Cascode Clamp Circuit for Low-Voltage Switching of High-Voltage LED Outputs
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Solution Overview
Problem
Conventional LED driver output stages require transistors with high voltage ratings, leading to increased area costs and inefficiencies due to the need for high voltage enable signals and parasitic diodes, which complicates the switching and protection of LEDs.
Innovation Solution
A clamp circuit is introduced, comprising cascode circuits with PMOS and NMOS transistors, a voltage regulator, and a back-to-back switch, which allows for reduced voltage ratings at the pins of the output stage, enabling efficient switching and protection of LEDs while minimizing area usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If transistors with high voltage ratings are used in the output stage, then the LEDs can be properly switched on and off with 6.5V rail voltage, but the transistor area increases significantly leading to higher cost
Solution Approach 1:
A clamp circuit is introduced as an intermediary component between the high voltage rail (6.5V) and the low voltage enable signals (3.3V). The clamp circuit includes a voltage regulator that generates a biased voltage and transistors (Q4, Q5, Q6) that work together to clamp the voltage at the drain of the main switching transistors (Q1, Q3), allowing them to operate with lower voltage ratings while still handling the 6.5V rail voltage through the combined voltage blocking capability of the clamp transistors and main transistors.
Solution Approach 2:
The voltage blocking function is segmented between multiple transistors. Instead of requiring a single transistor to block the full 6.5V, the voltage blocking is divided between the clamp circuit transistors (Q4, Q5, Q6) and the main switching transistors (Q1, Q3). This segmentation allows each transistor to have a lower voltage rating, reducing their individual area requirements while collectively handling the full rail voltage.
2Reliability
If high voltage enable signals are used to control the transistors, then the transistors can switch LEDs properly, but the complexity of the control circuit increases
Solution Approach 1:
The clamp circuit acts as an intermediary that translates the low voltage enable signals (3.3V) into appropriate voltage levels for controlling the main switching transistors. The voltage regulator and associated transistors in the clamp circuit generate the biased voltage needed to properly turn on and off the main transistors using only low voltage enable signals, eliminating the need for high voltage enable signal generation circuitry.
Solution Approach 2:
The clamp circuit automatically generates the necessary biased voltage and control signals for the main transistors using the available low voltage enable signals and the 6.5V rail voltage. The circuit self-regulates to provide the correct voltage levels for switching, eliminating the need for external high voltage signal generation and reducing overall system complexity.
3Reliability
If parasitic diodes are present in the transistors, then the transistors can provide inherent protection, but the voltage rating requirements increase leading to larger device area
Solution Approach 1:
The clamp circuit transistors (Q4, Q5, Q6) serve as intermediary protection elements that work in conjunction with the parasitic diodes of the main transistors. The clamp circuit provides an additional protection path that allows the main transistors to use lower voltage rated devices, as the combined protection mechanism of the clamp circuit and parasitic diodes provides sufficient inherent protection without requiring high voltage rated transistors.
Data Source
AI summary
An output stage for an LED driver is provided. In particular, a low voltage clamp, which uses several cascode circuits, is provided to protect low voltage switching transistors in the range of two times higher voltage application under both normal and fault conditions. Additionally, a circuit for regulating the bias voltage applied to each of the cascode circuits is provided to prevent damage during startup, while an internal voltage regulator is settling.