20 MHz-170MHz FSPLL PLL2013X
Samsung ASIC 6-11 STDM110
■Loop Filter:
The control signal that the phase detector generates for the charge pump may generate large excursions (rip-
ples) each time the VCO output is compared to the system clock. To avoid overloading the VCO, a low pass
filter samples and filters the high-frequency components out of the control signal. The filter is typically a sin-
gle-pole RC filter consisting of a resistor and capacitor.
■Voltage Controlled Oscillator (VCO):
The output voltage from the loop filter drives the VCO, causing its oscillation frequency to increase or de-
crease as a function of variations in voltage. When the VCO output matches the system clock in frequency
and phase, the phase detector stops sending a control signal to the charge pump, which in turn stabilizes the
input voltage to the loop filter. The VCO frequency then remains constant, and the PLL remains locked onto
the system clock.
Frequency Synthesis
Frequency synthesis uses the system clock as a base frequency to generate higher/lower frequency clocks
for internal logic.
For high speed applications in high-end designs, transmission line effects cause problems because of para-
sitic and impedance mismatch among various on-board components. These problems can be eliminated by
moving the high frequency to the chip level. On-chip clocks that are faster than the external system clock can
be synthesized by inserting a divider in the feedback path. The divider is placed after voltage controlled os-
cillator, as illustrated in Figure 6-11. The signal is running at M times the system clock frequency, so the PLL
matches the divider signal output to the system clock. This configuration reduces the problem of interfacing
to the system clock on the board, and it reduces the noise generated by the system clock oscillator and driver
for all the components in the system.
Design Considerations
The following design considerations apply:
• Phase tolerance and jitter are independent of the PLL frequency.
• Jitter is affected by the noise frequency in the power (VDDD/VSSD, VDDA/VSSA).
It increases when the noise level increases.
• A CMOS-level input reference clock is recommend for signal compatibility with the PLL circuit. Other
levels such as TTL may degrade the tolerances.
• The used of two, or more PLLs requires special design considerations. Please consult your application
engineer for more information.
• The following apply to the noise level, which can be minimized by using good analog power and ground
isolation techniques in the system:
- Use wide PCB traces for POWER (VDDD/VSSD, VDDA/VSSA, VBB) connections to the PLL core.
- Separate the traces from the chip’s VDDD/VSSD, VDDA/VSSA supplies.
- Use proper VDDD/VSSD, VDDA/VSSA de-coupling.
- Use good power and ground sources on the board.
- Use power VBB for minimize substrate noise.
• The PLL core should be placed as close as possible to the dedicated loop filter and analog power and
ground pins.
• It is inadvisable to locate noise-generating signals, such as data buses and high-current outputs,
near the PLL I/O cells.
• Other related I/O signals should be placed near the PLL I/O but do not have any pre-defined placement
restriction.