IDT / ICS LVDS PCI EXPRESS CLOCK SYNTHESIZER
7
ICS844204BK-245 REV. A JULY 9, 2007
ICS844204-245
CRYSTAL-TO-LVDS PCI EXPRESS CLOCK SYNTHESIZER W/SPREAD SPECTRUM
PRELIMINARY
LVCMOS TO XTAL INTERFACE
The XTAL_IN input can accept a single-ended LVCMOS
signal through an AC couple capacitor. A general interface
diagram is shown in Figure 3. The XTAL_OUT pin can be left
floating. The input edge rate can be as slow as 10ns. For
LVCMOS inputs, it is recommended that the amplitude be
reduced from full swing to half swing in order to prevent signal
interference with the power rail and to reduce noise. This
configuration requires that the output impedance of the driver
(Ro) plus the series resistance (Rs) equals the transmission
line impedance. In addition, matched termination at the crystal
input will attenuate the signal in half. This can be done in one
of two ways. First, R1 and R2 in parallel should equal the
transmission line impedance. For most 50
Ω applications, R1
and R2 can be 100
Ω. This can also be accomplished by
removing R1 and making R2 50
Ω.
FIGURE 3. GENERAL DIAGRAM FOR LVCMOS DRIVER TO XTAL INPUT INTERFACE
XTAL_IN
XTAL_OUT
VCC
R2
Ro
R1
Zo = 50
Rs
VCC
.1uf
VDD
Zo = Ro + Rs
FIGURE 4. P.C. BOARD FOR EXPOSED PAD THERMAL RELEASE PATH EXAMPLE
THERMAL RELEASE PATH
The expose metal pad provides heat transfer from the device to
the P.C. board. The expose metal pad is ground pad connected
to ground plane through thermal via. The exposed pad on the
device to the exposed metal pad on the PCB is contacted through
solder as shown in Figure 4. For further information, please refer
to the Application Note on Surface Mount Assembly of Amkor’s
Thermally /Electrically Enhance Leadframe Base Package, Amkor
Technology.
EXPOSED METAL PAD
PIN
SOLDER
EPAD
(GROUND PAD)
SOLDER
PIN
THERMAL VIA
PIN PAD
GROUND PLANE
PIN PAD