Bootstrap Current Steering Circuit for Low-Glitch DAC Switching
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Solution Overview
Problem
Conventional current steering DACs for laser diode drivers face challenges such as constant DC power dissipation, glitches due to stray capacitances, and slow settling times, which affect the quality of pico-projector output, while existing solutions either reduce output voltage swing or require precise capacitance values.
Innovation Solution
A circuit design featuring cascaded transistors and electronic switches with bootstrap switches and capacitors to manage bias voltages and stray capacitances, enabling high-speed switching with low power consumption and minimal idle current, achieving sharp rise/fall times and fast settling times.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If conventional switching transistors are used to control current output terminals, then current switching capability is achieved, but glitches occur due to stray capacitances between control terminals and output terminals
Solution Approach 1:
A bootstrap transistor is introduced as an intermediary component between the switching transistor and the current source. The bootstrap transistor's control terminal is coupled to the switching transistor's control terminal, and its current path is coupled in series with the switching transistor's current path. This intermediary structure isolates the stray capacitance effects from the output, preventing glitches while maintaining switching capability.
2Stability of the object's composition
If bias voltage is applied to control terminals of current source transistors to maintain constant current flow, then current stability is improved, but constant DC power dissipation occurs
Solution Approach 1:
The bootstrap transistor is activated periodically through the switching transistor's control signal rather than maintaining continuous bias voltage. The bootstrap transistor conducts during specific switching phases to transfer charge and maintain current stability only when needed, eliminating continuous DC power dissipation while preserving current stability during active periods.
3Loss of time
If switching transistors are driven to avoid simultaneous OFF state to prevent common node discharge, then settling time is improved, but device complexity increases
Solution Approach 1:
The bootstrap transistor serves as a mediator that manages the common node charging/discharging process. It provides a controlled path for charge transfer during switching transitions, allowing simpler switching control signals while maintaining fast settling times through the bootstrap transistor's regulated charge transfer mechanism.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides high dynamic performance with sharp rise/fall times, high-speed switching, and low power consumption, ensuring efficient operation with no idle power consumption and a wide output current full-scale range.
Implementation Method 1
A first circuit comprises: a first transistor (M1) and a second transistor (M3) having respective control terminals and current paths therethrough, the control terminal of the first transistor coupled to a first bias voltage node (VB1) and the control terminal of the second transistor coupled to a second bias voltage node (VB2); a first electronic switch (M2) having a selectively activatable current path therethrough; wherein the current paths through the first and second transistor and the current path through the first electronic switch are cascaded in an output current line between a circuit output node (Iout) and ground (GND), the output current line having an intermediate portion between the first and second transistor with a charge transfer node (Y) in the intermediate portion.
Data Source
AI summary
A circuit includes a first transistor and a second transistor having respective control terminals coupled to receive first and second bias voltages. A first electronic switch is coupled in series with, and between current paths of the first and second transistors to provide an output current line between a circuit output node and ground. A second electronic switch is selectively activated to a conductive state in order to provide a charge transfer current path between a bias node and a charge transfer node in the output current line. A third electronic switch is selectively activated to a conductive state in order to provide a charge transfer current path between the charge transfer node and the control terminal of the second transistor.