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CY28437OXC Datasheet(PDF) 11 Page - SpectraLinear Inc |
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CY28437OXC Datasheet(HTML) 11 Page - SpectraLinear Inc |
11 / 22 page CY28437 Rev 1.0, November 20, 2006 Page 11 of 22 The CY28437 requires a parallel resonance crystal. Substi- tuting a series resonance crystal will cause the CY28437 to operate at the wrong frequency and violate the ppm specifi- cation. For most applications there is a 300-ppm frequency shift between series and parallel crystals due to incorrect loading. Crystal Loading Crystal loading plays a critical role in achieving low ppm perfor- mance. To realize low ppm performance, the total capacitance the crystal will see must be considered to calculate the appro- priate capacitive loading (CL). Figure 2 shows a typical crystal configuration using the two trim capacitors. An important clarification for the following discussion is that the trim capacitors are in series with the crystal not parallel. It’s a common misconception that load capacitors are in parallel with the crystal and should be approximately equal to the load capacitance of the crystal. This is not true. Calculating Load Capacitors In addition to the standard external trim capacitors, trace capacitance and pin capacitance must also be considered to correctly calculate crystal loading. As mentioned previously, the capacitance on each side of the crystal is in series with the crystal. This means the total capacitance on each side of the crystal must be twice the specified crystal load capacitance (CL). While the capacitance on each side of the crystal is in series with the crystal, trim capacitors (Ce1,Ce2) should be calculated to provide equal capacitive loading on both sides. As mentioned previously, the capacitance on each side of the crystal is in series with the crystal. This mean the total capac- itance on each side of the crystal must be twice the specified load capacitance (CL). While the capacitance on each side of the crystal is in series with the crystal, trim capacitors (Ce1,Ce2) should be calculated to provide equal capacitance loading on both sides. Use the following formulas to calculate the trim capacitor values for Ce1 and Ce2. CL....................................................Crystal load capacitance CLe......................................... Actual loading seen by crystal using standard value trim capacitors Ce..................................................... External trim capacitors Cs .............................................. Stray capacitance (terraced) Ci ...........................................................Internal capacitance (lead frame, bond wires etc.) Table 4. Crystal Recommendations Frequency (Fund) Cut Loading Load Cap Drive (max.) Shunt Cap (max.) Motional (max.) Tolerance (max.) Stability (max.) Aging (max.) 14.31818 MHz AT Parallel 20 pF 0.1 mW 5 pF 0.016 pF 35 ppm 30 ppm 5 ppm Figure 2. Crystal Capacitive Clarification XTAL Ce2 Ce1 Cs1 Cs2 X1 X2 Ci1 Ci2 Clock Chip Trace 2.8pF Trim 33pF Pin 3 to 6p Figure 3. Crystal Loading Example Load Capacitance (each side) Total Capacitance (as seen by the crystal) Ce = 2 * CL – (Cs + Ci) Ce1 + Cs1 + Ci1 1 + Ce2 + Cs2 + Ci2 1 ( ) 1 = CLe |
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