Why Voltage Mode Driver in a SerDes?

The Nugget

Voltage mode drivers in SerDes systems are essential for achieving high output swing with low supply voltage, addressing the limitations of current mode drivers. While maintaining signal integrity, they significantly reduce power consumption, making them increasingly preferred in modern high-speed designs.

Make it stick

⚡ Supply voltage vs. output swing: Voltage mode drivers enable high output swings even under low supply voltages, unlike current mode drivers whose performance is hampered by saturation constraints.
🔄 Zero voltage drop: Ideal voltage mode drivers strive for zero voltage drop in active switch devices, maximizing output swing efficiency.
🔋 Power efficiency: Voltage mode drivers have a current power draw that is one-quarter that of current mode drivers, making them more power-efficient in SerDes applications.
🎯 Design considerations: Proper design of output impedance, pre-driver, and FFE can drastically impact power dissipation in both types of drivers.

Voltage mode drivers excel in high-speed SerDes systems by enabling larger output swings with lower supply voltages.
Current mode drivers have limitations due to saturation requirements that restrict their output swing when supply voltages are reduced.
Voltage mode drivers utilize series impedance matching, also termed source-series terminated (SST) drivers, to help maintain signal integrity.

The deliverable power in SerDes is heavily influenced by the output voltage swing and load resistance; a 50Ω load is common.
The power consumption of voltage mode drivers can be significantly lower, making them attractive for high-speed designs due to their reduced current draw.
Even if the actual power savings may not always align perfectly with estimates, voltage mode drivers can achieve approximately half the power of current mode drivers when designed properly.

The design of the pre-driver and feed-forward equalization (FFE) implementation plays a crucial role in overall power consumption and efficiency.
High output swing requirements can pose challenges, particularly when supply voltages fall below thresholds that accommodate necessary output levels.
The ideal operational conditions for voltage mode drivers rely on specific design strategies to ensure performance and minimize power waste.

"The voltage mode driver may help... its series voltage or supply allows for achieving a big output swing and low supply VDD."
"Since a CML driver’s both current source and input differential pair must stay in the saturation region, both VDS voltage drop must be greater than the device VDS,SAT voltage."
"The differential swing of the logic one or zero is driven by the voltage divider, which is half of the source voltage."
"With proper design, even though the power of the voltage mode driver is not really one-quarter of the CML, the ballpark power of the voltage mode driver could be one-half of the CML power."
"More and more high-speed SerDes apply the voltage mode driver recently not only to NRZ SerDes but also a PAM4 SerDes."

This summary contains AI-generated information and may have important inaccuracies or omissions.