Pass Gate Driver With Dual-Resistance Charging for LDO Stability
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Solution Overview
Problem
The stability of digital low dropout (LDO) regulators is affected by the 'dead time' introduced by the slow turn-on of pass gate switches due to the shallow ramp in gate voltage, which leads to transients and ringing in the regulated output voltage.
Innovation Solution
A driver circuit with a low-resistance charging path and a high-resistance discharging path is implemented, allowing for fast turn-on and slow slew rate control of pass gate switches, thereby reducing transients and stabilizing the output voltage.
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 transients and ringing in the regulated output voltage are prevented, but the turn-on time increases introducing dead time that affects 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 controlled slew rate. 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. When the pass gate transistor is off, the low-resistance path is activated for fast turn-on. When the transistor is on, the high-resistance path is activated for controlled slew rate, preventing transients. This dynamic adaptation resolves the contradiction between speed and stability
2Object-affected harmful factors
If a shallow ramp in gate voltage is used, then strong transients in current through pass gate switches are prevented, but the turn-on process becomes slow affecting regulator stability
Solution Approach 1:
The charging path is divided into two segments with different resistance characteristics. The low-resistance segment enables fast charging during turn-on, while the high-resistance segment provides controlled charging during conduction. This segmentation allows the system to achieve both fast turn-on speed and current transient prevention
Solution Approach 2:
The effective resistance of the charging path is changed based on the operating phase. During turn-on, the low-resistance path is used to achieve fast voltage rise. During conduction, the high-resistance path is used to limit the slew rate and prevent current transients. This parameter change resolves the contradiction between speed and transient suppression
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.