ICS722MLF datasheet(3/6 Pages) ICST

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LOW COST 27 MHZ 3.3 VOLT VCXO

MDS 722 A

Revision 121404

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ICS722

External Component Selection

The ICS722 requires a minimum number of external

components for proper operation.

Decoupling Capacitors

A decoupling capacitor of 0.01

µF should be connected

between VDD and GND on pins 2 and 4 as close to the

ICS722 as possible. For optimum device performance,

the decoupling capacitor should be mounted on the

component side of the PCB. Avoid the use of vias in the

decoupling circuit.

Series Termination Resistor

When the PCB trace between the clock output and the

load is over 1 inch, series termination should be used.

To series terminate a 50

Ω trace (a commonly used

trace impedance), place a 33

Ω resistor in series with

the clock line, as close to the clock output pin as

possible. The nominal impedance of the clock output is

Ω.

Quartz Crystal

The ICS722 VCXO function consists of the external

crystal and the integrated VCXO oscillator circuit. To

assure the best system performance (frequency pull

range) and reliability, a crystal device with the

recommended parameters (shown below) must be

used, and the layout guidelines discussed in the

following section shown must be followed.

The oscillation frequency of a quartz crystal is

determined by its “cut” and by the load capacitors

connected to it. The ICS722 incorporates on-chip

variable load capacitors that “pull” (change) the

frequency of the crystal. The crystal specified for use

with the ICS722 is designed to have zero frequency

error when the total of on-chip + stray capacitance is

14 pF.

Recommended Crystal Parameters:

Initial Accuracy at 25°C±20 ppm

Temperature Stability

±30 ppm

Aging

±20 ppm

Load Capacitance

14 pf

Shunt Capacitance, C0

7 pF Max

C0/C1 Ratio

250 Max

Equivalent Series Resistance

Ω Max

The external crystal must be connected as close to the

chip as possible and should be on the same side of the

PCB as the ICS722. There should be no via’s between

the crystal pins and the X1 and X2 device pins. There

should be no signal traces underneath or close to the

crystal. See application note MAN05.

Crystal Tuning Load Capacitors

The crystal traces should include pads for small fixed

capacitors, one between X1 and ground, and another

between X2 and ground. Stuffing of these capacitors

on the PCB is optional. The need for these capacitors is

determined at system prototype evaluation, and is

influenced by the particular crystal used (manufacture

and frequency) and by PCB layout. The typical required

capacitor value is 1 to 4 pF.

The procedure for determining the value of these

capacitors can be found in application note MAN05.

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