LDO Regulator Thin Pass Device Voltage Tracking
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Solution Overview
Problem
Low dropout (LDO) regulators face challenges in minimizing power loss in pass devices while maintaining regulation, especially during low voltage operations, as driving them into triode regions for reduced power loss results in open loop operation and loss of voltage tracking.
Innovation Solution
A circuital arrangement using a stack of transistors with a thin gate oxide input transistor and a thick gate oxide cascode transistor, along with a variable biasing circuit that adjusts the bias voltage based on supply and output voltages, allowing unequal voltage division across the stack to maintain regulation and reduce power loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If pass devices are driven into triode regions to reduce power loss, then power loss is reduced, but the LDO operates in open loop mode and loses voltage tracking capability
Solution Approach 1:
The pass device is segmented into two separate transistors: a thin pass device (first transistor) for low power loss operation and a thick cascode transistor (second transistor) for voltage withstanding and protection. This segmentation allows each transistor to be optimized for its specific function, enabling the system to operate in closed loop mode while maintaining reduced power loss.
Solution Approach 2:
Different regions of the pass device stack are assigned different qualities: the first transistor has a thin gate oxide layer optimized for low resistance and power loss, while the second transistor has a thick gate oxide layer optimized for high voltage withstanding. This local differentiation allows the system to achieve both low power loss and reliable voltage tracking simultaneously.
2Loss of energy
If a thin pass device is used to reduce power loss, then power loss is reduced, but the device cannot withstand high voltage during high voltage operation
Solution Approach 1:
The voltage withstanding function is segmented from the power loss function by introducing a separate thick cascode transistor. The thin pass device handles the low resistance requirement while the thick cascode transistor handles the high voltage withstanding requirement, allowing both functions to coexist without compromise.
Solution Approach 2:
The thick cascode transistor acts as an intermediary between the thin pass device and the high voltage supply. It protects the thin pass device from high voltage stress while allowing the thin device to operate efficiently at lower voltages, thus enabling both low power loss and high voltage withstanding capability.
3Reliability
If regulation is maintained during low voltage operation, then voltage tracking is preserved, but power loss increases due to pass device operation in saturation region
Solution Approach 1:
The biasing of the cascode transistor is made dynamic through a biasing circuit that adjusts the bias voltage based on the operating conditions (supply voltage and output voltage). This dynamic adjustment allows the system to maintain closed loop regulation when needed while optimizing for low power loss during low voltage operation, resolving the contradiction between regulation and power efficiency.
Data Source
AI summary
Systems, methods and apparatus for efficient control and biasing of pass devices that include at least one thin pass device and a remaining of thick pass devices. When operated at extreme high and low voltages, the at least one thin pass device maintains operation in its saturation region of operation while the remaining pass devices may be driven into their triode regions of operation. The thin and thick pass devices are arranged in a cascode configuration that includes a plurality of stacked devices. Biasing of the thin and thick cascode devices can be according to a voltage division scheme which protects the devices when the voltage across the stack is high, and provides a skewed voltage division across the stacked devices that promotes a higher gate-to-source voltage of the thick pass devices for a lower RON. In one exemplary case, gate length of the at least one thin pass device may be reduced to provide a lower gate-to-source voltage of the thin pass device during operation in the saturation region. An exemplary implementation of an LDO controlling the pass devices for providing RF power to a power amplifier is described.