Gate Capacitance Control in Load Switches
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Solution Overview
Problem
Existing load switches face issues with gate-oxide reliability due to undischarged external capacitors when the input power supply drops, leading to uncontrolled output voltage spikes when power is restored.
Innovation Solution
A circuit that disconnects the external capacitor from the transistor gate once the transistor is fully on, allowing for the capacitor to be discharged while the switch is still on, using an enable signal and a sleep latch to manage the connection and disconnection of the capacitor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If an external capacitor is directly connected to the gate to control rise time and in-rush current, then the rise time control is improved, but gate-oxide reliability deteriorates when input power drops
Solution Approach 1:
The gate control circuit is segmented into two independent parts: an internal gate capacitor directly connected to the gate for precise rise time control, and an external capacitor connected through a switch that can be independently controlled. This segmentation allows the external capacitor to be disconnected when input power drops, preventing gate-oxide damage while maintaining the internal capacitor's rise time control function.
Solution Approach 2:
The connection between the external capacitor and the gate is made dynamic through a switch that can change states based on input power conditions. When input power is normal, the switch connects the external capacitor for additional rise time control. When input power drops, the switch disconnects the external capacitor to prevent reliability issues, allowing the circuit to adapt to changing conditions.
2Device complexity
If the external capacitor remains connected during input power drop, then circuit simplicity is maintained, but uncontrolled output voltage spikes occur when power is restored
Solution Approach 1:
The switch is designed to disconnect the external capacitor from the gate before the input power is fully restored. This preliminary action ensures that the capacitor is in a known discharged state when power returns, preventing uncontrolled output voltage spikes while maintaining relatively simple circuit architecture.
Solution Approach 2:
The switch acts as an intermediary between the external capacitor and the gate, controlling their connection based on power conditions. This intermediary component enables precise control of capacitor discharge timing without requiring complex additional circuitry, balancing simplicity with effective spike prevention.
3Reliability
If a discharge circuit is added to power the capacitor discharge, then gate-oxide reliability is improved, but device complexity and power consumption increase
Solution Approach 1:
The discharge circuit is designed to automatically activate when input power drops, using the existing power supply conditions to discharge the external capacitor without requiring additional active power management components. The circuit self-regulates based on power availability, improving reliability while minimizing added complexity.
Data Source
AI summary
A switch for controlling a power supply and a method of operating the switch are disclosed. The switch includes a first transistor having a drain and a source connected between VIN and VOUT and a gate connected to be driven to a first voltage that is greater than VIN, an external capacitor operable, when connected to the gate of the first transistor, to control a rise time of VOUT, and a circuit coupled to the gate of the first transistor and to the external capacitor, the circuit connected to couple the external capacitor to the gate of the first transistor responsive to an enable signal turning on and to uncouple the external capacitor from the gate of the first transistor responsive to the voltage on the gate reaching the first voltage.