8
FN7174.3
May 9, 2008
must be accomplished with variable capacitance from the
varactor within this range. Crystal oscillators are more stable
than LC oscillators, which translates into lower jitter, but LC
oscillators can be pulled from their mid-point values further,
resulting in a greater capture and locking range. If the
incoming horizontal sync signal is known to be very stable,
then a crystal oscillator circuit can be used. If the H
SYNC
signal experiences frequency variations of greater than
about 300ppm, an LC oscillator should be considered, as
crystal oscillators are very difficult to pull this far. When
H
SYNC
input frequency is greater than CLK frequency ÷ N,
charge pump output (pin 7) sources current into the filter
capacitor, increasing the voltage across the varactor, which
lowers its capacitance, thus tending to increase VCO
frequency. Conversely, filter output pulls current from the
filter capacitor when H
SYNC
frequency is less than CLK ÷ N,
forcing the VCO frequency lower.
Loop Filter
The loop filter controls how fast the VCO will respond to a
change in filter output stimulus. Its components should be
chosen so that fast lock can be achieved, yet with a minimum
of VCO “hunting”, preferably in one to two oscillations of
charge pump output, assuming the VCO frequency starts
within capture range. If the filter is under-damped, the VCO
will over and under-shoot the desired operating point many
times before a stable lock takes place. It is possible to
under-damp the filter so much that the loop itself oscillates,
and VCO lock is never achieved. If the filter is over-damped,
the VCO response time will be excessive and many cycles will
be required for a lock condition. Over-damping is also
characterized by an easily unlocked system because the filter
can’t respond fast enough to perturbations in VCO frequency.
A severely over-damped system will seem to endlessly
oscillate, like a very large mass at the end of a long pendulum.
Due to parasitic effects of PCB traces and component
variables, it will take some trial and error experimentation to
determine the best values to use for any given situation. Use
the component tables as a starting point, but be aware that
deviation from these values is not out of the ordinary.
External Divide
DIV SELECT (pin 8) controls the use of the internal divider.
When high, the internal divider is enabled and EXT DIVIDER
(pin 13) outputs the CLK out divided by N. This is the signal
to which the horizontal sync input will lock. When divide
select is low, the internal divider output is disabled, and the
external divide becomes an input from an external divider, so
that a divisor other than one of the 8 pre-programmed
internal divisors can be used.
Normal Mode
Normal mode is enabled by pulling COAST (pin 9) low (below
1/3*V
CC
). If H
SYNC
and CLK ÷ N have any phase or
frequency difference, an error signal is generated and sent to
the charge pump. The charge pump will either force current
into or out of the filter capacitor in an attempt to modulate the
VCO frequency. Modulation will continue until the phase and
frequency of CLK ÷ N exactly match the H
SYNC
input. When
the phase and frequency match (with some offset in phase
that is a function of the VCO characteristics), the error signal
goes to zero, lock detect no longer pulses high, and the
charge pump enters a high impedance state. The clock is now
locked to the H
SYNC
input. As long as phase and frequency
differences remain small, the PLL can adjust the VCO to
remain locked and lock detect remains low.
Fast Lock Mode
Fast Lock mode is enabled by either allowing coast to float, or
pulling it to mid supply (between 1/3 and 2/3*V
CC
). In this
mode, lock is achieved much faster than in normal mode, but
the clock divisor is modified on the fly to achieve this. If the
phase detector detects an error of enough magnitude, the clock
is either inhibited or reset to attempt a “fast” lock of the signals.
Forcing the clock to be synchronized to the H
SYNC
input this
way allows a lock in approximately 2 H-cycles, but the clock
spacing will not be regular during this time. Once the near
lock condition is attained, charge pump output should be
very close to its lock-on value and placing the device into
normal mode should result in a normal lock very quickly.
Fast Lock mode is intended to be used where H
SYNC
becomes irregular, until a stable signal is again obtained.
Coast Mode
Coast mode is enabled by pulling the COAST (pin 9) high
(above 2/3*V
CC
). In coast mode, the internal phase detector
is disabled and filter out remains in high impedance mode to
keep filter out voltage and VCO frequency as constant as
possible. VCO frequency will drift as charge leaks from the
filter capacitor, and the voltage changes the VCO operating
point. Coast mode is intended to be used when noise or
signal degradation results in loss of horizontal sync for many
cycles. The phase detector will not attempt to adjust to the
resultant loss of signal so that when horizontal sync returns,
sync lock can be re-established quickly. However, if much
VCO drift has occurred, it may take as long to re-lock as
when restarting.
Lock Detect
LOCK DETECT (pin 12) will go low when lock is established.
Any DC current path from charge pump out will skew EXT
DIVIDER relative to HSYNC IN, tending to offset or add to
the 200ns internal delay, depending on which way the extra
current is flowing. This offset is called static phase error, and
is always present in any PLL system. If, when the part
stabilizes in a locked mode, lock detect is not low, adding or
subtracting from the loop filter series resistor R
2
will change
this static phase error to allow LDET to go low while in lock.
The goal is to put the rising edge of EXT DIVIDER in sync
with the falling edge of H
SYNC
+ 200ns (see “Timing
Diagrams” on page 5 and page 5). Increasing R
2
decreases
phase error, while decreasing R
2
increases phase error
(phase error is positive when EXT DIVIDER lags HSYNC.)
EL4584
