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ST LPS25HB Application Note

ST LPS25HB Application Note

Digital pressure sensors hardware guidelines for system integration
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AN4672
Application note
LPS22HB/LPS25HB digital pressure sensors:
hardware guidelines for system integration
Introduction
The purpose of this application note is to introduce guidelines for the hardware integration of
STMicroelectronics's LPS22HB and LPS25HB pressure sensors in the final customer's application.
August 2016
DocID027655 Rev 3
1/24
www.st.com

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  • Page 1 LPS22HB/LPS25HB digital pressure sensors: hardware guidelines for system integration Introduction The purpose of this application note is to introduce guidelines for the hardware integration of STMicroelectronics's LPS22HB and LPS25HB pressure sensors in the final customer's application. August 2016 DocID027655 Rev 3 1/24...

  • Page 2: Table Of Contents

    Sensor protection ................15 Reference design: integration and housing on a personal device ...................... 16 Use case and configuration example for the LPS22HB ....18 Main device settings ................ 18 Use case and configuration example for the LPS25HB ....20 Main device settings ................
  • Page 3 AN4672 List of tables List of tables Table 1: ODR configuration ........................18 Table 2: FIFO mode selection ........................19 Table 3: ODR configuration ........................20 Table 4: Temperature resolution configuration ..................21 Table 5: FIFO coefficient filter ........................21 Table 6: FIFO MEAN MODE configurations for different application scenarios ........
  • Page 4 List of figures AN4672 List of figures Figure 1: Pressure sensor system integration .................... 5 Figure 2: Pressure sensor integration and embodiment reference ............8 Figure 3: Pressure sensor integration and embodiment with vent channel ..........8 Figure 4: Heating isolation implemented for protecting the sensor ............9 Figure 5: Top view of the sensor housing: on the left a correct design with the heat isolation, on the right a wrong design ............................

  • Page 5: System Integration

    AN4672 System integration System integration The LPS22HB/LPS25HB pressure and temperature sensors' integration in application systems such as portable devices like smartphones, wearable devices, weather stations or industrial equipments shall be implemented without compromising the sensor performances. The system integration can be done by looking at the main mechanical and geometrical parameters and the factors that influence the sensor performance and thus optimizing those.
  • Page 6 System integration AN4672 Based on the considerations above, the design optimization consists of determining: the placement of the sensor in the system the sensor embodiment and housing sensor protection from dust, water, or chemical solvent by a sensor chamber, in presence of harsh environment The above elements are further described in the following section of this document.

  • Page 7: Mechanical Design Rules

    AN4672 Mechanical design rules Mechanical design rules For the mechanical design, the main constraints and features to be considered are described below, to provide a set of basic rules such as good design practices for a successfull integration of the sensor in the final application context. Sensor placement The sensor placement has a direct impact on the sensor performance as follows, in terms of sensor link to the environment, thermal propagation mechanism, and mechanical stress.

  • Page 8: Heat Propagation

    We report design guidelines for avoiding this effect, but, we remark that the increasing temperature impacts on performances and is strongly attenuated by the embedded temperature compensation of LPS22HB and LPS25HB devices. 8/24 DocID027655 Rev 3...

  • Page 9: Figure 4: Heating Isolation Implemented For Protecting The Sensor

    AN4672 Mechanical design rules From a physical point of view, these local sources act like a thermal capacitor placed in parallel to the thermal model of the LPS25HB and they can give a contribution to the local temperature that is different from the environmental one. Depending on the heat sources location and the heating mechanism propagation, we can distinguish the propagation related to different mechanisms as described below.

  • Page 10: Figure 5: Top View Of The Sensor Housing: On The Left A Correct Design With The Heat Isolation, On The Right

    Mechanical design rules AN4672 Figure 5: Top view of the sensor housing: on the left a correct design with the heat isolation, on the right a wrong design Heating conduction Thermal conduction mostly occurs through the metal lines on the PCB and PCB itself. In order to reduce this effect, we recommend adopting thin metal lines around the sensor, at appropriate distance among the sensor and potential heat sources, avoiding metal areas near and under the device.

  • Page 11: Figure 7: Sensor With A Bad Wiring On The Pcb

    AN4672 Mechanical design rules Figure 7: Sensor with a bad wiring on the PCB Figure 7: "Sensor with a bad wiring on the PCB "a wrong metal lines size is adopted, the bigger dimensions will provide higher level of heating conduction. In Figure 8: "Sensor wiring with wrong placement on the PCB", the wrong placement of the sensor, close to a...

  • Page 12: Mechanical Stress

    Mechanical design rules AN4672 2.1.3 Mechanical stress The sensor placement shall avoid any mechanical force applied on the sensor, direct due to a wrong mechanical system design, or indirect due to the user interaction with the system like in the case of wearable or portable device. Figure 9: Bad configuration for mechanical stress (a) Figure 10: Bad configuration for mechanical stress (b) Figure 11: Good configuration for avoiding mechanical stress and reducing the dead volume...

  • Page 13: Sensor Embodiment And Housing

    AN4672 Mechanical design rules Figure 12: Good configuration for avoiding mechanical stress and reducing the dead volume The figures above show correct and incorrect integration cases where, with the goal to reduce the dead volume around the sensor to improve the pressure response time, the embodiment structure is directly in contact with the sensor package, creating a mechanical stress that can deteriorate the sensor performance.

  • Page 14: Figure 13: Example Of Good Sensor Embodiment And Housing

    Mechanical design rules AN4672 Figure 13: Example of good sensor embodiment and housing Figure 14: Example of good sensor embodiment and housing with airflow channel Figure 14: "Example of good sensor embodiment and housing with airflow channel" describes a good design, related to the more efficient implementation with two vent apertures to get a better connection with the external environment under test that results in a higher efficiency in terms of response time and an extremely small dead volume.

  • Page 15: Sensor Protection

    AN4672 Mechanical design rules Figure 15: Example of a bad sensor embodiment and housing Sensor protection An optional filter can be adopted as sensor protection from dust, water, or chemical solvent by a sensor chamber, in presence of harsh environment or for water proof application. The key parameter for this kind of implementation is the appropriate choice of the membrane, according to the design requirements and taking into account that the membrane material will provide a slower response time, in particular in term of pressure response time.

  • Page 16: Reference Design: Integration And Housing On A Personal Device

    Reference design: integration and housing on a AN4672 personal device Reference design: integration and housing on a personal device The example below describes how the sensor placement is implemented by following the basic rules described in this document above; in other words by mounting the sensor as far as possible from the main heating sources present on the board like display LDO and microcontroller that represent the more critical sources of heating.

  • Page 17: Figure 17: Device Integration Reference In A Portable Device

    AN4672 Reference design: integration and housing on a personal device Figure 17: Device integration reference in a portable device DocID027655 Rev 3 17/24...

  • Page 18: Use Case And Configuration Example For The Lps22Hb

    Use case and configuration example for the LPS22HB The LPS22HB provides great flexibility to the designer and can be configured depending on the specific application requirements. Requisites such as resolution, power consumption, acquisition and measurement times can be set accordingly to the application context.LPS22HB can operate in two different modalities:...

  • Page 19: Table 2: Fifo Mode Selection

    AN4672 Use case and configuration example for the LPS22HB  Resolution  Current consumption  FIFO Mode Output Data Rate  Low Pass Filter In The register RES_CONF (0x1Ah), the LC_EN bit allows to select two possible configurations: Low Noise Mode (default) or Low Current Mode.

  • Page 20: Use Case And Configuration Example For The Lps25Hb

    Use case and configuration example for the AN4672 LPS25HB Use case and configuration example for the LPS25HB The LPS25HB provides great flexibility to the designer and can be configured depending on the specific application requirements. Requisites such as resolution, power consumption, acquisition and measurement times can be set accordingly to the application context.

  • Page 21: Table 4: Temperature Resolution Configuration

    AN4672 Use case and configuration example for the LPS25HB The main parameters that can be configured for the specific usage are as follows:  Resolution  FIFO moving average filter  Output Data Rate The register RES_CONF (0x10), bits AVGT [1: 0] e AVGP[ 1:0] allows to select the number of Pressure and Temperature samples acquired on each measurement cycle (oversampling) for implementing an average by the internal DSP.

  • Page 22: Table 6: Fifo Mean Mode Configurations For Different Application Scenarios

    Use case and configuration example for the AN4672 LPS25HB Based on these three main device settings, in order to make simpler the sensor use in the application context, some recommendations are provided, as a guidelines for different applications in the domain of portable device (PD) such as smartphone or wearable, of weather stations for applications like barometric pressure measurement, or more in general, of elevator and floor change detection functions, drop detection and indoor navigation as well.

  • Page 23: Revision History

    AN4672 Revision history Revision history Table 7: Document revision history Date Revision Changes 20-Aug-2015 Initial release. 16-Dec-2015 Document updated to include LPS22HB. Updated Section 4: "Use case and configuration 02-Aug-2016 example for the LPS22HB". DocID027655 Rev 3 23/24...
  • Page 24 ST products and/or to this document at any time without notice. Purchasers should obtain the latest relevant information on ST products before placing orders. ST products are sold pursuant to ST’s terms and conditions of sale in place at the time of order acknowledgement.

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