MK3724 Ver la hoja de datos (PDF) - Integrated Circuit Systems

Número de pieza

componentes Descripción

Lista de partido

MK3724

VCXO PLUS AUDIO CLOCK FOR STB

Integrated Circuit Systems 

MK3724

VCXO PLUS AUDIO CLOCK FOR STB

External Component Selection

The MK3724 requires a minimum number of external

components for proper operation.

Decoupling Capacitors

Decoupling capacitors of 0.01 µF should be connected

between VDD and GND on pins 3 and 4, pins 6 and 7,

and pins 11 and 14 as close to the MK3724 as

possible. For optimum device performance, the

decoupling capacitors 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 traces between the clock outputs and

the loads are 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

20 Ω.

Quartz Crystal

The MK3724 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 meeting ICS’

recommended parameters must be used, and the

layout guidelines discussed in the following section

must be followed.

See Application Note MAN05 for a full list of crystal

parameters.

The frequency of oscillation of a quartz crystal is

determined by its “cut” and by the load capacitors

connected to it. The MK3724 incorporates on-chip

variable load capacitors that “pull” (change) the

frequency of the crystal. The crystal specified for use

with the MK3724 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 MK3724. 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.

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 samples of the crystals which you plan to use in

production. You will also need measured initial

accuracy for each crystal at the specified crystal load

capacitance (CL).

To determine the value of the crystal capacitors:

1. Connect VDD to 3.3 V. Connect pin 5 to the second

power supply. Adjust the voltage on pin 5 to 0V.

Measure and record the frequency of the CLK output.

2. Adjust the voltage on pin 5 to 3.3 V. Measure and

record the frequency of the same output.

To calculate the centering error:

Error

=

106x

(---f--3--.-3---(--3---.-0---)--V----–-----f--t--a--r--g---e---t--)---+-----(--f--0---V-----–----f--t--a---r-g----e---t-)-

ft arg et

– 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 25 ppm 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 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:

MDS 3724 C

4

Revision 121904

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

Share Link: