MAX3673
Low-Jitter Frequency Synthesizer
with Selectable Input Reference
_______________________________________________________________________________________ 9
Pin Description (continued)
PIN NAME FUNCTION
37 OUTA3
38 OUTA3
Clock Output A3, Differential LVPECL
39 OUTA2
40 OUTA2
Clock Output A2, Differential LVPECL
43 DA Four-Level Control Input for A-Group Output Divider. See Table 2.
44 OUTA1
45 OUTA1
Clock Output A1, Differential LVPECL
46 OUTA0
47 OUTA0
Clock Output A0, Differential LVPECL
50 PLL_BYPASS
PLL Bypass Control, LVCMOS/LVTTL Input. Connect low or open for normal operation. Has
internal 90k pulldown to GND. Connect high to bypass the PLL, connecting the selected
reference clock directly to the clock outputs. In this mode, the clock qualification function
is not valid. To reduce spurious jitter in bypass mode, the internal VCO should be disabled
by shorting the CREG pin to GND.
51 RSVD3 Reserved. Connect to V
CC
.
54 RSVD4 Reserved. Leave pin open.
55 LOCK PLL Lock Indicator, LVCMOS/LVTTL Output. Low indicates PLL is locked.
56 IN1FAIL
REFCLK1 Failure Indicator, LVCMOS/LVTTL Output. Low indicates REFCLK1 fails the clock
qualification. Once a failed clock is detected, the indicator status is latched and updated
every 128 PFD cycles (~ 2μs).
— EP Exposed Pad. Connect to supply ground for proper electrical and thermal performance.
Detailed Description
The MAX3673 integrates two differential LVPECL refer-
ence inputs with a 2:1 mux, a PLL with configurable
dividers, nine differential LVPECL clock outputs, and a
selectable external feedback input for zero-delay buffer
applications (see the
Functional Diagram
).
The two reference clock inputs are continuously moni-
tored for clock failure by the internal PLL and associat-
ed logic. If the primary clock fails, the user can switch
over to the secondary clock using the 2:1 mux.
The PLL accepts reference input frequencies of 61.44,
122.88, 245.76, or 307.2MHz and generates output fre-
quencies of 61.44, 122.88, 153.6, 245.76, or 307.2MHz.
The nine clock outputs are organized into two groups
(A and B). Each group has a configurable frequency
divider and output-enable control.
Phase-Locked Loop (PLL)
The PLL contains a phase-frequency detector (PFD),
charge pump (CP) with a lowpass filter, and voltage-
controlled oscillator (VCO). The PFD compares the
divided reference frequency to the divided VCO output
at 61.44MHz, and generates a control signal to keep
the VCO phase and frequency locked to the selected
reference clock. Using a high-frequency VCO
(2.457GHz) and low-loop bandwidth (40kHz), the
MAX3673 attenuates reference clock jitter while main-
taining lock and generates low-jitter clock outputs at
multiple frequencies. Typical jitter generation is
0.3ps
RMS
(integrated 12kHz to 20MHz).
To minimize supply noise-induced jitter, the VCO sup-
ply (VCC_VCO) is isolated from the core logic and out-
put buffer supplies. Additionally, the MAX3673 uses an
internal low-dropout (LDO) regulator to attenuate noise
from the power supply. This allows the device to
achieve excellent power-supply noise rejection, signifi-
cantly reducing the impact on jitter generation.
Clock Failure Conditions
The MAX3673 clock failure detection is performed
using the combination of amplitude qualification and
PLL frequency and phase-error qualification. The failure
status is indicated for REFCLK0 and REFCLK1 at
