Pulse-Frequency Modulation Constant On-Time Peak-Current Servo
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing DC-DC switching converters with pulse-frequency modulation (PFM) in discontinuous mode struggle to accurately control peak current, especially with small inductor values, leading to inefficiencies and electromagnetic interference (EMI) issues due to uncontrolled pulse heights and delays in current limit comparators.
Innovation Solution
A peak-current servo system employing a pulse-frequency modulation (PFM) control signal with a constant on-time scheme, using a sampling circuit to compare the final coil current with a target value and adjust the on-time through a counter, allowing for precise control of peak current and pulse duration, independent of coil value or operating conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a simple PFM control scheme is used with discontinuous mode, then the device complexity is reduced, but the peak current control precision deteriorates
Solution Approach 1:
The patent implements a feedback mechanism where the final coil current value is sampled and compared to a target current value. The on-time is adjusted based on this comparison to bring the final coil current closer to the target value, creating a closed-loop control system that improves precision without excessive complexity
Solution Approach 2:
The patent replaces complex mechanical or analog current limiting mechanisms with a digital sampling and comparison approach. By sampling the final current value and using a comparator to adjust the on-time digitally, the system achieves precise current control with simpler overall architecture
2Quantity of substance
If a small value inductor is used, then the device size and energy storage requirements are reduced, but the peak current control accuracy deteriorates
Solution Approach 1:
The feedback loop samples the final coil current and compares it to the target value, adjusting the on-time to compensate for the faster current rise associated with small inductors. This ensures accurate peak current control regardless of inductor size
Solution Approach 2:
The system dynamically adjusts the on-time based on the actual current behavior observed in each cycle. For small inductors where current rises faster, the controller shortens the on-time accordingly, adapting to the dynamic characteristics of different inductor values
3Measurement precision
If the on-time is extended to achieve target current with small inductors, then the peak current accuracy is improved, but the pulse duration exceeds the clock period
Solution Approach 1:
The system uses feedback to observe both the final current value and the pulse duration. The counter adjusts the on-time to satisfy both constraints: achieving the target current while keeping the pulse duration within the clock period, balancing accuracy with timing requirements
Solution Approach 2:
The controller dynamically changes the on-time parameter based on observed performance. By adjusting this key parameter, the system optimizes both current accuracy and pulse duration, ensuring the pulse fits within the clock period while achieving target current
4Device complexity
If peak current is not well controlled, then the simplicity of PFM mode is maintained, but electromagnetic interference and efficiency deteriorate
Solution Approach 1:
The feedback mechanism controls peak current by sampling the final current value and adjusting the on-time accordingly. This prevents excessive current peaks that would generate EMI, while maintaining a relatively simple control structure based on existing PFM infrastructure
Solution Approach 2:
The system performs preliminary sampling of the final current value before adjusting the next on-time. This proactive approach prevents EMI-generating current overshoots by preparing the appropriate on-time setting in advance, addressing the issue before it manifests
Data Source
AI summary
The disclosure describes a DC-DC switching converter providing a peak-current servo, employing a pulse-frequency modulation (PFM) control signal and a constant on-time. A Buck, Boost, Buck-Boost, or similar switching converter that supports PFM mode is required, using a fixed on-time scheme for PFM. A final value of the coil current is sampled, and the sampled value of the coil current is compared to a target value for the coil current, to establish whether it is greater or less than the target value. The on-time of the high side device is adjusted to bring the final value of the coil current closer to the target value, using an adaptive coil current measurement.