CMOS Charge Pump Pre-Bias Circuit Reducing Switching Delay
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Solution Overview
Problem
Clock control applications face increased delay due to full discharge and subsequent recharging of diode-connected MOS transistors in charge pump circuits, which are controlled by clock signals, leading to inefficiencies in voltage node charging.
Innovation Solution
A CMOS charge pump circuit with a differential input stage featuring two parallel branches, each with a diode-connected MOS transistor and a complementary input MOS transistor, connected in series with a common tail current source, which provides a pre-bias scheme to prevent complete discharge and reduce switching delay.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the charge pump switches between positive and negative current controlled by the clock signal, then the charge pump can control the duty cycle of the crystal oscillator, but the full output current switching causes a larger than required voltage change in diode connected MOS transistors, increasing the delay of the charge pump
Solution Approach 1:
The patent applies preliminary action by pre-charging the diode-connected MOS transistors through a dedicated pre-charge current source before the main switching operation. This pre-biasing ensures that the transistors do not start from a fully discharged state, thereby reducing the charging time and overall delay of the charge pump circuit while maintaining duty cycle control capability
Solution Approach 2:
The patent segments the current path by introducing a separate pre-charge current source that operates independently from the main positive and negative current sources. This segmentation allows the pre-charge function to be performed without interfering with the main switching operation, effectively separating the pre-biasing function from the duty cycle control function
Data Source
Figure 1~2
AI summary
A charge pump CMOS circuit comprises a differential input stage with two parallel circuitbranches. Each of the parallelcircuitbranches has a diode-connected MOS transistor (MP3,MP4) connected in series with a complementary input MOS transistor (MNO,MNl). There is a common tail current source (IB) for both circuit branches. The diode-connected MOS transistors (MP3,MP4) each have their gate/drain node connected to corresponding current sources (I1, I2). The charge pump CMOS circuitis suitable for use in an oscillator.