Tiny 1D time-of-flight sensor for accurate proximity sensing

Supplied in a 2.2mmx3.6mmx1.0mm package, the new TMF8701 sensor fits in a narrow bezel, helping smartphone manufacturers to realise widescreen phone designs that have a high display screen to body ratio.

“Smartphone manufacturers have been clear about the drawbacks of today’s 1D time-of-flight sensors: they are too large, and their performance degrades noticeably in adverse lighting conditions and when the display screen is dirty,” said Dave Moon, Senior Product Marketing Manager in the Integrated Optical Sensors business line of ams. “The TMF8701 addresses all of these concerns, providing customers with a device in a smaller footprint which also offers exceptional rejection of contamination and interference.”

The TMF8701 integrates a VCSEL infrared emitter, multiple SPAD (single photon avalanche photodiode) light detectors, time-to-digital converter, and a histogram processing core. The device implements all histogram-based presence detection, distance measurement and proximity sensing algorithms on-chip.

The TMF8701 separately identifies reflections from fingerprint smudge contaminations on the display screen and optical reflections from objects beyond the cover glass, such as the user’s face, maintaining reliable performance even when the sensor’s aperture is dirty.

The Class 1 Eye Safe VCSEL emitter has excellent immunity to interference from ambient light and produces accurate distance measurement in all lighting conditions: the module achieves accuracy of ±5% when measuring distance in the range 20 to 60cm in normal lighting conditions. Even in bright sunlight (100klux), ±5% accuracy is maintained at a range of up to 35cm.

The low-power operation of the TMF8701, it draws only 940µA in proximity sensing mode when sampling at 10Hz, makes it the ideal companion to a smartphone’s face recognition system. Always on, it triggers the higher-power face recognition system to start up when the ToF sensor detects the presence of an object up to 60cm from the display screen.

The proximity sensing capability of the device can also be used to trigger the display and face recognition system to switch off when detecting a reflective surface at a distance of zero to ten centimetres from the screen.

The accuracy of the TMF8701’s distance measurements also supports the selfie camera’s LDAF (laser detect auto-focus) function, especially in low-light conditions.

The TMF8701 time-of-flight sensor is in mass production now. Unit pricing is $2.60 in an order quantity of 5,000 units.

An evaluation kit for the TMF8701 is available.