NB3V63143G Datasheet, PDF (18/21 Pages) ON Semiconductor – Operating Power Supply

NB3V63143G
HCSL Termination
HCSL is a differential signaling standard commonly used
in PCIe systems. The HCSL driver is typical 14.5 mA
switched current open source output that needs a 50 W
termination resistor to ground near the source, and generates
725 mV of signal swing. A series resistor (10 W to 33 W) is
optionally used to achieve impedance matching by limiting
overshoot and ringing due to the rapid rise of current from
the output driver. The open source driver has high internal
impedance; thus a series resistor up to 33 W does not affect
the signal integrity. This resistor can be avoided for VDDO
supply (1.8 V) of operation, unless impedance matching
requires it.
14 .5mA
2. 6mA
CLK 1
CLK0
50 W
50 W
Figure 17. Simplified HCSL Output Structure with Termination
Field Programming Kit and Software
The NB3V63143G can be programmed by the user using
the ‘Clock Cruiser Programmable Clock Kit’. This device
uses the 16L daughter card on the hardware kit. To design a
new clock, ‘Clock Cruiser Software’ is required to be
installed from the ON Semiconductor website. The user
manuals for the hardware kit Clock Cruiser Programmable
Clock Kit and Clock Cruiser Software can be found
following this link www.onsemi.com.
Recommendation for Clock Performance
Clock performance is specified in terms of Jitter in the
time domain. Details and measurement techniques of
Cycle−cycle jitter, period jitter, TIE jitter and Phase Noise
are explained in application note AND8459/D.
In order to have a good clock signal integrity for minimum
data errors, it is necessary to reduce the signal reflections.
The reflection coefficient can be zero only when the source
impedance equals the load impedance. Reflections are based
on signal transition time (slew rate) and due to impedance
mismatch. Impedance matching with proper termination is
required to reduce the signal reflections. The amplitude of
overshoots is due to the difference in impedance and can be
minimized by adding a series resistor (Rs) near the output
pin. Greater the difference in impedance, greater is the
amplitude of the overshoots and subsequent ripples. The
ripple frequency is dependant on the signal travel time from
the receiver to the source. Shorter traces results in higher
ripple frequency, as the trace gets longer the travel time
increases, reducing the ripple frequency. The ripple
frequency is independent of signal frequency, and only
depends on the trace length and the propogation delay. For
eg. On an FR4 PCB with approximately 150 ps/inch of
propogation rate on a 2 inch trace, the ripple frequency = 1
/ (150 ps * 2 inch * 5) = 666.6 MHz; [5 = number of times
the signal travels, 1 trip to receiver plus 2 additional round
trips].
PCB traces should be terminated when trace length >= tr/f
/ (2* tprate); tf/t = rise/ fall time of signal, tprate =
propagation rate of trace.
ÌÌÑ ^Overshoot
(Positive)
Ringing
ÌÌÌÑ Overshoot
(Negative)
Figure 18. Signal Reflection Components
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