PFD
CP
LF
VCO
Divider
CLKIN
÷
2
÷
2
÷
NR
÷
R
÷
OD
post-ODCLK
INTCLK
PLL CLK
Output CLK
Feedback
CLK
NF
NV
NS
Phase-Locked Loop Theory of Operation
539
SPNU563A – March 2018
Copyright © 2018, Texas Instruments Incorporated
Oscillator and PLL
The width of the up pulse and the down pulse depends on the difference in phase between the two inputs.
For example, when the reference input leads the feedback input by 10 ns, then an up pulse of
approximately 10 ns is generated (see
). On the other hand, when the reference input lags
the feedback input by 10 ns, then a down pulse of approximately 10 ns is generated. When the two inputs
are exactly in phase, the up pulse and down pulse become essentially zero-width. These pulses are fed to
the charge pump block, which meters charge into the low-pass loop filter.
The advantage of a phase-frequency detector over a phase-only detector is that it cannot lock to a
harmonic or subharmonic of the reference. This important property also ensures that the output frequency
of the VCO is always exactly 2 × NF times the reference frequency.
The reference feedback frequency is based upon the VCO frequency and the feedback divider. Fractional
multiplication is achieved by changing the feedback divider real-time in order to create the fractional
multiplication. As an example, if a multiplier of 100.5 is selected, the feedback divider divides by 100 and
101 in equal proportions; in this case, the PLLMUL bit field would be programmed as 99.5 (0x6380). This
fractional multiplication is useful when trying to achieve final frequencies that are non-integer to the input
frequency (a final frequency that is a prime number). The fractional portion of the divider should be small
compared to the multiplier and so it is recommended that the fractional portion relate to parts in 16,
implying that the last 4 bits should always be 0.
14.7.2 Charge Pump and Loop Filter
The charge pump (CP) add or remove charge from the loop filter based on the pulses coming from the
phase-frequency detector (PFD).
Two components of the filter output signal are summed together: an integral component and a
proportional component. The integral component maintains a DC level going to the VCO to set its
frequency, and the proportional component makes the VCO track changes in phase to minimize jitter. The
capacitors and resistors required for the filter are integrated in silicon.
14.7.3 Voltage-Controlled Oscillator
The output frequency of the VCO is proportional to its input control voltage, which is generated by the
charge pump via the integrated loop filter. If the VCO oscillates too slowly, the feedback phase begins to
lag the reference phase at the PFD, which increases the control voltage at the VCO. Conversely, if the
VCO oscillates too fast, the feedback phase begins to lead the reference phase at the PFD, which
decreases the control voltage at the VCO. These two actions keep the VCO running at the correct
frequency multiple of the reference.
Figure 14-11. PLL Modulation Block Diagram