9397 750 12522

© Koninklijke Philips Electronics N.V. 2005. All rights reserved.

Product data sheet

Rev. 01 — 13 October 2005

9 of 17

Philips Semiconductors

PCK9447

3.3 V/2.5 V 1 : 9 LVCMOS clock fan-out buffer

9.

Application information

9.1 Driving transmission lines

The PCK9447 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 17

Ω (V

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 50

Ω resistance to V

of DC current, and thus only a single terminated line can be driven by each output of the

PCK9447 clock driver. For the series terminated case, however, there is no DC current

draw, thus the outputs can drive multiple series terminated lines. Figure 11, illustrates an

output driving a single series terminated line versus two series terminated lines in parallel.

When taken to its extreme, the fan-out of the PCK9447 clock driver is effectively doubled

due to its capability to drive multiple lines.

The waveform plots of Figure 12 show simulation results of an output driving a single line

versus two lines. In both cases the drive capability of the PCK9447 output buffer is more

than sufficient to drive 50

Ω transmission lines on the incident edge. Note from the delay

measurement in the simulations a delta of only 43 ps 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 PCK9447. The output waveform in

Figure 12 shows a step in the waveform; this step is caused by the impedance mismatch

seen looking into the driver. The parallel combination of the 33

Ω 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:

Fig 11. Single versus dual transmission lines

Zo = 50 Ω

002aaa718

RS = 33 Ω

Zo = 50 Ω

RS = 33 Ω

PCK9447

OUTPUT

BUFFER

OutB1

OutB0

17

Ω

Zo = 50 Ω

RS = 33 Ω

PCK9447

OUTPUT

BUFFER

OutA

17

Ω

IN

IN

V

L

V

S

Z

O

R

S

R

O

Z

0

++

--------------------------------





=