Average Output Inductor Current Indicator Circuit
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Solution Overview
Problem
Conventional methods for measuring average output inductor current in switched-mode voltage converters, especially those operating in discontinuous conduction mode, rely on output inductor DCR or series sampling resistors, which are not constant, result in power losses, and require low pass filtering, and existing solutions for continuous conduction mode are unsuitable for discontinuous conduction mode.
Innovation Solution
A circuit and method that calculates the average output inductor current using peak current and counts from a differential amplifier and counter system, eliminating the need for DCR or series sampling resistors, by determining peak current based on sampled inductor current and clock pulses, and calculating average current using these counts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional DCR or series sampling resistor methods are used to measure average output inductor current, then measurement can be obtained, but power losses increase and measurement precision deteriorates due to DCR variation with temperature and part-to-part variability
Solution Approach 1:
The patent extracts the current measurement function from the power path by using a separate sense resistor Rs placed in parallel with the output inductor Lo. This allows current measurement without inserting a resistor into the main power flow, eliminating the power loss issue while maintaining measurement capability through the relationship iL = iR + C*dvout/dt
Solution Approach 2:
The patent introduces an intermediary capacitor C connected in parallel with the output inductor, which mediates the relationship between inductor current and measurable quantities. By measuring the voltage across the sense resistor and using the capacitor's current-voltage relationship, the inductor current can be determined without direct measurement, avoiding the need for power-dissipating series resistors
2Measurement precision
If series sampling resistor is used to measure average output inductor current, then current measurement is achieved, but device complexity increases due to required low pass filtering
Solution Approach 1:
The patent extracts the measurement function from the power path and implements it through a parallel sense resistor configuration with capacitor, eliminating the need for complex filtering circuits that would be required with series sampling resistor methods
Solution Approach 2:
The patent replaces the mechanical/electrical filtering approach with a mathematical relationship-based measurement system. By using the known relationship iL = iR + C*dvout/dt and measuring dvout/dt through voltage sampling, the system obtains average current information without requiring physical low-pass filtering hardware
3Ease of manufacture
If output inductor DCR is used for current measurement, then measurement is obtained, but measurement precision deteriorates due to DCR variation with temperature and part-to-part variability
Solution Approach 1:
The patent introduces a dedicated sense resistor Rs as an intermediary measurement element with stable, known resistance value. This replaces the unreliable DCR of the power inductor with a precision measurement resistor, enabling accurate current sensing through the parallel configuration and capacitor relationship without being affected by temperature or manufacturing variations in the power inductor
Solution Approach 2:
The patent creates a measurement copy of the current path through the parallel sense resistor and capacitor combination. Instead of measuring through the power inductor's DCR directly, it creates an equivalent measurement path that replicates the current information with higher precision, using the relationship between the sense resistor current and capacitor voltage derivative
Data Source
AI summary
In one implementation, a circuit for producing an average output inductor current indicator in a voltage converter is configured to start a counter when a high side power switch turns on, to sense a sample current through an output inductor of the voltage converter after the high side power switch turns off and when a low side power switch is on, and to register a first count of the counter when the low side power switch turns off. The circuit is further configured to register a second count of the counter when the high side power switch subsequently turns on, and to produce the average output inductor current indicator based on the sample current and the first and second counts of the counter.