VSC8540RT datasheet(69/87 Pages) MICROCHIP

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DS60001603E-page 69

VSC8540RT

The ideality factor, n, represents the deviation from ideal diode behavior as exemplified by the diode equation:

where, IS = saturation current, q = electronic charge, VD = voltage across the diode, k = Boltzmann constant, and T =

absolute temperature (Kelvin).

8.4

AC Characteristics

The following sections discuss AC characteristics.

8.4.1

REFERENCE CLOCK

The following table lists the AC specifications for the REFCLK reference clock.

8.4.1.1

XTAL Reference Clock

When using the 25 MHz crystal clock input option (REFCLK_SEL[1:0] = 00), the additional specifications listed in the

following table are required.

TABLE 8-16:

REFCLK

Parameter

Symbol

Minimum

Typical

Maximum

Unit

Condition

REFCLK frequency,

REFCLK_SEL[1:0]= 11

–100 ppm 125

100 ppm

MHz

REFCLK frequency,

REFCLK_SEL[1:0]= 10

–100 ppm 50

100 ppm

MHz

REFCLK frequency,

REFCLK_SEL[1:0]= 01

–100 ppm 25

100 ppm

MHz

Rise time and fall time

tR, tF

1.5

20% to 80%, 5.1 pF load

Duty cycle

Phase jitter—Gaussian

psRMS Bandwidth from 10 kHz to

10 MHz

Total jitter, peak-to-peak

200

psPP

10K samples.

TABLE 8-17:

XTAL REFCLK

Parameter

Symbol

Minimum

Typical

Maximum

Unit

CL1

where

C1EXT =C1+CINEXT

C2EXT = C2 + COUTEXT

CINEXT represents input (XTAL1) I/O, bond pad, package pin, and routing

parasitic capacitance

and COUTEXT represents output (XTAL2) I/O, bond pad, package pin, and routing

parasitic capacitance.

For a reference tank circuit, see Figure 5-8.

8pF

Crystal ESR



IFW

IS e

qVD

 nkT





–



C1EXT C2EXT



C1EXT C2EXT

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