Drain-Follower Gate Driver for Rail-to-Rail Low-Voltage PMOS Drive
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Solution Overview
Problem
Existing gate driver circuits, such as source follower circuits, are unsuitable for low voltage applications due to input-to-output voltage drop issues and are inefficient for driving large PMOS transistors, while alternative solutions like differential amplifiers increase complexity, cost, and area usage.
Innovation Solution
A gate driver circuit utilizing a drain follower configuration with a MOS driver transistor and capacitive dividers, allowing a field-induced gate voltage without DC bias, enabling a rail-to-rail output with low output impedance and maintaining speed/bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a source follower circuit is used, then the output impedance is low, but the input-to-output voltage drop makes it unsuitable for low voltage applications
Solution Approach 1:
The patent inverts the conventional source follower configuration by using a drain follower with the MOS transistor connected in a common-source arrangement. The source is connected to ground through a current sink, the gate receives the input signal, and the drain provides the output. This inversion eliminates the inherent voltage drop of source followers while maintaining low output impedance through the capacitive feedback network.
Solution Approach 2:
The patent employs capacitive feedback by connecting a capacitor between the drain (output) and gate (input) of the MOS transistor. This feedback mechanism dynamically adjusts the gate voltage to compensate for voltage drops and maintain the desired output voltage level, enabling rail-to-rail operation in low voltage applications while preserving low output impedance.
2Ease of operation
If a p-type source follower is used, then the output voltage can be driven close to ground level, but it cannot drive the output voltage to within 500 mV of ground level for rail-to-rail circuits
Solution Approach 1:
By inverting to a drain follower configuration with common-source topology, the circuit achieves superior voltage swing capability. The MOS transistor operates in saturation region with the source at a fixed potential (ground through current sink), allowing the drain voltage to swing from near ground to near the supply voltage, achieving true rail-to-rail operation that exceeds the 500 mV limitation of conventional p-type source followers.
Solution Approach 2:
The capacitive feedback network dynamically controls the gate voltage to extend the output voltage swing range. As the output voltage changes, the feedback capacitor adjusts the gate potential to maintain proper transistor operation throughout the entire rail-to-rail range, enabling the output to reach within 500 mV of both ground and supply voltage levels.
3Use of energy by moving object
If a differential amplifier in unity gain configuration is used, then the voltage drop issue is resolved, but the complexity, die area, cost and speed/bandwidth are compromised
Solution Approach 1:
The patent extracts only the essential functionality needed to resolve the voltage drop issue from the complex differential amplifier. By using a single MOS transistor in drain follower configuration with capacitive feedback, it achieves the voltage swing capability of a differential amplifier without requiring the complex multi-transistor differential pair structure, thereby reducing circuit complexity while maintaining the ability to drive rail-to-rail outputs.
Solution Approach 2:
The capacitive feedback network acts as an intermediary that provides the necessary voltage adjustment without requiring a complex amplifying structure. The capacitor dynamically couples the output to the gate, providing the equivalent voltage boosting function of a differential amplifier in unity gain configuration but with a single transistor, thus eliminating the need for complex biasing and matching circuits.
4Use of energy by moving object
If a differential amplifier in unity gain configuration is used, then the voltage drop issue is resolved, but the die area and cost increase
Solution Approach 1:
The patent extracts only the essential voltage swing functionality from the differential amplifier structure, implementing it with a single MOS transistor and capacitive feedback. This eliminates the need for multiple transistors, current mirrors, and complex biasing circuits that consume die area, reducing the overall circuit footprint while maintaining the ability to resolve the voltage drop issue in low voltage applications.
Solution Approach 2:
The single MOS transistor in the drain follower configuration performs multiple functions simultaneously: it provides voltage amplification, impedance transformation, and feedback control. This multi-functionality replaces the specialized components of a differential amplifier (input transistors, current mirrors, bias circuits), significantly reducing die area while achieving the same voltage drop compensation effect.
5Use of energy by moving object
If a differential amplifier in unity gain configuration is used, then the voltage drop issue is resolved, but the speed/bandwidth is reduced
Solution Approach 1:
The patent removes the bandwidth-limiting elements of the differential amplifier configuration, such as large compensation capacitors and complex pole-zero cancellation networks. By using a single transistor with minimal capacitive feedback, the circuit achieves voltage swing capability without the bandwidth penalties associated with differential amplifier unity gain configurations, preserving high-speed operation.
Solution Approach 2:
The capacitive feedback network provides dynamic voltage adjustment that adapts to the signal frequency. The capacitor's impedance varies with frequency, providing automatic gain and phase compensation that maintains bandwidth. This dynamic behavior replaces the static, bandwidth-limited operation of a differential amplifier in unity gain configuration, enabling high-speed operation across a wide frequency range.
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 proposed circuit achieves a rail-to-rail output with low current and area efficiency, suitable for low voltage applications, without compromising speed or bandwidth, and is simpler, cheaper, and more cost-effective.
Implementation Method 1
a gate driver circuit comprises a drain follower with a MOS driver transistor having its gate connected to an interconnection node of a capacitive divider. A first capacitor of the capacitive divider is connected between the drain and the gate of the MOS driver transistor and a second capacitor is connected between the gate of the MOS driver transistor and an input of the gate driver circuit
Implementation Method 2
Since a MOS device is 'field driven' by the application of a voltage to its gate which induces a conductive channel underneath the gate with the resulting electric field
Implementation Method 3
Since the connection between the output and the first input of the differential amplifier closes a negative feedback loop, the voltage at the output of the drain follower (i.e. at the drain of the common-source amplifier transistor) is forced to assume whatever value it must be to modulate the conductance of the MOS power transistor as needed for an output voltage swing from rail to rail
Data Source
AI summary
A circuit arrangement with a gate driver circuit for a power transistor is disclosed which is suitable for low voltage applications, permitting a rail-to-rail output without a loss in speed/bandwidth, which is very simple, low cost, low current and area efficient. The gate driver circuit comprises a drain follower with a MOS driver transistor having the gate connected to an interconnection node of a capacitive divider. A first capacitor of the capacitive divider is connected between the drain and the gate and a second capacitor is connected between the gate and an input of the gate driver circuit. The gate driver has the required low impedance for driving the gate of the power transistor.