ICS841S02I Datasheet, PDF (11/17 Pages) Integrated Device Technology – PCI EXPRESS™ CLOCK GENERATOR

ICS841S02I
PCI EXPRESS™ CLOCK GENERATOR
APPLICATION INFORMATION
PRELIMINARY
POWER SUPPLY FILTERING TECHNIQUES
As in any high speed analog circuitry, the power supply pins are
vulnerable to random noise. To achieve optimum jitter
performance, power supply isolation is required. The ICS841S02I
provides separate power supplies to isolate any high switching
noise from the outputs to the internal PLL. V_{DD_SRC}, V_DDA and V_{DD_REF}
should be individually connected to the power supply
plane through vias, and 0.01µF bypass capacitors should be used
for each pin. Figure 1 illustrates this for a generic V_{DD_SRC} pin and
also
shows
that
V
DDA
requires
that
an
additional10Ω
resistor
along with a 10µF bypass capacitor be connected to the V_DDA pin.
3.3V
V
DD
.01μF 10Ω
V_DDA
.01μF
10μF
FIGURE 1. POWER SUPPLY FILTERING
USING THE ON-BOARD CRYSTAL OSCILLATOR
The ICS841S02I features a fully integrated Pierce oscillator to
minimize system implementation costs. The recommended
operation of the ICS841S02I is with a 25MHz, 18pF parallel
resonant crystal. See Table 7 for complete crystal specifications.
For proper operation, a minimum of 10pF capacitance on each
crystal pin is required. The capacitor values shown in Figure 2
TABLE 7. RECOMMENDED CRYSTAL SPECIFICATIONS
Parameter
Value
Crystal Cut
Fundamental AT Cut
Resonance
Parallel Resonance
Shunt Capacitance (C_L)
Load Capacitance (C_O)
Equivalent Series Resistance (ESR)
5-7pF
18pF
20-50Ω
are typical values for the recommended crystal as show in
Table 7. The specific values may be adjusted to trim the frequency
for the individual board layouts if desired.
The crystal and optional trim capacitors should be located as
close to the ICS841S02I XTAL_IN and XTAL_OUT pins as
possible to minimize board level parasitics.
25 MHz
T³B^3pD^F
XT AL_IN
T¹B^8pD^F
XTAL_OUT
FIGURE 2. CRYSTAL OSCILLATOR WITH TRIM CAPACITOR
IDT^™ / ICS^™ PCI EXPRESS CLOCK GENERATOR
11
ICS841S02BGI REV. C NOVEMBER 1, 2007