LED Driver Segmented Current Control for Smooth Dimming
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Solution Overview
Problem
Existing LED driver systems are unable to provide sufficient small light intensity steps, particularly in applications like wake-up lights, where the initial dimming steps are visible and affect performance.
Innovation Solution
A LED driver system that includes a current source and a controller, which supplies a regulated average current during a constant current period and decaying current during a decay period, allowing the parallel switch to be opened during the decay period to achieve a higher number of light intensity levels by controlling the duration and timing of the open period.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If PWM control with 16 bit signal is used to dim LED, then dimming ratio of 65535:1 is achieved, but the initial step from zero to 1/65535 is clearly visible and affects performance
Solution Approach 1:
The dimming process is divided into two distinct phases: a constant current period where the current source maintains a regulated average current, and a decay period where the current naturally decays to zero. This segmentation allows different control strategies to be applied in each phase, enabling smooth transitions and eliminating visible steps in the light intensity output.
Solution Approach 2:
The system dynamically switches between two operating modes: during the constant current period, the parallel switch is controlled via PWM to modulate LED current; during the decay period, the parallel switch is opened to allow the decaying current to flow through the LED. This dynamic adaptation of control strategy based on the operating phase enables continuous, smooth dimming without visible steps.
2Ease of operation
If parallel switch is opened during constant current period to create light intensity levels, then dimming control is achieved, but visible light steps occur at low intensities
Solution Approach 1:
The system employs periodic switching between the constant current period and decay period, creating a cyclic dimming waveform. By controlling the duration and timing of the open period within the decay phase, the system generates a high number of light intensity levels while maintaining smooth transitions, as the periodic nature allows for precise control of the average current through the LED over time.
3Reliability
If current source supplies constant current during entire period, then LED operates at defined operating point, but cannot achieve high number of light intensity levels
Solution Approach 1:
The system changes the current parameter dynamically by introducing a decay period where the current naturally decays from its regulated value to zero. This parameter change enables the generation of multiple light intensity levels during the decay phase, while the constant current period maintains stable LED operation at the defined operating point. The combination of these two parameter regimes achieves both reliability and high-resolution dimming control.
Data Source
AI summary
A LED driver system (1) comprises a current source (4) which supplies a power supply current (i) to a parallel arrangement of a parallel switch (20) and a LED (10). The parallel switch (20) short-circuits the LED (10) when it is closed. A controller (15) generates during use: (i) a first control signal (E) to control the current source (4) to change from a regulating mode wherein the power supply (i) current is regulated to have a non-zero average level into a decaying mode wherein the power supply current (i) decays during a decay period (Td), and (ii) a second control signal (CSO) to control the parallel switch (20) to open during an open period (To) comprising a sub-period of the decay period (Td).