ICS9220 datasheet(12/17 Pages) IDT

12 / 17 page

ICS9220

1227G—11/05/07

VTH

Input

Clock Generator

Input

XDR

Clock Generator

a. Differential input

b. Single-ended input

Supply Voltage

CLK_INC

CLK_INT

Figure 1. Differential and single-ended reference clock inputs

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 Channel

Symbol

Parameter

Value

Tolerance

Unit

Termination resistor

39.2

+/- 1%

Ω

Termination resistor

66.5

+/- 1%

Ω

Termination resistor

93.1

+/- 1%

Ω

Termination resistor

49.9

+/- 1%

Ω

RRC

Swing control resistor

200

+/- 1%

Ω

VTS

Source termination voltage

2.5

+/-5%

Termination voltage

1.2

+/-5%

Notes:

1 A different set of resistors is used in Figure 2 when testing

for maximum output current of the clock driver (IOLABS).

These resistors are: R1 = 23Ω, R2 = 36.5Ω, R3 = 52.3Ω,

RT=28Ω, RRC = 118Ω

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