Clock Buffers, Drivers
1. What are the Basic Functions of Clock Buffers, Drivers?
Signal Fan-out and Drive Enhancement
The clock buffer replicates a single input clock signal into multiple outputs through a non-PLL structure, providing fan-out capability to meet the system’s multi-component synchronization requirements, while strengthening signal drive capabilities to reduce distortion. For example, Renesas’ clock buffer supports output frequencies up to 3.2GHz and covers signal types such as LVCMOS and LVDS.
Format and Level Conversion
Supports conversion of different signal formats (such as LVPECL to LVDS) and level standards to adapt to diverse circuit design requirements. Some devices also support mixed output modes for flexible configuration.
2. What are the Key Parameters of Clock Buffers, Drivers?
1) Performance Parameters
Number of Output Channels: Mainstream products can provide 5-27 output channels to meet the needs of high-density systems;
Jitter and Phase Noise: The jitter added by the buffer itself will be superimposed on the original clock source, directly affecting the timing margin of the high-speed system, and its additional jitter value (such as 50fs level) needs to be paid attention to;
Transmission Delay: Determined by the load resistance and output capacitance, the delay can be reduced by optimizing the tail current and slew rate.
2) Electrical Characteristics
Power Consumption and Voltage: Support 1.2V to 3.3V power supply, low power design reduces system energy consumption;
Duty Cycle Stability: Directly affects the symmetry of the clock signal, and the output duty cycle error must be less than ±2%.
3. What are Clock Buffers, Drivers Used for?
High-speed Communication System
Used in 5G base stations, data centers, and other scenarios, the timing accuracy of high-speed interfaces (such as PCIe and Ethernet) is guaranteed by reducing jitter and noise.
Multi-chip Synchronization
Use a single master clock source with a buffer instead of a multi-crystal oscillator solution to reduce costs and solve synchronization problems.
4. What is the Typical Structure of Clock Buffers, Drivers?
Buffer Circuit Design
Usually, an even-numbered inverter cascade structure is used, and the driver size is gradually increased to balance power consumption and delay, and optimize the driver ratio (such as 2.718:1).
Zero Delay Buffer
PLL is integrated into the special design to eliminate the phase difference between input and output, which is suitable for scenarios sensitive to delay.
5. Clock Buffers, Drivers FAQs
1) What is the core function of the Clock Buffer?
It is mainly used to copy a single clock signal into multiple outputs (fan-out), support format conversion (such as differential to single-ended), level conversion, and reduce the risk of distortion during long-distance signal transmission.
Some models (such as zero delay buffers) achieve input and output clock phase alignment through internal feedback loops to reduce timing deviation.
2) How to classify Clock Buffer?
Fan-out Buffer: simply copy the clock signal, typical applications such as Renesas Electronics’ LVCMOS buffer, support more than 10 fan-outs.
Zero Delay Buffer: Synchronizes input and output clock phases through PLL, suitable for scenarios with strict timing requirements.
3) Which parameters affect system timing?
Output-to-Output Skew: The maximum time difference between multiple output clocks, which directly affects the timing margin of high-speed interfaces (such as setup/hold time).
Additive Jitter: The jitter introduced by the buffer itself, superimposed on the clock source jitter, the formula is total jitter = √(source jitter² + additive jitter²).
Drive Capability: The number of loads and wiring length must match the fan-out capability of the buffer.
4) How to quantify the impact of additive jitter?
If the clock source jitter is 300fs, adding a 50fs buffer only increases the total jitter to 304fs; but when the source jitter drops to 50fs, the total jitter will deteriorate to 70.7fs3.
5) Which indicators should be paid attention to when selecting?
Input Type: Supports flexibility of interfaces such as LVDS, HCSL, and LVCMOS.
Output Quantity and Format: Select the single-ended/differential output ratio according to the system load requirements.
Power Consumption and Temperature Range: Wide temperature and low power consumption models should be preferred for high-temperature or high-density scenarios.
6) How to reduce timing deviation?
Prefer buffers with output skew <50ps, and reduce PCB trace differences through symmetrical layout.
In FPGA development, combine the DONT_TOUCH attribute to retain the key buffer layout.
7) How to handle multi-clock domain synchronization?
Zero delay buffers lock the input clock phase through internal PLL to avoid cross-clock domain data conflicts.