LED Driver Circuit with Dummy Load for Low Current Dimming
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Solution Overview
Problem
Commercially available LED driver ICs struggle to accurately regulate current at low levels, leading to issues with fully turning off LEDs or achieving low brightness levels, as they experience non-linearity and inefficiency in current control, especially at currents below 5% or 10% of the maximum current.
Innovation Solution
The introduction of a 'dummy load' circuit in parallel with the LED string, which supplies a constant current at low levels to ensure accurate regulation, making the current supplied by the drive circuit a non-linear function at one range and a linear function at another, thereby improving dimming performance at low brightness levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If current-based analog dimming is used to provide constant current to LEDs without pulsing, then the fundamental issues associated with PWM dimming are solved, but the light output becomes non-linearly proportional to current due to diminishing efficiency at higher currents and junction temperature increases
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the reference current based on temperature compensation factors and calibration data. The system modifies the current parameter in response to changing junction temperatures to maintain linear brightness-current relationship across different operating conditions.
Solution Approach 2:
The patent implements feedback through temperature sensing and calibration mechanisms that monitor LED junction temperature and adjust the reference current accordingly. This closed-loop approach compensates for efficiency variations and maintains accurate brightness control despite thermal effects.
2Use of energy by moving object
If commercially-available LED driver ICs are used with small control voltage range (0-2V), then the IC can be powered by low voltage power supply, but the current regulation becomes inaccurate at low current levels (below 5-10% of maximum)
Solution Approach 1:
The patent transitions from direct low-voltage control to a two-stage approach: first generating a high-voltage reference current that maintains regulation accuracy, then scaling it down through a DAC. This dimensional transformation allows the system to operate with low control voltages while maintaining high current regulation accuracy through the intermediate high-voltage reference stage.
Solution Approach 2:
The patent introduces a high-voltage reference current as an intermediary between the low-voltage control signal and the LED current. This intermediate reference current serves as a mediator that preserves regulation accuracy while enabling low-voltage operation of the control circuitry.
3Ease of operation
If current is reduced to achieve low brightness levels, then dimming function is implemented, but the LED driver IC cannot fully turn off the LED or achieve very low brightness due to minimum current regulation limits
Solution Approach 1:
The patent applies partial action by using a dummy load to absorb the minimum regulation current that cannot be reduced to zero by the LED driver IC. This allows the LED current to be fully reduced while the dummy load maintains the required minimum current for accurate regulation, enabling complete LED off capability and very low brightness levels.
4Measurement precision
If dummy load circuit is added in parallel with LED string, then accurate current regulation at low levels is achieved, but device complexity increases
Solution Approach 1:
The patent applies universality by designing the dummy load circuit to serve multiple functions: providing accurate current regulation at low levels, enabling complete LED off capability, compensating for temperature effects, and working across the full brightness range. This multi-functionality justifies the added circuit complexity by eliminating multiple separate problems.
Data Source
AI summary
By diverting a small amount of current from a string of LED(s) powered by a LED driver at low current levels in a process of dimming the LED string, performance of the LED string light emission is improved.