Self-Isolating Output Driver for Powered-Down Bus Isolation
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Solution Overview
Problem
Push-pull output drivers in integrated circuits can interfere with communication on a bus when powered down, as they may cause the drain diode of p-channel field-effect transistors (PFETs) to become forward biased, leading to unwanted current draw and interference.
Innovation Solution
Incorporating isolation circuitry that monitors the voltage on the bonding pad and applies appropriate voltages to the body and gate of the PFETs to prevent forward biasing, ensuring the driver remains off when the internal supply voltage falls below the bonding pad voltage, thereby preventing interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the output driver is powered down to save energy, then power consumption is reduced, but the drain diode of PFETs becomes forward biased causing unwanted current draw and communication interference
Solution Approach 1:
The isolation circuit proactively applies a preliminary reverse bias voltage to the drain diode of the PFET before the driver is powered down. This preliminary anti-action prevents the forward biasing that would otherwise occur during power-down conditions, thereby blocking unwanted current paths and eliminating communication interference while maintaining low power consumption.
Solution Approach 2:
An isolation circuit is introduced as an intermediary between the power supply and the PFET driver. This intermediary circuit monitors the power-down condition and actively manages the biasing state of the PFET drain diode, preventing harmful current flow without requiring the main driver to remain powered. The isolation circuit acts as a mediator that enables safe power-down while blocking interference paths.
2Reliability
If the driver remains powered on to prevent forward biasing, then communication interference is avoided, but power consumption increases
Solution Approach 1:
The system is segmented into two independent functional blocks: the main output driver and the isolation circuit. The isolation circuit can operate independently with minimal power consumption to maintain the reverse bias condition on the PFET drain diode, while the main driver can be fully powered down. This segmentation allows communication reliability to be maintained through the isolation circuit without requiring the entire driver to remain powered on.
Solution Approach 2:
The isolation circuit is designed to automatically detect power-down conditions and self-activate to apply the necessary reverse bias voltage to the PFET drain diode. This self-service mechanism ensures that communication reliability is maintained without requiring external control signals or keeping the main driver powered, thereby minimizing overall power consumption while preventing interference.
Data Source
AI summary
Push-pull integrated circuit output drivers may interfere with communication by other entities on a bus when an integrated circuit is powered down. When there is no power and/or when the bonding pad is externally driven above the internal supply voltage, the substrate/body/well of the p-channel field effect transistor (PFET) of the output driver is biased to prevent its drain diode from becoming forward biased thereby preventing interference with communication on the bus. Also, when there is no power, driver is powered down or pull up is disabled, the gate of the driver PFET is driven to a voltage that ensures the driver PFET remains off when the bonding pad is externally driven above the internal supply voltage.