SI5321 Datasheet, PDF (21/34 Pages) List of Unclassifed Manufacturers – SONET/SDH PRECISION CLOCK MULTIPLIER IC

Si5321
detected. When the input clock returns and is validated,
the device exits digital hold mode by re-locking to the
input clock without executing another self-calibration.
2.9. Bias Generation Circuitry
The Si5321 makes use of an external resistor to set
internal bias currents. The external resistor allows
precise generation of bias currents, which significantly
reduces power consumption and variation as compared
with traditional implementations that use an internal
resistor. The bias generation circuitry requires a 10 k
(1%) resistor connected between REXT and GND.
2.10. Differential Input Circuitry
The Si5321 provides a differential input for the clock
input, CLKIN. This input is internally-biased to a voltage
of V_ICM (see Table 2 on page 6) and may be driven by a
differential or single-ended driver circuit. For
transmission line termination, the termination resistor is
connected externally as shown.
2.11. Differential Output Circuitry
The Si5321 utilizes a current mode logic (CML)
architecture to drive the differential clock output,
CLKOUT.
For single-ended output operation simply connect to
either CLKOUT+ or CLKOUT– and leave the unused
signal unconnected.
2.12. Power Supply Connections
The Si5321 incorporates an on-chip voltage regulator to
power the device from a 3.3 V supply. The voltage
regulator requires an external compensation circuit of
one resistor and one capacitor to ensure stability over
all operating conditions.
Internally, the Si5321 V_DD33 pins are connected to the
on-chip voltage regulator input and to the device’s
LVTTL I/O circuitry. The V_DD25 pins supply power to the
core DSPLL circuitry, and are also used for connection
of the external compensation circuit.
The regulator’s compensation circuit is a resistor and a
capacitor in series between the V_DD25 node and ground.
Typically, the resistor is incorporated into the capacitor’s
equivalent series resistance (ESR). The target RC time
constant for this combination is 15 to 50 s. The
capacitor used in the Si5321 evaluation board is a 33 f
tantalum capacitor with an ESR of 0.8 . This gives an
RC time constant of 26.4 s. The Venkel part number
TA6R3TCR336KBR is an example of a capacitor that
meets these specifications. (See Figure 5.)
To get optimal performance from the Si5321 device, the
power supply noise spectrum must comply with the plot
in Figure 9. This plot shows the power supply noise
tolerance mask for the Si5321. The customer should
provide a 3.3 V supply that does not have noise density
in excess of the amount shown in the diagram.
However, the diagram cannot be used as spur criteria
for a power supply that contains single tone noise.
V_n (V/Hz)
2100
42
10 kHz
500 kHz
f
100 Mhz
Figure 9. Power Supply Noise Tolerance Mask
Rev. 2.5
21