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MK3724GL Datasheet(PDF) 4 Page - Integrated Circuit Systems |
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MK3724GL Datasheet(HTML) 4 Page - Integrated Circuit Systems |
4 / 8 page VCXO PLUS AUDIO CLOCK FOR STB MDS 3724 C 4 Revision 121904 In te gr ated Circuit Systems ● 525 Ra ce Street, San Jose, CA 9512 6 ● tel (4 08) 297-1 201 ● www.icst.com MK3724 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: 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: Error 10 6 x f3.3 3.0 ()V f tet arg – () f 0V f tet arg – () + f tet arg ---------------------------------------------------------------------------------------- error xtal – = |
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