XIO2001_16 Datasheet, PDF (128/141 Pages) Texas Instruments – PCI Express to PCI Bus Translation Bridge

XIO2001
SCPS212I – MAY 2009 – REVISED JANUARY 2016
www.ti.com
Power Supply Recommendations (continued)
and the fifth harmonic is 6.25 GHz. Finding inductors and capacitors with a series resonance frequency above
6.25 GHz is both difficult and expensive. Components with a series resonance frequency in the 4 to 6 GHz
range are a good compromise.
• The inductor(s) associated with the filter must have a DC resistance low enough to pass the required current
for the connected power terminals. The voltage drop across the inductor must be low enough to meet the
minus 10% voltage margin requirement associated with each XIO2001 power terminal. Power supply output
voltage variation must be considered as well as voltage drops associated with any connector pins and circuit
board power distribution geometries.
• The Q versus frequency curve associated with the inductor must be appropriate to reduce power terminal
noise to less than the maximum peak-to-peak amplitude requirement for the XIO2001. Recommending a
specific inductor is difficult because every system design is different and therefore the noise frequencies and
noise amplitudes are different. Many factors will influence the inductor selection for the filter design. Power
supplies must have adequate input and output filtering. A sufficient number of bulk and bypass capacitors are
required to minimize switching noise. Assuming that board level power is properly filtered and minimal low
frequency noise is present, frequencies less than 10 MHz, an inductor with a Q greater than 20 from
approximately 10 MHz to 3 GHz should be adequate for most system applications.
• The series component(s) in the filter may either be an inductor or a ferrite bead. Testing has been performed
on both component types. When measuring PCI-Express link jitter, the inductor or ferrite bead solutions
produce equal results. When measuring circuit board EMI, the ferrite bead is a superior solution.
NOTE
The XIO2001 reference schematics include ferrite beads in the analog power supply
filters.
• When designing filters associated with power distribution, the power supply is a low impedance source and
the device power terminals are a low impedance load. The best filter for this application is a T filter. See
Figure 35 for a T-filter circuit. Some system may require this type of filter design if the power supplies or
nearby components are exceptionally noisy. This type of filter design is recommended if a significant amount
of low frequency noise, frequencies less than 10 MHz, is present in a system.
• For most applications a Pi filter will be adequate. See Figure 35 for a Pi-filter circuit. When implementing a Pi
filter, the two capacitors and the inductor must be located next to each other on the circuit board and must be
connected together with wide low impedance traces. Capacitor ground connections must be short and low
impedance.
• If a significant amount of high frequency noise, frequencies greater than 300 MHz, is present in a system,
creating an internal circuit board capacitor will help reduce this noise. This is accomplished by locating power
and ground planes next to each other in the circuit board stackup. A gap of 0.003 mils between the power
and ground planes will significantly reduce this high frequency noise.
• Another option for filtering high-frequency logic noise is to create an internal board capacitor using signal
layer copper plates. When a component requires a low-noise power supply, usually the Pi filter is located near
the component. Directly under the Pi filter, a plate capacitor may be created. In the circuit board stack-up,
select a signal layer that is physically located next to a ground plane. Then, generate an internal 0.25 inch by
0.25 inch plate on that signal layer. Assuming a 0.006 mil gap between the signal layer plate and the internal
ground plane, this will generate a 12 pF capacitor. By connecting this plate capacitor to the trace between the
Pi filter and the component’s power pins, an internal circuit board high frequency bypass capacitor is created.
This solution is extremely effective for switching frequencies above 300 MHz.
Figure 35 illustrates two different filter designs that may be used with the XIO2001 to provide lownoise power to
critical power pins.
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