Si5351A/B/C

3.1.2. External Clock Input (CLKIN)

The external clock input is used as a clock reference for the PLLs when generating synchronous clock outputs. CLKIN can accept any frequency from 10 to 100 MHz. A divider at the input stage limits the PLL input frequency to 30 MHz.

3.1.3. Voltage Control Input (VC)

The VCXO architecture of the Si5350B eliminates the need for an external pullable crystal. Only a standard, low- cost, fixed-frequency (25 or 27 MHz) AT-cut crystal is required.

The tuning range of the VCXO is configurable allowing for a wide variety of applications. Key advantages of the VCXO design in the Si5351 include high linearity, a wide operating range (linear from 10 to 90% of VDD), and reliable startup and operation. Refer to Table 3 on page 5 for VCXO specification details.

A unique feature of the Si5351B is its ability to generate multiple output frequencies controlled by the same control voltage applied to the VC pin. This replaces multiple PLLs or VCXOs that would normally be locked to the same reference. An example is illustrated in Figure 9 on page 15.

3.2. Synthesis Stages

The Si5351 uses two stages of synthesis to generate its final output clocks. The first stage uses PLLs to multiply the lower frequency input references to a high-frequency intermediate clock. The second stage uses high- resolution MultiSynth fractional dividers to generate frequencies in the range of 1 MHz to 100 MHz. It is also possible to generate two unique frequencies up to 160 MHz on two or more of the outputs.

A crosspoint switch at the input of the first stage allows each of the PLLs to lock to the CLKIN or the XTAL input. This allows each of the PLLs to lock to a different source for generating independent free-running and synchronous clocks. Alternatively, both PLLs could lock to the same source. The crosspoint switch at the input of the second stage allows any of the MultiSynth dividers to connect to PLLA or PLLB. This flexible synthesis architecture allows any of the outputs to generate synchronous or non-synchronous clocks, with spread spectrum or without spread spectrum, and with the flexibility of generating non-integer related clock frequencies at each output.

Since the VCXO already generates a high-frequency intermediate clock, it is fed directly into the second stage of synthesis. The MultiSynth high-resolution dividers synthesize the VCXO center frequency to any frequency in the range of ~391 kHz to 160 MHz. The center frequency is then controlled (or pulled) by the VC input. An interesting feature of the Si5351 is that the VCXO output can be routed to more than one MultiSynth divider. This creates a VCXO with multiple output frequencies controlled from one VC input as shown in Figure 5.

Frequencies down to 8 kHz can be generated by applying the R divider at the output of the Multisynth (see Figure 5 below).

XA

Fixed Frequency XB Crystal (non-pullable)

Control VC Voltage

OSC

VCXO

Multi

Synth

0

Multi

Synth

1

Multi

Synth

2

The clock frequency

R0 CLK0 generated from CLK0 is controlled by the VC input

R1 CLK1

Additional MultiSynths

R2 CLK2 can be “linked” to the VCXO to generate

additional clock

frequencies

Figure 5. Using the Si5351 as a Multi-Output VCXO

3.3. Output Stage

An additional level of division (R) is available at the output stage for generating clocks as low as 8 kHz. All output drivers generate CMOS level outputs with separate output voltage supply pins (VDDOx) allowing a different voltage signal level (1.8, 2.5, or 3.3 V) at each of the four 2-output banks.

12

Preliminary Rev. 0.95

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Silicon Laboratories SI5351A/B/C Synthesis Stages, Output Stage, External Clock Input Clkin, Voltage Control Input VC
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SI5351A/B/C specifications

Silicon Laboratories SI5351A/B/C is a versatile, low-power clock generator and frequency synthesizer that has gained widespread popularity in various applications, including telecommunications, consumer electronics, and industrial control systems. These devices are primarily designed to provide precise clock frequency generation with low phase noise and jitter, making them ideal for high-performance applications.

One of the standout features of the SI5351 is its ability to generate multiple output frequencies simultaneously. Capable of producing up to three independent programmable outputs, the SI5351A/B/C can generate frequencies ranging from 8 kHz to 160 MHz. With its integrated phase-locked loop (PLL) technology, it achieves excellent frequency stability and accuracy, simplifying the design of frequency-dependent systems.

The device operates under a supply voltage range of 1.8V to 3.6V, allowing it to be used in battery-powered applications without excessive power consumption. The SI5351’s low current draw, typically as low as 25 mA, is especially beneficial in portable devices, extending battery life and enhancing overall efficiency. Furthermore, it features a programmable output driver, which can be set to various drive strengths, ensuring compatibility with a wide array of load requirements.

Configuration and control of the SI5351 are user-friendly, implemented via an I2C interface. This allows for straightforward integration into microcontroller-based designs. Moreover, the device includes an on-chip memory that stores settings, which streamlines the reconfiguration process when power cycling, minimizing setup time for developers.

Another significant advantage of the SI5351A/B/C is its output jitter performance, which is typically below 1 ps, resulting in clean output signals essential for high-speed data communications and precise timing applications. The SI5351’s integration of multiple synthesizer stages contributes to its impressive phase noise characteristics, making it suitable for demanding RF applications.

Additionally, the SI5351 devices offer programmable frequency stepping, allowing users to define custom frequency increments, which is particularly useful in applications requiring precise tuning or modulation. This flexibility, combined with its compact size and simple interface, makes the SI5351A/B/C an ideal choice for engineers seeking a reliable, cost-effective solution for generating clock signals in a myriad of electronic systems.

In summary, Silicon Laboratories SI5351A/B/C provides a robust, low-power solution for high-precision clock generation, characterized by its programmable outputs, low jitter, easy configurability, and broad frequency range, making it an excellent choice for both commercial and industrial applications across various sectors.
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