pck9446 NXP Semiconductors, pck9446 Datasheet - Page 10

• Manufacturer Part Number: pck9446
• Description: 2.5 V And 3.3 V Lvcmos Clock Fan-out Buffer
• Manufacturer: NXP Semiconductors

Philips Semiconductors
PCK9446_1
Product data sheet
The waveform plots of
versus two lines. In both cases the drive capability of the PCK9446 output buffer is more
than sufficient to drive 50
measurements 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 PCK9446. The output waveform in
shows a step in the waveform; this step is caused by the impedance mismatch seen
looking into the driver. The parallel combination of the 36
impedance does not match the parallel combination of the line impedances. The voltage
wave launched down the two lines will equal:
At the load end the voltage will double, due to the near unity reflection coefficient, to 2.5 V.
It will then increment towards the quiescent 3.0 V in steps separated by one round trip
delay (in this case 4.0 ns).
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 multiple lines, the situation in
should be used. In this case the series terminating resistors are reduced such that when
the parallel combination is added to the output buffer impedance the line impedance is
perfectly matched.
V
Z
R
R
Fig 4. Single versus dual waveforms.
o
s
o
L
=
=
=
=
36
17
50
V
S
------------------------------
R
s
36
+
50
Z
R
o
o
+
Z
voltage
o
(V)
Figure 4
Rev. 01 — 10 April 2006
3.0
2.0
1.0
0.5
=
0
0
3.0
transmission lines on the incident edge. Note from the delay
----------------------------- -
18
show simulation results of an output driving a single line
IN
+
25
17
4
+
25
2.5 V and 3.3 V LVCMOS clock fan-out buffer
OutA
t
OutB
t
d
d
=
8
= 3.8956 ns
= 3.9386 ns
1.25 V
12
time (ns)
002aaa679
© Koninklijke Philips Electronics N.V. 2006. All rights reserved.
series resistor plus the output
16
PCK9446
Figure 5
Figure 4
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