Pass Gate Driver With Dual-Path Charging for Fast Stable Turn-On
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Solution Overview
Problem
Digital low dropout (LDO) regulators experience 'dead time' due to slow turn-on of pass gate switches, which affects stability due to parasitic inductance and results in strong transients and ringing in the regulated output voltage.
Innovation Solution
A driver circuit with a low-resistance charging path, a high-resistance charging path, and an inverter configuration that enables fast turn-on and slow slew rate, reducing transients by quickly charging the pass gate transistor to its threshold voltage and then maintaining a slow ramp to prevent strong transients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a shallow ramp in gate voltage is used to turn on pass gate switches, then strong transients and ringing in the regulated output voltage are prevented, but the turn-on time increases and introduces dead time affecting stability
Solution Approach 1:
The charging path for the gate voltage is segmented into two distinct paths: a low-resistance charging path for rapid voltage rise and a high-resistance charging path for slow slew rate control. This segmentation allows the system to achieve both fast turn-on and transient suppression by selectively activating different paths based on the required operation phase
Solution Approach 2:
The driver circuit dynamically switches between different charging paths based on the gate voltage level. An inverter monitors the gate voltage and automatically transitions from the low-resistance path to the high-resistance path when the threshold voltage is reached, optimizing both speed and stability without manual intervention
2Object-generated harmful factors
If a slow turn-on with shallow ramp is used, then current transients through pass gate switches are reduced, but the dead time increases affecting LDO regulator stability
Solution Approach 1:
The low-resistance charging path is activated in advance to quickly charge the gate voltage to the threshold level, preparing the pass gate switch for conduction. This preliminary fast charging action reduces the overall dead time while the subsequent high-resistance path maintains stable operation, preventing the stability issues associated with prolonged slow turn-on
3Loss of time
If fast turn-on is implemented to reduce dead time, then stability of digital LDO regulator improves, but strong transients and ringing occur in the regulated output voltage
Solution Approach 1:
The high-resistance charging path acts as an intermediary mechanism that takes over from the low-resistance path once the threshold voltage is reached. This intermediary path provides a controlled slew rate that prevents direct coupling of fast switching edges to the output, thereby eliminating ringing while maintaining the stability benefits of fast initial turn-on
Data Source
AI summary
A driver includes a low-resistance charging path between a supply voltage rail and a first output node, a high-resistance charging path between the supply voltage rail and the first output node, an inverter coupled to the first output node and configured to enable and disable the low-resistance charging path, and a high-resistance discharging path between the first output node and a second output node. The first output node is coupled to a control terminal of a pass gate transistor in some implementations. The low-resistance charging path charges a voltage on the first output node to a threshold voltage of the pass gate transistor, and the high-resistance charging path charges the voltage on the first output node greater than the threshold voltage of the pass gate transistor. The high-resistance discharging path discharges the voltage on the first output node.