ICS85314-01 Datasheet, PDF (9/15 Pages) Integrated Circuit Systems – LOW SKEW, 1-TO-5 DIFFERENTIAL-TO-2.5V/3.3V LVPECL FANOUT BUFFER

Integrated
Circuit
Systems, Inc.
PRELIMINARY
ICS85314-01
LOW SKEW, 1-TO-5
DIFFERENTIAL-TO-2.5V/3.3V LVPECL FANOUT BUFFER
APPLICATION INFORMATION
WIRING THE DIFFERENTIAL INPUT TO ACCEPT SINGLE ENDED LEVELS
Figure 2 shows how the differential input can be wired to accept single ended levels. The reference voltage V_REF ~ V_CC/2 is
generated by the bias resistors R1, R2 and C1. This bias circuit should be located as close as possible to the input pin. The ratio of
R1 and R2 might need to be adjusted to position the V_REF in the center of the input voltage swing. For example, if the input clock
swing is only 2.5V and V_CC = 3.3V, V_REF should be 1.25V and R2/R1 = 0.609.
V_CC
CLK_IN
C1
0.1uF
R1
1K
+
V_REF _-
R2
1K
FIGURE 2 - SINGLE ENDED SIGNAL DRIVING DIFFERENTIAL INPUT
TERMINATION FOR LVPECL OUTPUTS
The clock layout topology shown below is a typical termina-
tion for LVPECL outputs. The two different layouts mentioned
are recommended only as guidelines.
FOUT and nFOUT are low impedance follower outputs that gen-
erate ECL/LVPECL compatible outputs. Therefore, terminating
resistors (DC current path to ground) or current sources must
be used for functionality. These outputs are designed to drive
50Ω transmission lines. Matched impedance techniques should
be used to maximize operating frequency and minimize signal
distortion. Figures 3A and 3B show two different layouts which
are recommended only as guidelines. Other suitable clock lay-
outs may exist and it would be recommended that the board
designers simulate to guarantee compatibility across all printed
circuit and clock component process variations.
Z
o
=
50Ω
F_OUT
F_IN
Z_o = 50Ω
50Ω
1
RTT =
Z_o
(V_OH + V_OL / V_CC –2) –2
50Ω
RTT
V_CC - 2V
F
OUT
5
2
Z
o
Z_o = 50Ω
3.3V
5
2
Z
o
F_IN
Z
o
=
50Ω
3
2 ^Zo
3
2
Z
o
FIGURE 3A - LVPECL OUTPUT TERMINATION
FIGURE 3B - LVPECL OUTPUT TERMINATION
85314AG-01
www.icst.com/products/hiperclocks.html
REV. B JUNE 21, 2002
9