Negative Voltage Protection for LIN Bus Interface Devices
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Solution Overview
Problem
High voltage PMOS-based LIN bus interfaces face challenges with negative voltage protection, leading to parasitic PNP device turn-on and substrate noise, which affects slew rate control and increases noise in the LIN driver chip.
Innovation Solution
Incorporating a protective device between the power transistor and the LIN bus, controlled by a circuit that turns on when the bus voltage is above a certain level and turns off when it's at or below that level, to limit negative voltage excursions and prevent parasitic PNP device activation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If high voltage PMOS devices are used to replace bipolar transistors in LIN interfaces, then cost is reduced, but parasitic PNP device turn-on occurs and substrate noise increases
Solution Approach 1:
An NPN transistor is introduced as an intermediary device between the PMOS power transistor and the LIN bus. The NPN transistor acts as a mediator that controls the timing of the PMOS transistor's operation, preventing parasitic PNP device turn-on while allowing the cost-effective PMOS device to be used. The NPN transistor's collector is connected to the PMOS gate, and its base is controlled by the control circuit to enable/disable the PMOS at appropriate times.
2Reliability
If high voltage diodes are used to protect PMOS devices from negative voltages, then negative voltage protection is provided, but parasitic PNP device turn-on is prevented and substrate noise increases
Solution Approach 1:
The NPN transistor serves as an intermediary that provides negative voltage protection without the harmful side effects of high voltage diodes. By controlling the NPN transistor's base through the control circuit, the PMOS transistor is enabled only when bus voltage is above the lower voltage level, providing protection while maintaining slew rate control and preventing parasitic turn-on.
Solution Approach 2:
The control circuit monitors the LIN bus voltage level and provides feedback control to the NPN transistor's base. When the bus voltage drops to or below the lower voltage level, the control circuit activates the NPN transistor to disable the PMOS power transistor, preventing parasitic PNP device turn-on. This feedback mechanism ensures protection while maintaining controlled slew rates.
3Reliability
If the protective device is always on to protect from negative voltages, then negative voltage protection is ensured, but slew rate control is lost and parasitic PNP device turns on
Solution Approach 1:
The protective scheme transitions from a static always-on protective device to a dynamic controlled system. The NPN transistor's state is dynamically adjusted based on the LIN bus voltage level, being activated only when protection is needed (bus voltage at or below lower voltage level). This dynamic control maintains both protection and slew rate control capabilities.
Solution Approach 2:
The control circuit implements feedback control by monitoring the LIN bus voltage and adjusting the NPN transistor's state accordingly. When bus voltage is above the lower voltage level, the NPN remains off allowing normal PMOS operation and slew rate control. When bus voltage drops to or below the lower voltage level, the NPN turns on to provide protection, thus maintaining both protection and controlled slew rate.
Data Source
AI summary
A bus interface bus is described. A first logical state is conveyed over the bus by a higher voltage level and a second logical state is conveyed by a lower voltage level. An output stage of the interface includes a power transistor configured to drive the lower voltage level onto the bus to convey the second logical state, and a protective device between the power transistor and the bus. The protective device couples the power transistor to the bus when turned on and limits negative voltage excursions at the power transistor when turned off. A control circuit of the interface is configured to turn on the protective device when the bus voltage is above the lower voltage level and to turn off the protective device when the bus voltage is at or below the lower voltage level.