Current DAC Slewing Circuit with Independent Delay and Slew Control
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Solution Overview
Problem
Conventional slewing circuits for LED output stages lack programmability in rise and fall times, leading to significant propagation delays that fail to meet performance requirements.
Innovation Solution
A circuit comprising a current mirror, reference current source, switching transistor, resistor, capacitor, and current steering inverter, which adjusts current levels to manage propagation delay and slew rate, utilizing adjustable current sources and sensing circuits to control the switching transistor and current mirror.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the parameters are designed to achieve desired proper rise/fall time, then the rise/fall time is improved, but propagation delay becomes significantly long and cannot meet performance requirements
Solution Approach 1:
The patent applies preliminary action by pre-charging the capacitor C to a voltage level (e.g., 3.3V) before the switching transistor needs to switch. This pre-charging is done through a dedicated current path that is active before the main switching event, so when the switching transistor needs to turn on or off, the capacitor is already prepared, reducing the propagation delay while maintaining the desired rise/fall times.
Solution Approach 2:
The patent segments the current paths and timing control into separate stages. There is a pre-charge phase with one current level, and a switching phase with another current level. The circuit uses separate transistors and current sources for different stages of the switching process, allowing optimization of each stage independently to reduce overall propagation delay while achieving proper rise/fall times.
2Device complexity
If conventional slewing circuits are used, then the circuit structure is simple, but programmability of rise and fall times is missing
Solution Approach 1:
The patent introduces dynamic control by making the current levels programmable through digital control signals. The first and second current sources are controlled by control signals that can be programmed to achieve different rise and fall times. This dynamic adjustability allows the circuit to adapt to different applications while maintaining a relatively simple overall structure.
Solution Approach 2:
The patent changes the electrical parameters (current levels) of the circuit to achieve programmability. By controlling the magnitude of currents in different phases (pre-charge current, charging current, discharging current) through digital control, the circuit can be programmed to achieve specific rise and fall times without fundamentally changing the circuit topology.
3Loss of time
If higher current is applied to reduce propagation delay, then propagation delay is reduced, but slew rate control becomes difficult
Solution Approach 1:
The patent uses periodic or staged action by applying different current levels at different times during the switching process. First, a higher current is applied for pre-charging to reduce propagation delay, then a controlled current is applied for the actual slewing phase to maintain proper slew rate. This time-staged current application allows both fast response and controlled slewing.
Solution Approach 2:
The patent applies preliminary action by using a higher current level in the pre-charge phase to quickly prepare the capacitor voltage, reducing propagation delay. Then, in the subsequent slewing phase, the current is reduced to a controlled level that ensures proper slew rate. This preliminary high-current action followed by controlled current achieves both goals.
Data Source
AI summary
A circuit for independently controlling slew and propagation delay of a current DAC is provided. The circuit applies dual slope technique with feed-back control the gate (or control electrode) of a switching transistor to make propagation delay independent control from rise/fall slew rate. This allows one to adjust propagation delay and current slew rate separately to achieve better performance.