FS8170
Page 11
May 2003
Multi-function Lock Detect Output (FOLD)
A digital lock detect function is included with the phase detector through an internal digi-
tal filter to produce a logic level output which is available on the FOLD output pin. The
criterion of lock indication depends on the period of the crystal oscillator reference. The
lock dectect output is HIGH whenever the phase error between phase detector inputs is
less than 2 times of the crystal period for more than three consecutive comparison cycles,
otherwise is low. Note that LD becomes HIGH during the power saving mode. The LD
output is depicted in Fig. 3 as well.
Power-down Control (EN)
By setting the pin EN to LOW, the chip enters into power-down mode, reducing the cur-
rent consumption. During the power-down mode, the phase detector output, DO, is set to
its high impedance. Normal operation mode resumes when EN is switched to HIGH. To
prove a smooth start-up condition, an intermittent control circuit is activated when the
device returns to normal operation. Due to the unknown relationship between f
V and fR
after returning from power-down, the PFD output is unpredictable and may give rise to a
significant jump in the VCO’s frequency which will result in an increased lock-up time.
To prevent this, the FS8170 employs an intermittent control circuit to limit the magnitude
of the error signal generated by the phase detector when it returns to normal operation,
thus ensuring a much quicker return to the fully phase-locked condition.
Table 8: Setting for the pin EN
EN
Status
H
Normal operation mode
L
Power-down mode
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