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NB3N502DG Datasheet(PDF) 4 Page - ON Semiconductor |
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NB3N502DG Datasheet(HTML) 4 Page - ON Semiconductor |
4 / 5 page NB3N502 http://onsemi.com 4 APPLICATIONS INFORMATION High Frequency CMOS/TTL Oscillators The NB3N502, along with a low frequency fundamental mode crystal, can build a high frequency CMOS/TTL output oscillator. For example, a 20 MHz crystal connected to the NB3N502 with the 5X output selected (S1 = L, S0 = H) produces a 100 MHz CMOS/TTL output clock. External Components Decoupling Instructions In order to isolate the NB3N502 from system power supply, noise de−coupling is required. The 0.01 mF decoupling capacitor has to be connected between VDD and GND on pins 2 and 3. It is recommended to place de−coupling capacitors as close as possible to the NB3N502 device to minimize lead inductance. Control input pins can be connected to device pins VDD or GND, or to the VDD and GND planes on the board. Series Termination Resistor Recommendation A 33 W series terminating resistor can be used on the CLKOUT pin. Crystal Load Capacitors Selection Guide The total on−chip capacitance is approximately 12 pF per pin (CIN1 and CIN2). A parallel resonant, fundamental mode crystal should be used. The device crystal connections should include pads for small capacitors from X1/CLK to ground and from X2 to ground. These capacitors, CL1 and CL2, are used to adjust the stray capacitance of the board to match the nominally required crystal load capacitance (CLOAD (crystal)). Because load capacitance can only be increased in this trimming process, it is important to keep stray capacitance to a minimum by using very short PCB traces (and no vias) between the crystal and device. Crystal load capacitors, if needed, must be connected from each of the pins X1 and X2 to ground. The load capacitance of the crystal (CLOAD (crystal)) must be matched by total load capacitance of the oscillator circuitry network, CINX, CSX and CLX, as seen by the crystal (see Figure 3 and equations below). R CIN1 12 pF CIN2 12 pF G Internal to Device X2 X1/CLK CS1 CS2 CL1 CL2 Crystal CLOAD1 = CIN1 + CS1 + CL1 [Total capacitance on X1/CLK] CLOAD2 = CIN2 + CS2 + CL2 [Total capacitance on X2] CIN1 [ CIN2 [ 12 pF (Typ) [Internal capacitance] CS1 [ CS2 [ 5 pF (Typ) [External PCB stray capacitance] CLOAD1,2 = 2 S CLOAD (Crystal) CL2 = CLOAD2 − CIN2 − CS2 [External load capacitance on X2] CL1 = CLOAD1 − CIN1 − CS1 [External load capacitance on X1/CLK] Example 1: Equal stray capacitance on PCB CLOAD (Crystal) = 18 pF (Specified by the crystal manufacturer) CLOAD1 = CLOAD2 = 36 pF CIN1 = CIN2 = 12 pF CS1 = CS2 = 6 pF CL1 = 36 − 12 − 6 = 18 pF CL2 = 36 − 12 − 6 = 18 pF Example 2: Different stray capacitance on PCB trace X1/CLK vs. X2 CLOAD (Crystal) = 18 pF CLOAD1 = CLOAD2 = 36 pF CIN1 = CIN2 = 12 pF CS1 = 4 pF & CS2 = 8 pF CL1 = 36 − 12 − 4 = 20 pF CL2 = 36 − 12 − 8 = 16 pF Figure 3. Using a Crystal as Reference Clock |
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