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ICS663 Datasheet PDF : 7 Pages
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ICS663
PLL BUILDING BLOCK
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 = (---R----Z----2⋅---Kπ-----O⋅---N--⋅---I--C---P----)
Damping factor, ζ= R---2--Z- K-----O-----⋅---I-N-C---P----⋅---C-----1
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.,
BW ≤ (REFIN) ⁄ 20
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:
10,000 = R-----Z--2--⋅-π--2---⋅0---20---0--⋅-0--2---.-5-
Choosing a damping factor of 0.7 (a minimal damping
factor than can be used to ensure fast lock time),
damping factor equation becomes:
0.7 = 2----5---,-2--0---0---0- --2---0---0----⋅-2---20---.0-5----⋅----C----1
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.,
C2 ≅ C1 ⁄ 20
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
and RZ = 25 kΩ
MDS 663 D
6
Revision 062904
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