Under-Display Ambient Light Sensing with Blanking and Charge Hold
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Solution Overview
Problem
Incorporating ambient light sensors into electronic devices is challenging due to space constraints and potential interference from other components, such as stray light emitted by displays, which degrades the accuracy of ambient light measurements.
Innovation Solution
An ambient light sensor is integrated into electronic devices with a photosensitive element, an integrating stage, and an accumulation stage, using an amplifier with input offset voltage management and input swapping circuits to prevent charge injection errors during transitions between blanking and non-blanking periods, allowing accurate ambient light measurement even under interference conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If the ambient light sensor is placed under the display to save space, then the device space utilization is improved, but the sensor accuracy deteriorates due to stray light interference from the display
Solution Approach 1:
The patent implements periodic blanking of display rows during which the ambient light sensor takes measurements. By periodically turning off specific display rows (emission blanking) and synchronizing sensor measurements with these blanking periods, the system eliminates stray light interference while maintaining the space-saving under-display sensor configuration. The display controller alternates between emitting light for display purposes and blanking for accurate ambient light sensing.
2Device complexity
If the amplifier's input offset voltage is not compensated, then the circuit complexity is reduced, but charge injection errors increase during transitions between blanking and non-blanking periods
Solution Approach 1:
The patent applies preliminary compensation for the amplifier's input offset voltage by swapping the connections of the photosensitive element to the amplifier inputs before actual measurement. This preliminary swapping action equalizes the offset voltage effects on both differential inputs, preventing charge injection errors during subsequent blanking/non-blanking transitions without requiring complex real-time correction circuits.
Solution Approach 2:
The patent employs feedback mechanisms where the system monitors and measures the offset voltage effects during operation. By implementing differential measurement techniques and using the known blanking periods to characterize offset behavior, the system feeds back correction information to compensate for offset-induced charge injection, maintaining measurement accuracy while managing circuit complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution enables accurate ambient light measurement by minimizing interference from stray display light, maintaining sensor accuracy, and optimizing space usage within electronic devices.
Implementation Method 1
an ambient light sensor may include a photosensitive element, an integrating stage having an amplifier, and an accumulation stage configured to receive signals from the integrating stage
Data Source
AI summary
An electronic device may have a display with an array of pixels configured to display images for a user. The electronic device may have an ambient light sensor for gathering ambient light information. The ambient light sensor may be at least partially covered by the display. The ambient light sensor may be switched into use during blanking periods when a portion of the display overlapping with the display is temporarily turned off. The ambient light sensor may include a photodetector, a first stage with an amplifier, first and second capacitors selectively coupled across the amplifier, and a second accumulation stage. During blanking time, the first capacitor may integrate photodetector current. During non-blanking, the charge stored in the first capacitor may be transferred to the second accumulation stage while the second capacitor applies an offset voltage to the photodetector to prevent undesired charge injection into the photodetector.