LTC6945 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC6945' PDF : 28 Pages View PDF
LTC6945
APPLICATIONS INFORMATION
INTRODUCTION
A PLL is a complex feedback system that may conceptually
be considered a frequency multiplier. The system multiplies
the frequency input at REF± and outputs a higher frequency
at RF±. The PFD, charge pump, N divider, and external VCO
and loop filter form a feedback loop to accurately control
the output frequency (see Figure 15). The R and O dividers
are used to set the output frequency resolution.
LTC6945
REF±
(fREF)
fPFD
R_DIV
÷R
KPFD
N_DIV
÷N
RF±
O_DIV
(fRF)
÷O
ICP CP
VCO±
(fVCO)
LOOP FILTER
RZ
CP
CI
LF(s)
KVCO
6945 F15
Figure 15. PLL Loop Diagram
OUTPUT FREQUENCY
When the loop is locked, the frequency fVCO (in Hz)
produced at the output of the VCO is determined by the
reference frequency fREF, and the R and N divider values,
given by Equation 2:
fVCO
=
fREF •N
R
(2)
Here, the PFD frequency fPFD produced is given by the
following equation:
fPFD
=
fREF
R
(3)
and fVCO may be alternatively expressed as:
fVCO = fPFD • N
The output frequency fRF produced at the output of the O
divider is given by Equation 4:
fRF
=
fVCO
O
(4)
Using the above equations, the output frequency resolution
fSTEP produced by a unit change in N is given by Equation 5:
fSTEP
=
fREF
R•O
(5)
LOOP FILTER DESIGN
A stable PLL system requires care in selecting the external
loop filter values. The Linear Technology PLLWizard ap-
plication, 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 constraints. Using
Equations 2, 3, 4 and 5, change fREF, N, R and O until
the application frequency constraints are met. Use the
minimum R value that still satisfies the constraints.
2. Select the loop bandwidth BW constrained by fPFD. A
stable loop requires that BW is less than fPFD by at least
a factor of 10.
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
(6)
or :
RZ
=
2•π
ICP
•BW •N
• K VCO
where KVCO is in Hz/V, ICP is in Amps, and RZ is in Ohms.
KVCO is the VCO’s frequency tuning sensitivity, and
may be determined from the VCO specifications. Use
ICP = 11.2mA to lower in-band noise unless component
values force a lower setting.
6945f
18
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