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ICS663 Datasheet(PDF) 6 Page - Integrated Circuit Systems |
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ICS663 Datasheet(HTML) 6 Page - Integrated Circuit Systems |
6 / 7 page PLL BUILDING BLOCK MDS 663 D 6 Revision 062904 In te gr ated Circuit Systems ● 525 Ra ce Street, San Jose, CA 9512 6 ● tel (4 08) 297-1 201 ● www.icst.com ICS663 Determining the Loop Filter Values The loop filter components consist of C1, C2, and RZ. Calculating these values is best illustrated by an example. Using the example in Figure 1, we can synthesize 20 MHz from a 200 kHz input. The phase locked loop may be approximately described by the following equations: Bandwidth Damping factor, where: KO = VCO gain (MHz/Volt) Icp = Charge pump current (µA) N = Total feedback divide from VCO, including the internal VCO post divider C1 = Loop filter capacitor (Farads) RZ = Loop filter resistor (Ohms) As a general rule, the bandwidth should be at least 20 times less than the reference frequency, i.e., In this example, using the above equation, bandwidth should be less than or equal to 10 kHz. By setting the bandwith to 10kHz and using the first equation, RZ can be determined since all other variables are known. In the example of Figure 1, N = 200, comprising the divide by 2 on the chip (VCO post divider) and the external divide by 100. Therefore, the bandwidth equation becomes: and RZ = 25 kΩ Choosing a damping factor of 0.7 (a minimal damping factor than can be used to ensure fast lock time), damping factor equation becomes: and C1 = 1.25 nF (1.2 nF is the nearest standard value). The capacitor C2 is used to damp transients from the charge pump and should be approximately 1/20th the size of C1, i.e., Therefore, C2 = 60 pF (56 pF nearest standard value). To summarize, the loop filter components are: C1 = 1.2 nf C2 = 56 pf Rz = 25 kΩ RZ KO ICP ⋅⋅ () 2 π N ⋅ ------------------------------------ = ζ RZ 2 ------ KO ICP C1 ⋅⋅ N ------------------------------- = BW REFIN () 20 ⁄ ≤ 10,000 RZ 200 2.5 ⋅⋅ 2 π 200 ⋅ -------------------------------- = 0.7 25 000 , 2 ------------------ 200 2.5 C1 ⋅⋅ 200 --------------------------------- = C2 C1 20 ⁄ ≅ |
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