Exponential ADC Circuit for Accurate LED Brightness PWM
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Solution Overview
Problem
Existing analog to digital converters (ADCs) used to control LED brightness suffer from accuracy issues when converting linear analog signals to logarithmic PWM signals, particularly due to reliance on absolute current values, comparator accuracy, and the physical and power consumption costs of lookup tables or microprocessors.
Innovation Solution
The proposed solution involves a ramp-type ADC coupled with an exponential function circuit, where an internal gated clock signal is used to generate an M-bit exponential output from an N-bit linear input, providing high accuracy and repeatability by multiplying the output of an M-bit register by a constant factor, resulting in an exponential PWM signal with consistent accuracy across the range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a lookup table is used to convert linear ADC output to exponential PWM signal, then conversion accuracy is improved, but die area increases exponentially with resolution
Solution Approach 1:
The patent extracts only the essential exponential conversion function from a complete lookup table, implementing it through a simplified circuit using a capacitor, resistor, and Schmitt trigger. This extracts the core functionality while eliminating the excessive die area requirement of a full lookup table, achieving exponential conversion with minimal components.
Solution Approach 2:
The patent changes the operational parameters by using an RC time constant circuit to generate the exponential voltage decay curve, rather than storing pre-computed values. By adjusting the RC time constant, the system dynamically generates exponential values on-the-fly, avoiding the need for large static lookup tables while maintaining conversion accuracy.
2Adaptability or versatility
If a microprocessor with software lookup table is used, then conversion flexibility is improved, but power consumption and device size increase substantially
Solution Approach 1:
The patent replaces the mechanical/electronic system of a microprocessor executing software with an analog electronic circuit that performs exponential conversion directly in hardware. This substitution eliminates the need for software interpretation and microprocessor operation, dramatically reducing power consumption while maintaining conversion functionality.
Solution Approach 2:
The RC circuit automatically generates the exponential voltage decay curve without external control or processing. The capacitor naturally charges and discharges through the resistor, creating the exponential waveform autonomously based on physical laws, eliminating the need for microprocessor intervention and reducing power requirements.
3Measurement precision
If BJT exponential characteristic is used for logarithmic conversion, then high resolution is achieved, but accuracy deteriorates due to reliance on absolute current values and comparator accuracy
Solution Approach 1:
The patent introduces a Schmitt trigger as an intermediary element that converts the analog exponential voltage decay into a clean digital PWM signal. This intermediary provides hysteresis-based noise immunity and sharp transition edges, improving accuracy by eliminating comparator uncertainty and ensuring clean digital output levels without relying on absolute current value precision.
4Device complexity
If logarithmic ramp comparison is used to generate exponentially spaced PWM signals, then PWM signal generation is simplified, but accuracy decreases as duty cycle increases due to finite comparator resolution
Solution Approach 1:
The patent employs the natural exponential curvature of the RC voltage decay curve to generate the PWM signal. The exponential voltage curve provides a non-linear time base that compensates for the linear comparator resolution limitations, maintaining accuracy across the full duty cycle range by using the inherent mathematical properties of exponential decay rather than linear ramp comparison.
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AI summary
A method and system of an analog to digital conversion having an exponential result are provided. An analog input signal is received by the ramp ADC. The analog input signal is converted into an N-bit digital signal having a linear relationship with the analog input signal. An internal gated clock signal is generated based on the received first clock signal. The gated clock signal is used as an input to an M-bit register. An output of the M-bit register is multiplied by a predetermined factor. The product of the multiplication is provided as an input to the M-bit register. The output of the M-bit register provides an M-bit output having an exponential relationship with the analog input signal.