Ambient Light Sensing Made Easy with X-FAB

X-FAB is the first foundry to offer a ready-to-use ambient light sensing solution for its customers.  Ambient light sensing is widely used in applications such as tablet PCs, television sets, laptop computers and smart phones. X-FAB is providing fully characterized photodiodes and IP for embedded light sensing applications. Those photodiodes feature best-in-class sensitivity across a wide spectral range. In addition, X-FAB has optimized its technology for 1/f low noise performance to enable signal conditioning circuitry with optimal linearity and sensitivity.

 
 

Ambient Light Sensor - Block Diagram

 
 

Ambient Light Sensing

Ambient light sensors have to detect the brightness of the ambient like the human eye. The human eye is sensitive in a range from xlux up to 10,000 lux for wavelengths from 500nm to 600nm. The spectrum of silicon covers the sensitivity of the human eye. But silicon has a much higher sensitivity for infrared wavelengths. To create an ambient light sensor in silicon the infrared part has to be suppressed by filtering or calculation.

 
 

Photo Diode (Stacked)

Photo Diodes for ambient light sensors need a high sensitivity for visible light. IR portion of the light should be suppressed. Due to the modularity of X-FAB technologies different diodes with different doping profiles are available for using in ambient light sensors. 

As low cost version a stacked photo diode can be used. The photo current at the p+ Anode shows a much smaller portion of  IR- versus a standard Si photo diode.

Examples for stacked diodes:

  • CX08: WELLDIODE
  • XC06: dpdnw, dpw
  • XH035: dwplw
 
 

Photo Diode (Combined)

Creating a more accurate sensor can be done by using a combined photo diode, for instance diodes dlw and dwplw in XH035. Diode dlw (CH0) is sensitive for the full spectrum from UV to IR light. Diode dwplw (CH1) is especially sensitive for IR light. A sensitive curve which is close to the human eye sensitivity can be calculated by using CH0 and CH1.

Example for combined diodes:

  • CX08: WELLDIODE/NWD
  • XC06: dp/dnw
  • XH035: dwl/dwplw
 
 

Amplifier (AMP)

Ambient light sensors needs a low noise amplification of the  photo diode current. In case of a voltage or digital output the current of the photo diode has to be converted by a trans impedance amplifier to a voltage.

X-FAB offers a high selection of primitive devices which can be used for low noise amplifiers. Beside high ohmic MOS transistors other good matching devices are available for designing transimpedance, operation and other amplifiers. Fast bipolar transistors can be used for high speed low noise amplifiers.  The selection of primitive devices is completed by accurate standard models. X-FAB’s technology  libraries contain a selection of different pre-designed operational amplifiers.


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Analog-to-Digital Converter (ADC)

Depending on requirements Analog Digital Converters can be designed by using the standard core or additional modules for reduced noise, trimming devices or memory.

Use of junction isolated devices makes the ADC design robust versus substrate noise.
Zener zaps and fuses are available for trimming of parameters like offset during wafer probing or final test. Several robust ADCs (SAR until 10Bit) can be selected from X-FABs analog standard cell library.


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Logic Control

Different digital libraries optimized for speed, low power, low noise and low threshold available in different technologies allowing optimized designs. For low noise/low leakage applications special junction isolated digital libraries can be used. To store firmware for controller or other data a portfolio of non-volatile memories can be used. Unique for X-FAB is the possibility to use EEPROM and FLASH on the same chip. Hard-, software co-design allows fast, reliable and flexible creation of memory cells.


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Interface

There are I/O libraries for core or pad limited designs available. For high ESD required applications special high ESD resistance I/O libraries can be selected. RF I/Os supporting different metal layers allowing flexible designs. Special high-voltage I/Os are also selectable.


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