Voltage Reference Pre-charge Circuit for Fast Startup
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Solution Overview
Problem
Voltage reference circuits in low-power or disabled states face challenges with prolonged power-up wait times due to dielectric absorption, leading to errors in ADC measurements, and back-feed issues when transitioning between power states, which are not effectively addressed by existing solutions.
Innovation Solution
A voltage reference circuit with a selectable low power state and switching circuit that pre-charges and discharges the filter capacitor using a comparator-controlled switching mechanism, preventing back-feed and reducing power-up wait times by coupling the capacitor to a pre-charge or discharge path based on voltage thresholds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the voltage reference filter capacitor is charged during power-down state, then the power-up wait time is reduced, but the capacitor voltage may exceed the power supply rail causing back-feed current into other circuits
Solution Approach 1:
A precharge transistor is introduced as an intermediary component between the power supply rail and the filter capacitor. This transistor acts as a controlled mediator that allows capacitor charging during power-down state while preventing uncontrolled back-feed current, thus resolving the contradiction between reduced wait time and prevention of harmful back-feed effects
Solution Approach 2:
The precharge transistor is configured to automatically control the charging of the filter capacitor based on the power state of the voltage reference buffer amplifier. During power-down, the transistor self-activates to charge the capacitor, and during normal operation, it self-deactivates to prevent back-feed, eliminating the need for external control circuitry
2Measurement precision
If the voltage reference buffer amplifier operates at high current levels, then the output voltage precision is maintained, but the power consumption increases significantly
Solution Approach 1:
The voltage reference buffer amplifier is designed with dynamic current control capability, allowing it to operate at high current levels only when precision is required (during normal operation), and switch to low current mode during power-down state. The precharge transistor dynamically manages the capacitor charging current to maintain precision while minimizing overall power consumption
Solution Approach 2:
The filter capacitor is precharged to the reference voltage during the power-down state before the buffer amplifier is activated. This preliminary action ensures that when the amplifier starts operating at high current, the capacitor is already at the correct voltage, maintaining output precision while allowing the amplifier to consume high power only when necessary for precision operation
3Measurement precision
If the RC filter time constant is increased, then noise reduction is improved, but the settling time of the voltage reference output increases
Solution Approach 1:
The filter capacitor is precharged to the reference voltage during power-down state, performing the useful charging action in advance. When the system transitions to normal operation, the capacitor is already at or near the target voltage, significantly reducing the settling time while maintaining the long RC time constant for effective noise reduction during operation
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enables fast startup of voltage references with reduced power consumption and prevents back-feed, ensuring accurate ADC measurements and stable system operation.
Implementation Method 1
A comparator compares a voltage across the voltage reference filter capacitor to a threshold voltage and controls a switching circuit that selectively couples a first terminal of the voltage reference filter capacitor to a pre-charge source
Implementation Method 2
When the voltage reference transitions from the low power state to the normal power state, the switching circuit decouples the pre-charge source from the first terminal of the voltage reference filter capacitor and couples the first terminal of the voltage reference filter capacitor to a discharge path, to reduce the voltage on the voltage reference filter capacitor
Implementation Method 3
the switching circuit selectively couples a first terminal of the voltage reference filter capacitor to a pre-charge source having a voltage approximately equal to a target reference voltage of the voltage reference
Data Source
AI summary
A power managed voltage reference quickly provides accurate operation when enabled and also avoids back-charging power supply rails when disabled. When disabled, the voltage reference filter capacitor is decoupled from the voltage reference buffer and coupled to a pre-charge source having a voltage magnitude greater than the reference voltage. When the voltage reference is enabled, the capacitor is coupled to a discharge path and the voltage across the capacitor is detected to determine when to decouple the capacitor from the discharge path and couple the capacitor to the voltage reference buffer. The capacitor voltage is also detected while disabling the voltage reference. Back-charging the pre-charge supply is prevented by coupling the capacitor to the discharge path until the magnitude of the capacitor voltage is less than the lowest voltage specified for the pre-charge supply, then coupling the capacitor to the pre-charge supply to prepare for enabling the voltage reference.