MAX3676ESLASHD datasheet(9/16 Pages) MAXIM

9 / 16 page

622Mbps, 3.3V Clock-Recovery and

Data-Retiming IC with Limiting Amplifier

_______________________________________________________________________________________

The comparator is configured with an active-high LOP

output. An on-chip, 6k

Ω 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

(fL) fixed at 250kHz. The external capacitor, CF, 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, fZ, is a function of external capaci-

tor CF, and can be approximated according to:

For an overdamped system (fZ/fL) <0.25, the jitter peak-

ing (MP) of a second-order system can be approximat-

ed by:

For example, using CF = 0.22µF results in a jitter peak-

ing of 0.27dB. Reducing CF below 0.22µF may result in

PLL instability. The recommended value for CF 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 10-10 for

long strings of consecutive zeros and ones. Measured

results show that the MAX3676 can tolerate 1200 con-

secutive ones or zeros. Decreasing CF reduces the

number of tolerated consecutive identical zeros and

ones. CF 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 CLOL)

until the loop reacquires lock (Figure 4).

0log 1+









π()

90 CF

100

10k

100k

CF = 0.22µF

fZ = 8.04kHz

CF = 2.2µF

fZ = 804Hz

10M

f (Hz)

100

10k

100k

CF = 0.22µF

CF = 2.2µF

-3

H(J2

πf) (dB)

10M

f (kHz)

Figure 2. Open-Loop Transfer Function

Figure 3. Closed-Loop Transfer Function

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