LTC6948I View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC6948I
Linear Technology
'LTC6948I' PDF : 36 Pages View PDF
LTC6948
Applications Information
Alternatively, to calculate the numerator step size NUMSTEP
needed to produce a given frequency step fSTEP(FRAC), use
Equation 9:
NUMSTEP
=
fSTEP(FRAC) •R • O • 218
fREF
(9)
The output frequency resolution fSTEP(INT) produced by a
unit change in N while in integer mode is given by Equa-
tion 10:
fSTEP(INT
)
=
fREF
R•O
(10)
Loop Filter Design
A stable PLL system requires care in designing the external
loop filter. The Linear Technology FracNWizard applica-
tion, available from www.linear.com, aids in design and
simulation of the complete system.
The loop design should use the following algorithm:
1) Determine the output frequency fRF and frequency step
size fSTEP based on application requirements. Using
Equations 3, 5, 7, and 8, change fREF, N, R, and O until
the application frequency constraints are met. Use the
minimum R value that still satisfies the constraints.
Then calculate B using Equation 1 and Tables 8 and 9.
2) Select the open loop bandwidth BW constrained by
fPFD and oversampling ratio OSR. The OSR is the ratio
of fPFD to BW (see Equation 11):
OSR = fPFD
(11)
BW
or
BW
=
fPFD
OSR
where BW and fPFD are in Hz.
A stable loop, both in integer and fractional mode,
requires that the OSR is greater than or equal to 10.
Further, in fractional mode, OSR must be high enough
to allow the loop filter to reduce modulator quantization
noise to an acceptable level.
Choosing a higher order loop filter when using the Δ∑
modulator allows for a smaller OSR, and thus a larger
loop bandwidth. Linear Technology’s FracNWizard helps
choose the appropriate OSR and BW values.
3) Select loop filter component RZ and charge pump cur-
rent ICP based on BW and the VCO gain factor, KVCO.
BW (in Hz) is approximated by the following equation:
BW
≅
ICP
• RZ • KVCO
2• π •N
(12)
or
RZ
=
2•π
ICP
• BW • N
• KVCO
where KVCO is in Hz/V, ICP is in Amps, and RZ is in
Ohms. KVCO is obtained from the VCO Tuning Sensitiv-
ity in the Electrical Characteristics. Use ICP = 5.6mA to
lower in-band noise unless component values force a
lower setting.
4) Select loop filter components CI and CP based on BW
and RZ. A reliable second-order loop filter design can
be achieved by using the following equations for the
loop capacitors (in Farads).
CI
=
2
•
π
3.5
• BW
•
RZ
(13)
CP
=
7
•
π
•
1
BW
•
RZ
(14)
Use FracNWizard to aid in the design of higher order
loop filters.
6948f
26
For more information www.linear.com/LTC6948
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