전자부품 데이터시트 검색엔진 |
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ICS728 데이터시트(PDF) 3 Page - Integrated Circuit Systems |
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ICS728 데이터시트(HTML) 3 Page - Integrated Circuit Systems |
3 / 8 page LOW COST 27 MHZ 3.3 VOLT VCXO MDS 728 B 3 Revision 012505 Integr ated Circuit System s l 525 Ra ce Stree t , Sa n Jose, CA 951 26 l te l (4 08) 297 -1 201 l www.ics t.co m ICS728 External Component Selection The ICS728 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) 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 When the PCB trace between the clock output (CLK, pin 5) 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 20 Ω. Quartz Crystal The ICS728 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 frequency of oscillation of a quartz crystal is determined by its “cut” and by the load capacitors connected to it. The ICS728 incorporates on-chip variable load capacitors that “pull” (change) the frequency of the crystal. The crystal specified for use with the ICS728 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 35 Ω Max The third overtone mode of the crystal and all spurs must be >100 ppm distant from 3x the fundamental resonance measured with a physical load of 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 ICS728. 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 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 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 ICS728 to 3.3 V. Connect pin 3 of the ICS728 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: 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 Error 10 6x f 3.0V f tet arg – () f 0V f tet arg – () + f tet arg ------------------------------------------------------------------------------ error xtal – = |
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