Melexis Announces Pressure Sensing IC for Engine EVAP Systems

October 23, 2019 by Gary Elinoff

The MLX90821 measures pressures that dip down to 50 mbars in the EVAP systems of internal combustion or hybrid engines.

It is becoming more and more common for governments to prohibit vehicles from discharging fuel vapor into the atmosphere. To comply with this requirement, vehicle manufacturers are installing EVAP (evaporation) systems to measure, capture, store, and ultimately dispose of the vapor in a responsible manner.

Melexis has designed a relative pressure sensor IC that can operate in and measure very low pressures in automotive applications. The MLX90821 can detect even the smallest vapor leaks—which is a central part of EVAP systems.


MLX 90821 SoC

The system-in-package IC solution is designed with EVAP systems for hybrid vehicles in mind. Image from Melexis

The device is an SoC (system on a chip) employing MEMS (Microelectromechanical system) technology, an analog signal chain, and digital signal processing to measure fuel vapor pressure over a range of 50 to 700 millibars.


The Growing Need to Measure Vapor Pressure

As hybrid vehicles become more widespread, ever more sophisticated vapor pressure measurement systems will become increasingly necessary since the measurement process is more complex than it is with ICEs (internal combustion engines).

“The MLX90821 is a relative pressure sensor that has the range, reliability, stability, and accuracy to cover EVAP systems for both internal combustion and hybrid engines,” commented Laurent Otte, Pressure Sensors Product Line Manager at Melexis. “This gives Tier 1 and module manufacturers the ability to standardize on a single design for both types of engine, using a sensor IC that is factory calibrated but can also be customized for each individual application. Its robust, easy-to-seal packaging is certified to operate between -40℃ and +150℃ and comes with full PPAP support.”


The MLX90821. Image from Melexis


Device Fabrication

The SoCs MEMS sensor is a square diaphragm fabricated through back-side etching. As it reacts to a pressure difference between its top and bottom side, the internal strain increases. An implanted piezoresistive element reacts to the strain and serves as the unit’s pressure transducer. 

This arrangement simultaneously allows for the sensing element to be exposed directly to the vapor being measured while simultaneously providing the tough protection required in this harsh environment. Along with the IC's robust EMC protection, this simplifies the task of implementing EVAP systems in ICE vehicles as well as in hybrid vehicles.


Block Diagram for the MLX90821
Block Diagram for the MLX90821. Image from Melexis


Both analog and SENT (Single Edge Nibble Transmission) outputs are available. The DSP incorporates a 16-bit MCU and offers the ability to make full use of the SENT interface to add diagnostic messaging and to apply custom calibration. 

The MLX90821 is calibrated during the manufacturing stage with internal EEPROM storing the pressure transfer curve and clamping levels. If the SENT output is utilized, the measurement can also contain temperature readings. 

Because of its small size, the MLX90821 can be used in any module size, and its SoC sophistication makes deployment of this device easy. Automotive diagnostic features such as clamping levels, broken track diagnostics, and multiple internal fault diagnostics are available.

The MLX90821 operates over a -40°C to +150°C temperature range. Typical supply voltage and current are 5 volts and 8.5 Ma, respectively. The device is available in a SOIC16 WB cavity package.



Do you work with relative pressure sensor ICs in your design process? Let us know in the comments below.