ICS9220B Datasheet, PDF (12/16 Pages) Integrated Device Technology – Programmable RambusTM XDRTM Clock Generator

ICS9220B
Programmable Rambus^TM XDR^TMClock Generator
Clock Output Drivers
Figure 2 shows the clock driver equivalent circuit. The differential driver produces a specified voltage swing on the channel by
switching the currents going into ODCLK_T and ODCLK_C. The external resistor RRC at the IREFY pin sets the maximum
current. The minimum current is zero.
The voltage at the IREFY pin, VIREFY, is by design equal to 1 V nominally, and the driver current is seven times the current flowing
through RRC. So, the output low current can be estimated as IOL = 7/ RRC.
The driver output characteristics are defined together with the external resistors, R1, R2, and R3. The output clock signals are
specified at the measurement points indicated in Figure 2. Table 5 shows example values for the resistors.
R1, R2, and R3 and the clock driver output impedance, ZOUT, must match the impedance of the channel, ZCH , to minimize
secondary reflections. ZOUT is specified as 1000 Ohms, minimum to accomplish this. The effective impedance can be estimated
by:
(1000R1/(1000+R1)+R2) R3/(1000R1/(1000+R1)+R2+R3)
Pull-up resistor RT terminates the transmission line at the load to minimize clock signal reflection signal reflections. Table 5
shows the resistor values for establishing and effective source termination impedance of 49.2 Ohms to match a 50 Ohm
channel. The termination voltages are 2.5 V for VTS and 1.2 V for VT. The resistor values R1 = 38.3 Ohms, R2 = 19.1 Ohms, R3
= 54.9 Ohms and RRC = 200 Ohms can be used to match a 28 Ohm channel.
Table 5. Example Resistor Values and Termination Voltages for a 50 Ohm Channel¹
Symbol
Parameter
Value Tolerance
Unit
R₁
Termination resistor
39.2
+/- 1%
Ω
R₂
Termination resistor
66.5
+/- 1%
Ω
R₃
Termination resistor
93.1
+/- 1%
Ω
R_T
Termination resistor
49.9
+/- 1%
Ω
R_RC
Swing control resistor
200
+/- 1%
Ω
V_TS
Source termination voltage
2.5
+/-5%
V
V_T
Termination voltage
1.2
+/-5%
V
Notes:
1 A different set of resistors is used in Figure 2 when testing
for maximum output current of the clock driver (I_OLABS).
These resistors are: R₁ = 23Ω, R₂ = 36.5Ω, R₃ = 52.3Ω,
R_T=28Ω, R_RC = 118Ω
Supply Voltage
CLK_INC
CLK_INT
Input
XDR
Clock Generator
a. Differential input
CLK_INT
VTH
Input
XDR
Clock Generator
b. Single-ended input
Figure 1. Differential and single-ended reference clock inputs
IDT^TM Programmable Rambus^TM XDR^TMClock Generator
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1427A—01/26/10