SI5350B-B Datasheet, PDF (12/32 Pages) Silicon Laboratories – FACTORY-PROGRAMMABLE ANY-FREQUENCY CMOS CLOCK GENERATOR

Si5350B-B
4. Configuring the Si5350B
The Si5350B is a factory-programmed custom clock generator that is user definable with a simple to use web-
based utility (www.silabs.com/ClockBuilder). The ClockBuilder utility provides a simple graphical interface that
allows the user to enter input and output frequencies along with other custom features as described in the following
sections. All synthesis calculations are automatically performed by ClockBuilder to ensure an optimum
configuration. A unique part number is assigned to each custom configuration.
4.1. Crystal Inputs (XA, XB)
The Si5350B uses a fixed-frequency non-pullable standard AT-cut crystal as a reference to synthesize its output
clocks and to provide the frequency stability for the VCXO.
4.1.1. Crystal Frequency
The Si5350B can operate using either a 25 MHz or a 27 MHz crystal.
4.1.2. Internal XTAL Load Capacitors
Internal load capacitors are provided to eliminate the need for external components when connecting a XTAL to the
Si5350B. The total internal XTAL load capacitance (C_L) can be selected to be 0, 6, 8, or 10 pF. XTALs with
alternate load capacitance requirements are supported using additional external load capacitance  2 pF (e.g., by
using  4 pF capacitors on XA and XB) as shown in Figure 5.
XA
XB
Optional additional
external load
capacitance
(< 2 pF)
Optional internal
load capacitance
0, 6, 8,10 pF
Figure 5. External XTAL with Optional Load Capacitors
4.2. Output Clocks (CLK0–CLK7)
The Si5350B is orderable as a 3-output (10-MSOP) or 8-output (20-QFN) clock generator. Output clocks CLK0 to
CLK5 can be ordered with two clock frequencies (F1_x, F2_x) which are selectable with the optional frequency
select pins (FS0/1). See “4.3.3. Frequency Select (FS_0, FS_1)” for more details on the operation of the frequency
select pins. Each output clock can select its reference either from the PLL or from the VCXO.
4.2.1. Output Clock Frequency
Outputs can be configured at any frequency from 2.5 kHz up to 200 MHz. However, only two unique frequencies
above 112.5 MHz can be simultaneously output. For example, 125 MHz (CLK0), 130 MHz (CLK1), and 150 MHz
(CLKx) is not allowed. Note that multiple copies of frequencies above 112.5 MHz can be provided, for example,
125 MHz could be provided on four outputs (CLKS0-3) simultaneously with 130 MHz on four different outputs
(CLKs4-7).
4.2.2. .Spread Spectrum
Spread spectrum can be enabled on any of the clock outputs that use PLLA as its reference. Spread spectrum is
useful for reducing electromagnetic interference (EMI). Enabling spread spectrum on an output clock modulates its
frequency, which effectively reduces the overall amplitude of its radiated energy. Note that spread spectrum is not
available on clocks synchronized to PLLB or to the VCXO.
The Si5350B supports several levels of spread spectrum allowing the designer to choose an ideal compromise
between system performance and EMI compliance. An optional spread spectrum enable pin (SSEN) is
configurable to enable or disable the spread spectrum feature. See “4.3.1. Spread Spectrum Enable (SSEN)” for
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