4.6 LOCK DETECT DIGITAL FILTER

The Lock Detect Digital Filter compares the difference between the phase of the inputs of the phase detector to a RC generated

delay of approximately 15nS. To enter the locked state (Lock = HIGH) the phase error must be less than the 15nS RC delay for

5 consecutive reference cycles. Once in lock (Lock = HIGH), the RC delay is changed to approximately 30nS. To exit the locked

state (Lock = LOW), the phase error must become greater than the 30nS RC delay. When the PLL is in the power down mode,

Lock is forced LOW. A flow chart of the digital filter is shown at right.

DS100831-4

www.national.com

18