Si5351A/B/C

3. Functional Description

The Si5351 is a versatile I2C programmable clock generator that is ideally suited for replacing crystals, crystal oscillators, VCXOs, PLLs, and buffers. A block diagram showing the general architecture of the Si5351 is shown in Figure 3. The device consists of an input stage, two synthesis stages, and an output stage.

The input stage accepts an external crystal (XTAL), a clock input (CLKIN), or a control voltage input (VC) depending on the version of the device (A/B/C). The first stage of synthesis multiplies the input frequencies to an high-frequency intermediate clock, while the second stage of synthesis uses high resolution MultiSynth fractional dividers to generate the desired output frequencies. Additional integer division is provided at the output stage for generating output frequencies as low as 8 kHz. Crosspoint switches at each of the synthesis stages allows total flexibility in routing any of the inputs to any of the outputs.

Because of this high resolution and flexible synthesis architecture, the Si5351 is capable of generating synchronous or free-running non-integer related clock frequencies at each of its outputs, enabling one device to synthesize clocks for multiple clock domains in a design.

CLKIN

XA

XTAL

XB

VC

Input

Synthesis

Synthesis

Stage

Stage 1

Stage 2

 

 

Multi

 

 

Synth

 

 

0

 

 

Multi

Div

 

Synth

PLL A

1

 

Multi

 

(SSC)

 

Synth

 

 

2

 

PLL B

Multi

OSC

(VCXO)

Synth

 

3

 

 

 

 

Multi

 

 

Synth

 

 

4

 

 

Multi

 

 

Synth

VCXO

 

5

 

Multi

 

 

Synth

 

 

6

 

 

Multi

 

 

Synth

 

 

7

 

Output

 

Stage

R0

VDDOA

CLK0

 

R1

CLK1

 

R2

VDDOB

CLK2

 

R3

CLK3

 

R4

VDDOC

CLK4

 

R5

CLK5

 

R6

VDDOD

CLK6

 

R7

CLK7

 

Figure 3. Si5351 Block Diagram

3.1. Input Stage

3.1.1. Crystal Inputs (XA, XB)

The Si5351 uses a fixed-frequency standard AT-cut crystal as a reference to the internal oscillator. The output of the oscillator can be used to provide a free-running reference to one or both of the PLLs for generating asynchronous clocks. The output frequency of the oscillator will operate at the crystal frequency, either 25 MHz or 27 MHz. The crystal is also used as a reference to the VCXO to help maintain its frequency accuracy.

Internal load capacitors (CL) are provided to eliminate the need for external components when connecting a crystal to the Si5351. Options for internal load capacitors are 6, 8, or 10 pF. Crystals with alternate load capacitance requirements are supported using additional external load capacitors as shown in Figure 4. Refer to application note AN551 for crystal recommendations.

CL XA

CL

XB

Optional CL Additional external

load capacitors

(< 2 pF)

CL Selectable internal load capacitors 6 pF, 8 pF, 10 pF

Figure 4. External XTAL with Optional Load Capacitors

Preliminary Rev. 0.95

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Silicon Laboratories SI5351A/B/C specifications Functional Description, Input Stage, Crystal Inputs XA, XB

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.