Ω|| 50Ω RS = 33Ω|| 33Ω R0 = 17Ω VL = 3.0 ( 25 ÷ (16.5+17+25) = 1.28V At the load end the voltage will double, due to the near unity reflection coefficient, to 2.5V. It will then increment towards the quiescent 3.0V in steps separated by one round trip delay (in this case 4.0nS). Since this step is well above the threshold region it will not cause any false clock triggering; however, designers may be uncomfortable with unwanted reflections on the line. To better match the impedances when driving PCS2I99448 OUTPUT BUFFER 17Ω Z0=50Ω RS=33Ω PCS2I99448 OUTPUT BUFFER 17Ω Z0=50Ω RS=33Ω Z0=50Ω RS=33Ω CCLK or PCLK CLK _ STOP Q0 to Q11 3.0 2.5 2.0 1.5 1.0 0.5 0 24 6 8 10 12 14 TIME (nS) In OutA tD = 3.8956 OutB tD = 3.9386 |
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