LMH1983SQ/NOPB National Semiconductor, LMH1983SQ/NOPB Datasheet - Page 27

• Manufacturer Part Number: LMH1983SQ/NOPB
• Description: IC VID CLK GEN MULTI RATE 40LLP
• Manufacturer: National Semiconductor
• Type: Clock Generatorr

Specifications of LMH1983SQ/NOPB

• Applications: Video Equipment
• Mounting Type: Surface Mount
• Package / Case: 40-LLP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Loop Filter Capacitors
The most common types of capacitors used in many circuits
today are ferroelectric ceramic capacitors such as X7R, Y5V,
X5R. Y5U, etc. These capacitors suffer from piezoelectric ef-
fects, which generate an electrical signal in response to me-
chanical vibration, stress and shock. This effect can adversely
affect the jitter performance when presented to the control in-
put to the VCXO. The easiest way to eliminate this effect is to
use tantalum capacitors which do not exhibit the piezoelectric
effect.
Lock Determination
There are four bits in register 2 that indicate the lock status of
the four PLLs. Lock determination for PLL1 can be controlled
through two registers: LockStepSize (register 0x2D) and the
Loss of Lock Threshold Register. The LockStepSize register
sets the amount of variation that is permitted on the VC_LPF
pin while still considering the device to be locked. If the ref-
erence to the LMH1983 has a lot of jitter on it, then the device
may be reluctant to declare lock if LockStepSize is set too low.
The second register which controls the lock state declaration
External Loop Filter detail
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of PLL1 is Register 0x1C — Loss of Lock Threshold. This
register sets a number of cycles on the H
seen before loss of lock is declared. For some reference sig-
nals, there can be several missing H
refresh, so it is suggested that this register be loaded with a
value greater than 6. Pin 11, NO_LOCK, gives the lock status
of the LMH1983. The status of the NO_LOCK pin can be read
from register 0x01, and is a logical OR of the four individual
NO_LOCK status bits of the four PLLs, and is masked by the
bits in the PLL Lock mask (register 0x1D), and is also masked
if an individual PLL is powered down.
Lock Time Considerations
The lock time of the LMH1983 is dominated by the lock time
of PLL1. The other PLLs have much higher loop bandwidths,
and as a result lock much more quickly than does PLL1,
therefore lock time considerations are all focussed on PLL1.
The lock time for a PLL is dependent upon the loop bandwidth,
the equation for which is listed above in the PLL1 Loop Re-
sponse Design Equations section. The LMH1983 also allows
a 'Fastlock' mode, in which the bandwidth is increased by in-
creasing the charge pump current when the loop is unlocked,
then at a time programmed by the user after lock is declared,
I
point. The result is both fast lock time and very low residual
jitter.
Another issue to watch when considering lock time is if you
have enabled 'drift lock' as described in the section on TOF1
Alignment. If drift lock is enabled, and there is a significant
difference in the phase of TOF1 relative to the F
this case, the VCXO is slewed to ramp the clock rate up or
down until the two framing signal are brought into alignment.
It is possible for this to take a long time (tens of seconds)
Separate from the time that it takes for the PLL to lock, there
is a circuit which determines how to set the NOLOCK output
pin. The LMH1983 PLL operates by adjusting the voltage that
is applied to the VCXO control pin to lock the VCXO to a har-
monic of the incoming reference. When the device is not
locked, the PLL will be pulling the VCXO control voltage to
one extreme or the other of it’s range to slew the voltage into
lock, once the phase differences between the VCXO and the
reference are small, the device begins to nudge the control
voltage one way or the other to maintain the phase difference.
An adjustment might be necessary either because of the
VCXO drifting, or because of jitter on the reference.
CP1
is throttled back to drop the bandwidth to the desired set
IN
pulses during vertical
IN
input that must be
www.national.com
IN
signal. In