ICS853001 Datasheet, PDF (9/16 Pages) Integrated Circuit Systems – 1:1, DIFFERENTIAL LVPECL-TO-2.5V, 3.3V, 5V LVPECL/ECL BUFFER

Integrated
Circuit
Systems, Inc.
ICS853001
1:1, DIFFERENTIAL LVPECL-TO-
2.5V, 3.3V, 5V LVPECL/ECL BUFFER
TERMINATION FOR 3.3V 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
FOUT
FIN
Z_o = 50Ω
50Ω
RTT =
1
((V_OH + V_OL) / (V_CC – 2)) – 2
Z_o
50Ω
V_CC - 2V
RTT
FOUT
3.3V
125Ω
125Ω
Z_o = 50Ω
FIN
Z_o = 50Ω
84Ω
84Ω
FIGURE 3A. LVPECL OUTPUT TERMINATION
FIGURE 3B. LVPECL OUTPUT TERMINATION
TERMINATION FOR 5V LVPECL OUTPUT
This section shows examples of 5V LVPECL output termina-
tion. Figure 4A shows standard termination for 5V LVPECL. The
termination requires matched load of 50Ω resistors pull down to
V_CC - 2V = 3V at the receiver. Figure 4B shows Thevenin equiva-
lence of Figure 4A. In actual application where the 3V DC power
supply is not available, this approached is normally used.
5V
PEC L
Zo = 50 Ohm
Zo = 50 Ohm
R1
R2
50
50
3V
5V
+
-
PECL
FIGURE 4A. STANDARD 5V PECL OUTPUT TERMINATION
5V
PECL
Zo = 50 Ohm
Zo = 50 Ohm
5V
R3
R4
84
84
+
-
R1
R2
125
125
PECL
FIGURE 4B. 5V PECL OUTPUT TERMINATION EXAMPLE
853001AG
www.icst.com/products/hiperclocks.html
9
REV. A JANUARY 29, 2005