LVDS Pre-Driver Circuit for Adjustable Output Slew Rate
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Solution Overview
Problem
Conventional differential transmission line drivers have fast slew rates due to large current capabilities, leading to high electromagnetic interference (EMI) and limited transmission line length, which is undesirable in systems like multipoint low voltage differential signaling (MLVDS).
Innovation Solution
A differential transmission line driver with a pre-driver circuit that controls slew rate using a current source and sink to generate constant charging and discharging currents, and cascode-connected transistors to minimize current variation, allowing for adjustable slew rates and reduced EMI. The pre-driver circuit includes capacitors charged and discharged with these currents to produce output control voltages that control the output driver circuit, enabling slow slew rates for longer transmission lines.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a simple input buffer circuit with large drive capability is used, then the drive capability is improved, but the slew rate becomes fast causing high EMI and limited transmission line length
Solution Approach 1:
The driver is divided into two separate circuits: a simple input buffer for drive capability and a pre-driver for slew rate control. This segmentation allows each circuit to perform its specialized function independently, resolving the contradiction between drive capability and EMI reduction.
Solution Approach 2:
The pre-driver acts as an intermediary circuit between the simple input buffer and the output switches. It controls the slew rate of the output signal by regulating the current flow, thereby reducing EMI while maintaining the drive capability of the overall system.
2Power
If a simple input buffer circuit with large drive capability is used, then the drive capability is improved, but the permitted transmission line length is limited
Solution Approach 1:
The driver is divided into two separate circuits: a simple input buffer for drive capability and a pre-driver for slew rate control. This segmentation allows each circuit to perform its specialized function independently, resolving the contradiction between drive capability and EMI reduction.
Solution Approach 2:
The pre-driver acts as an intermediary circuit between the simple input buffer and the output switches. It controls the slew rate of the output signal by regulating the current flow, thereby reducing EMI while maintaining the drive capability of the overall system.
3Device complexity
If a simple input buffer circuit is used, then the circuit complexity is reduced, but the slew rate varies greatly with fabrication process
Solution Approach 1:
The pre-driver acts as an intermediary circuit between the simple input buffer and the output switches. It controls the slew rate of the output signal by regulating the current flow, thereby reducing EMI while maintaining the drive capability of the overall system.
Solution Approach 2:
The pre-driver uses adjustable current sources to control the slew rate parameter. By changing the current magnitude, the slew rate can be precisely controlled and made consistent across different fabrication processes, overcoming the variability inherent in simple buffer circuits.
Data Source
AI summary
A differential signal driver includes a pre-driver configured to generate a constant charging current and a constant discharging current. A first capacitor of the pre-driver is charged with the charging current when a differential input signal has a first state, and discharged with the discharging current when the differential input signal has a second state, thereby developing a first output control voltage on the first capacitor. A second capacitor of the pre-driver is discharged with the discharging charging current when the differential input signal has the first state, and charged with the charging current when the differential input signal has the second state, thereby developing a second output control voltage on the second capacitor. An output driver circuit generates a differential output signal in response to the first and second output control voltages. The slew rate of the differential output signal is controlled by the charging and discharging currents.