MOS Device Turn-Off Acceleration via Drain Tie-Backs
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Solution Overview
Problem
Operational transconductance amplifiers (OTAs) face challenges in quickly driving MOS devices due to large parasitic capacitance and high output impedance, resulting in slow turn-on and turn-off times, which is undesirable for both performance and low power applications.
Innovation Solution
The implementation of drain tie-backs, where a first drain of the MOS device is connected to its gate and a second drain is connected to the base of a bipolar device, providing negative feedback and increasing current to accelerate the turn-off process, while maintaining stability during load transitions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If high bias current is provided to OTA to drive the gate at high speed, then turn-on and turn-off time is reduced, but power consumption increases
Solution Approach 1:
The circuit dynamically switches between two drive modes: during normal operation, the OTA provides low-current drive to maintain low power consumption; during fast transitions (turn-on/turn-off), a second current source activates to provide high current for rapid gate voltage changes. This dynamic adaptation resolves the contradiction by providing high speed only when necessary.
Solution Approach 2:
The high current drive is activated periodically or event-driven based on transition detection, rather than continuously. The circuit monitors the gate voltage and enables the second current source only during transition periods, achieving fast switching when needed while maintaining low power during steady-state operation.
2Device complexity
If OTA with high output impedance is used to drive the MOS device gate, then circuit complexity is reduced, but turn-on and turn-off time increases
Solution Approach 1:
The drive function is segmented into two independent current sources: the first current source (OTA) provides controlled, low-power drive with negative feedback for stability; the second current source provides additional current specifically for fast transitions. This segmentation allows each component to be optimized for its specific function without compromising the other.
Solution Approach 2:
A current switching mechanism acts as an intermediary between the two current sources and the gate. This intermediary selectively connects or disconnects the second current source based on transition conditions, coordinating the actions of both current sources to achieve fast switching while maintaining overall circuit simplicity.
3Speed
If negative feedback is applied to lower impedance at the gate node, then frequency response is improved, but turn-off time during heavy to light load transition is not sufficiently reduced
Solution Approach 1:
The circuit detects the transition condition (heavy to light load) in advance and proactively activates the second current source before the turn-off process completes. This preliminary action ensures that sufficient current is available to rapidly discharge the gate capacitance during the critical turn-off phase, overcoming the limitations of negative feedback alone.
Solution Approach 2:
The parasitic capacitance that normally slows down turn-off is converted into a benefit by using it as a sensing mechanism. The circuit monitors the gate voltage and uses the capacitance discharge process itself to trigger the activation of the second current source, turning the harmful effect into a useful trigger signal for enhanced drive.
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
This configuration significantly reduces the turn-off time of the MOS device, minimizing output voltage overshoot and allowing the circuit to operate efficiently in low power applications by maintaining low impedance and high frequency response.
Implementation Method 1
a second drain is connected to the base of a bipolar device, providing additional current through the beta multiplication
Data Source
AI summary
An OTA driving an MOS device needs to turn it off quickly to minimize overshoot during a heavy to light load state. The drains of the MOS device can be used to accelerate turning off the MOS device. A first drain is connected to the gate of the MOS device. A second drain is connected to a base of a bipolar device. The emitter of the bipolar device is connected to the MOS device gate. Notably, this bipolar device is active during the heavy to light load state. Therefore, any current provided by the second drain is then multiplied by the beta of the bipolar device. The increased current generated on the emitter of the bipolar transistor and provided to the gate of the MOS device can advantageously accelerate the turnoff of that MOS device. The first drain can provide minimal additional current during the heavy to light load state.