R-2R Output Driver Slew Control With Reduced Crowbar Current
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Solution Overview
Problem
High-speed DDR SDRAM output drivers experience undesirable crowbar current and lack control over slew rate during rapid transitions between logic states, leading to increased power consumption and potential electromagnetic interference.
Innovation Solution
The output driver incorporates a pull-up and pull-down network with R-2R resistive ladder structures and control circuits to selectively activate or deactivate resistive branches through switching transistors, reducing crowbar current and controlling slew rate by managing current distribution during transitions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If rapid switching between pull-up and pull-down transistors is used to achieve high-speed data transfer, then data transfer speed is improved, but crowbar current increases causing excessive power consumption
Solution Approach 1:
The control circuit deactivates the pull-up transistor before activating the pull-down transistor during logic state transitions. This preliminary action ensures that the pull-up transistor is already OFF before the pull-down transistor turns ON, eliminating the overlap condition that causes crowbar current while maintaining rapid switching capability for high-speed data transfer
Solution Approach 2:
The control circuit monitors the switching state of the transistors and adjusts their activation timing accordingly. By providing feedback control over the transistor switching sequence, the circuit optimizes the transition timing to prevent simultaneous conduction of pull-up and pull-down transistors, thereby reducing crowbar current while maintaining high-speed operation
2Productivity
If high-speed switching is used to achieve double data rate transmission, then bandwidth is improved, but electromagnetic interference increases
Solution Approach 1:
By deactivating the pull-up transistor before activating the pull-down transistor, the circuit eliminates the high-current crowbar condition that generates electromagnetic interference. This preliminary switching action reduces EMI while maintaining the high-speed switching necessary for double data rate transmission and achieving approximately twice the bandwidth of SDR SDRAM
Solution Approach 2:
The control circuit converts the potentially harmful simultaneous switching event into a beneficial sequence by timing the transistor transitions to avoid overlap. This transforms what would be a harmful crowbar current event into a controlled transition sequence that reduces EMI while maintaining high-speed performance
3Device complexity
If simple transistor switching is used to maintain device simplicity, then device complexity is reduced, but control over slew rate is lost
Solution Approach 1:
The control circuit performs multiple functions using a unified switching control mechanism. It simultaneously manages the deactivation timing of pull-up transistors, activation timing of pull-down transistors, and slew rate control for multiple output drivers. This multi-functional approach provides slew rate control capability without proportionally increasing device complexity
Solution Approach 2:
The control circuit dynamically adjusts the switching timing of transistors based on the required slew rate. By making the switching时序 dynamic and adjustable rather than fixed, the circuit achieves versatile slew rate control while maintaining relatively simple device structure through adaptive timing control
Data Source
AI summary
An output driver that includes a pull-up network comprising a first plurality of resistive branches forming a first R-2R resistive ladder structure, wherein the resistive branches of the pull-up network are coupled to a high voltage supply through pull-up switching transistors. The output driver may further include a pull-down network comprising a second plurality of resistive branches forming a second R-2R resistive ladder structure, wherein the resistive branches of the pull-down network are coupled to a low voltage supply through pull-down switching transistors. The output driver includes a control circuit to selectively activate or deactivate each of the first plurality of resistive branches and to selectively activate or deactivate each of the second plurality of resistive branches.