Datasheet

MAX3676
622Mbps, 3.3V Clock-Recovery and
Data-Retiming IC with Limiting Amplifier
_______________________________________________________________________________________ 9
The comparator is configured with an active-high LOP
output. An on-chip, 6kΩ pull-up resistor is provided to
reduce the external part count.
Setting the Loop Filter
The MAX3676 is designed for both regenerator and
receiver applications. Its fully integrated PLL is a classic
second-order feedback system, with a loop bandwidth
(f
L
) fixed at 250kHz. The external capacitor, C
F
, can be
adjusted to set the loop damping. Figures 2 and 3 show
the open-loop and closed-loop transfer functions. The
PLL zero frequency, f
Z
, is a function of external capaci-
tor C
F
, and can be approximated according to:
For an overdamped system (f
Z
/f
L
) <0.25, the jitter peak-
ing (M
P
) of a second-order system can be approximat-
ed by:
For example, using C
F
= 0.22μF results in a jitter peak-
ing of 0.27dB. Reducing C
F
below 0.22μF may result in
PLL instability. The recommended value for C
F
is 2.2μF
to guarantee a maximum jitter peaking of less than
0.1dB.
The MAX3676 is optimally designed to acquire lock and
to provide a bit-error rate (BER) of less than 10
-1 0
for
long strings of consecutive zeros and ones. Measured
results show that the MAX3676 can tolerate 1200 con-
secutive ones or zeros. Decreasing C
F
reduces the
number of tolerated consecutive identical zeros and
ones. C
F
must be a low-TC, high-quality capacitor of
type X7R or better.
Lock Detect
The MAX3676’s LOL monitor indicates when the PLL is
locked. Under normal operation, the loop is locked and
the LOL output signal is high. When the MAX3676 loses
lock, a fast negative-edge transition occurs on LOL.
The output level remains at a low level (held by C
LOL
)
until the loop reacquires lock (Figure 4).
M = 0log 1+
P
2
f
f
Z
L
⎛
⎝
⎜
⎞
⎠
⎟
f =
1
2
Z
π() 90 C
F
100 1k 10k 100k
C
F
= 0.22μF
f
Z
= 8.04kHz
C
F
= 2.2μF
f
Z
= 804Hz
1M 10M
f (Hz)
OPEN-LOOP GAIN
100 1k 10k 100k
C
F
= 0.22μF
C
F
= 2.2μF
0
-3
H(J2πf) (dB)
1M 10M
f (kHz)
CLOSED-LOOP GAIN
Figure 2. Open-Loop Transfer Function
Figure 3. Closed-Loop Transfer Function