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MK3725

DUAL OUTPUT 3.3 VOLT VCXO

Integrated Circuit Systems 

MK3725

DUAL OUTPUT 3.3 VOLT VCXO

External Component Selection

The MK3725 requires a minimum number of external

components for proper operation.

Decoupling Capacitor

A decoupling capacitor of 0.01µF must be connected

between VDD (pin 2 & 7) and GND (pin 4), as close to

these pins 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

Use series termination when the PCB trace between

the clock outputs and the loads are over 1 inch. To

series terminate a 50Ω trace (a commonly used trace

impedance), place a 33Ω resistor in series with the

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

The nominal impedance of the clock output is 20Ω.

Quartz Crystal

The MK3725 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 (as described in application

note MAN05) must be used, and the layout guidelines

discussed in the following section must be followed.

The frequency of oscillation of a quartz crystal is

determined by its “cut” and by the load capacitors

connected to it. The MK3725 incorporates on-chip

variable load capacitors that “pull” (change) the

frequency of the crystal. The crystal specified for use

with the MK3725 is designed to have zero frequency

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

14 pF.

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 MK3725. There should be no vias between

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

should be no signal traces underneath or close to the

crystal.

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.

To determine the need for and value of the crystal

adjustment capacitors, you will need a PC board of

your final layout, a frequency counter capable of about

1 ppm resolution and accuracy, two power supplies,

and some samples of the crystals which you plan to

use in production, along with measured initial accuracy

for each crystal at the specified crystal load

capacitance, CL.

To determine the value of the crystal capacitors:

1. Connect VDD of the MK3725 to 3.3 V. Connect pin 3

of the MK3725 to the second power supply. Adjust the

voltage on pin 3 to 0V. Measure and record the

frequency of the CLK output.

2. Adjust the voltage on pin 3 to 3.3 V. Measure and

record the frequency of the same output.

To calculate the centering error:

Error = 106x (---f--3--.--0---V----–-----f--t--a--r--g---ef--t-t-a-)--r-+g---e--(-t--f-0---V-----–----f--t--a---r--g---e---t-)- – errorxtal

Where:

ftarget = nominal crystal frequency

errorxtal =actual initial accuracy (in ppm) of the crystal

being measured

If the centering error is less than ±25 ppm, no

adjustment is needed. If the centering error is more

than 25ppm negative, the PC board has excessive

stray capacitance and a new PCB layout should be

considered to reduce stray capacitance. (Alternately,

the crystal may be re-specified to a higher load

capacitance. Contact ICS MicroClock for details.) If the

centering error is more than 25 ppm positive, add

identical fixed centering capacitors from each crystal

pin to ground. The value for each of these caps (in pF)

is given by:

External Capacitor =

2 x (centering error)/(trim sensitivity)

MDS 3725 A

3

Revision 100803

Integrated Circuit Systems z 525 Race Street, San Jose, CA 95126 z tel (408) 297-1201 z www.icst.com

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