SI5338 Datasheet, PDF (25/170 Pages) Silicon Laboratories – I2C-PROGRAMMABLE ANY-FREQUENCY, ANY-OUTPUT QUAD CLOCK GENERATOR

Si5338
3.9.6. Spread Spectrum
To help reduce electromagnetic interference (EMI), the
Si5338 supports spread spectrum modulation. The
output clock frequencies can be modulated to spread
energy across a broader range of frequencies, lowering
system EMI. The Si5338 implements spread spectrum
using its patented MultiSynth technology to achieve
previously unattainable precision in both modulation
rate and spreading magnitude as shown in Figure 15.
Spread spectrum can be applied to any output clock,
any clock frequency, and any spread amount. The
device supports center spread (±0.1% to ±5%) and
down spread (–0.1% to –5%). In addition, the device
has extensive on-chip voltage regulation so that power
supply variations do not influence the device’s spread-
spectrum clock waveforms.
20
+/- 0%
10
+/- 1%
0
+/- 2.5%
+/- 5%
-10
-20
-30
-40
-50
4. Applications of the Si5338
Because of its flexible architecture, the Si5338 can be
configured to serve several functions in the timing path.
The following sections describe some common
applications.
4.1. Free-Running Clock Generator
Using the internal oscillator (Osc) and an inexpensive
external crystal (XTAL), the Si5338 can be configured
as a free-running clock generator for replacing high-end
and long-lead-time crystal oscillators found on many
printed circuit boards (PCBs). Replacing several crystal
oscillators with a single IC solution helps consolidate the
bill of materials (BOM), reduces the number of
suppliers, and reduces the number of long-lead-time
components on the PCB. In addition, since crystal
oscillators tend to be the least reliable aspect of many
systems, the overall FIT rate improves with the
elimination of each oscillator.
Up to four independent clock frequencies can be
generated at any rate within its supported frequency
range and with any of supported output types. Features,
such as frequency increment and decrement and phase
adjustments on a per-output basis, provide
unprecedented flexibility for PCB designs. Figure 16
shows the Si5338 configured as a free-running clock
generator.
-60
XTAL
Osc ^ref PLL
M0
R0
F₀
-70
-80
-10%
-8%
-6%
-4%
-2%
0%
2%
4%
6%
8%
10%
Relative Frequency
Figure 15. Configurable Spread Spectrum
M1
R1
F₁
M2
R2
F₂
Si5338
M3
R3
F₃
Figure 16. Si5338 as a Free-Running Clock
Generator
Rev. 0.6
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