ST LPS22HB Installation and operating instructions

August 2016
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www.st.com
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.

Contents
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Contents
1System integration..........................................................................5
2Mechanical design rules.................................................................7
2.1 Sensor placement.............................................................................7
2.1.1 Exposition to the environment............................................................ 7
2.1.2 Heat propagation................................................................................ 8
2.1.3 Mechanical stress............................................................................. 12
2.2 Sensor embodiment and housing....................................................13
2.3 Sensor protection............................................................................15
3Reference design: integration and housing on a personal
device......................................................................................................16
4Use case and configuration example for the LPS22HB..............18
4.1 Main device settings........................................................................18
5Use case and configuration example for the LPS25HB..............20
5.1 Main device settings........................................................................20
6Revision history ............................................................................23

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List of tables
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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.......................................22
Table 7: Document revision history ..........................................................................................................23

List of figures
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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..........................................................................................................................................10
Figure 6: Sensor with a correct sensor placement on the PCB to get the appropriate isolation from heat
sources .....................................................................................................................................................10
Figure 7: Sensor with a bad wiring on the PCB........................................................................................11
Figure 8: Sensor wiring with wrong placement on the PCB .....................................................................11
Figure 9: Bad configuration for mechanical stress (a)..............................................................................12
Figure 10: Bad configuration for mechanical stress (b)............................................................................12
Figure 11: Good configuration for avoiding mechanical stress and reducing the dead volume (a) .........12
Figure 12: Good configuration for avoiding mechanical stress and reducing the dead volume (b) .........13
Figure 13: Example of good sensor embodiment and housing................................................................14
Figure 14: Example of good sensor embodiment and housing with airflow channel ...............................14
Figure 15: Example of a bad sensor embodiment and housing...............................................................15
Figure 16: Integration of the digital pressure sensor device in a sensor chamber with two vent apertures
..................................................................................................................................................................16
Figure 17: Device integration reference in a portable device ...................................................................17
Figure 18: FIFO moving average filter scheme ........................................................................................21

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System integration
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1 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.
The typical sensor integration scenario is described in Figure 1: "Pressure sensor system
integration" where the embodiment of the sensor has to be designed in order to get as
much as possible the correspondence between the pressure (Px) and temperature (Tx)
conditions of the environment under test, and (Ps, Ts) that represent the conditions around
the sensor sensing area, nearby the air inlet houses.
Figure 1: Pressure sensor system integration
Therefore, in order to get a reliable and consistent measurement, all the parameters
involved in the mechanical design must be dimensioned to get the maximum sensor
exposition to the external environment, to get a faster response time, in terms of pressure
and temperature, compatible with the required design specifications.
Every change in the condition under test must be reflected as a sensor consistent
measurement, also in the case of fast pressure and temperature variations. Therefore, the
integration design must guarantee the environment conditions matching with the sensing
area conditions not only in “steady-state” (static conditions) but also in dynamic conditions.
Deviations between the conditions under test and the conditions around the sensing area
are also influenced by heating sources, like other devices close the sensing area, the self-
heating of the sensor. Changes in temperature are critical because not only the
temperature is influenced but, changes in temperature will also determine pressure
deviations and, as a consequence, a slower response of the system.

System integration
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Based on the considerations above, the design optimization consists of determining:
1. the placement of the sensor in the system
2. the sensor embodiment and housing
3. 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.

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Mechanical design rules
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2 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.
2.1 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.
2.1.1 Exposition to the environment
To maximize the exposition with the environment where to measure pressure and
temperature, the sensor has to be placed in static and dynamic working condition.
In static conditions, or steady-state, after a change of the pressure and the temperature
environment and their stabilization, sensing conditions must be the same as the conditions
under test, or very close to the target value, depending on the application tolerance and
specifications.
In dynamic conditions, in the presence of fast changes of the conditions under test, the
sensor must be able to provide a reliable measurement output able to follow the dynamic of
the environment. At the end of the sensor integration design, the overall response time will
be modified, and the final performance shall match the target specifications. In general,
target is to avoid design with a response time lower than the product specifications. In
order to maximize the sensor performance in static and dynamic conditions after system
integration, depending on the design specifications the below guidelines are suggested,
with reference to Figure 2: "Pressure sensor integration and embodiment reference":
1. Place the sensor to get the best connection with the environment under test, as close
as possible to the vent aperture
2. Large dead volume will increase the response time, with a bigger contribute to the
pressure response time; therefore is recommended to minimize the volume, trying to
shape a tailored housing around the sensor geometry
3. Vent aperture should be as large as possible.
4. The depth of the vent aperture must be minimized.
As a reference for integration design, Figure 2: "Pressure sensor integration and
embodiment reference" describes an example of the above recommendations. In order to
maximize the environment connection and therefore to get a fast response time, the
volume around the sensor (dead volume) is minimized and the vent size aperture has the
same order of magnitude of the sensing area. A filter membrane protection has been
added, for protecting the sensor from water or harsh environment.

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Figure 2: Pressure sensor integration and embodiment reference
A different implementation , more expensive but more efficient in terms of sensing
performances is the design with an air flow structure, described in Figure 3: "Pressure
sensor integration and embodiment with vent channel". The design with multiple vent
apertures is a more expensive solution, but providing , depending on the design
specifications, a faster response time.
Figure 3: Pressure sensor integration and embodiment with vent channel
It is useful to underline that the sensor can work properly even if it is placed in customer's
application system without considering any dedicated hole (vent aperture) unless that one
is not hermetically sealed. The design guidelines reported above are for getting out top
performances.
2.1.2 Heat propagation
The presence of heating sources near the sensor can deteriorate the performances by
modifying pressure and temperature measurement as well as generating thermal gradients
around the sensing area affecting the correct measurement in static and dynamic
conditions.
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.

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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.
Heating convection
Local thermal sources around the sensor can modify the pressure and temperature
measurement by heating radiation.
Typical sources are as follows:
other sensors and devices like close the pressure sensor
power management devices
processors and microcontrollers
LCD displays that, in particular provide a significant temperature gradient between the
environment and the dead volume inside the system
Therefore the sensor has to be placed at the correct distance from these sources, and to
guarantee the appropriate isolation, it is recommended to adopt inside the embodiment,
heating isolation structures as described in Figure 4: "Heating isolation implemented for
protecting the sensor". It also suggested, according to the specific layout to implement as
well vent aperture close the heat source, acting as cooling channels.
Figure 4: Heating isolation implemented for protecting the sensor
Looking at a section of the sensor housing, Figure 5: "Top view of the sensor housing: on
the left a correct design with the heat isolation, on the right a wrong design" shows a good
design with the heating isolation structure on the left; the heat source is far from the sensor
and a thermal protection structure is placed in the middle. On the right, a wrong design is
described, determining the sensor heating because of the heat radiation coming from the
component nearby.

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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.
A good design rule is provided in Figure 6: "Sensor with a correct sensor placement on the
PCB to get the appropriate isolation from heat sources". As an example of good design, it
describes the positioning of the devices on the left, generating heat as far as possible from
the sensor, and in Figure 8: "Sensor wiring with wrong placement on the PCB" a wrong
layout with the devices generating heat too close to the sensor. In both cases thinner metal
lines are adopted.
Figure 6: Sensor with a correct sensor placement on the PCB to get the appropriate isolation
from heat sources

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Figure 7: Sensor with a bad wiring on the PCB
In 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
device generating too much heating deteriorates the sensor performance.
Figure 8: Sensor wiring with wrong placement on the PCB
In both cases of thermal mechanism propagation, the infrared based thermal analysis of
the whole system, running in different working condition, is the right approach for identifying
the appropriate sensor location.

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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
(a)

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Figure 12: Good configuration for avoiding mechanical stress and reducing the dead volume
(b)
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. A minimal clearance has to be
maintained as in Figure 11: "Good configuration for avoiding mechanical stress and
reducing the dead volume (a)" and Figure 12: "Good configuration for avoiding mechanical
stress and reducing the dead volume (b)" to avoid any force applied on the sensor and
minimize the dead volume as well.
2.2 Sensor embodiment and housing
The sensor embodiment in the system shall match as much as possible the
recommendations highlighted above for the sensor placement and, on top of that, has to
provide all the features of specific application like waterproof, water resistant or resistant to
harsh environment, in case it is required.
Furthermore, the customer device design shall guarantee the air circulation from the
environment till the sensing area, first from the environment (outside) to the customer
device (inside), then internally from the aperture to the sensor housing and sensing
element as well. More efficient is the air circulation in this path, better the performances will
be.
The air path shall be well identified and sized in order to maximize the airflow, and as a
result, the final performance of the integrated system.
The pictures below represent a summary of a good case versus a bad case of sensor
embodiment and housing. In Figure 13: "Example of good sensor embodiment and
housing" a good design is described including also an optional filter membrane and PCB
cut to increase the thermal decoupling, that is a solution for specific case where the
devices around the pressure sensor are generating too much heating.

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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. In this
example the vent apertures size is an order magnitude lower than the sensor dimension,
for example an aperture of 0.5 mm provides a good response time and an excellent level of
integration for PD application.
In Figure 15: "Example of a bad sensor embodiment and housing" a wrong design is
described, with a very low efficiency of the final design, in terms of response time and the
sensor under the effects of heating coming from other devices.

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Figure 15: Example of a bad sensor embodiment and housing
2.3 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. The
sensor integration should protect the sensor from the light as well, therefore inside the
application; the sensor should be housed in a dark place, where the light cannot reach the
sensing element.

Reference design: integration and housing on a
personal device
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3 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. In Figure 16: "Integration
of the digital pressure sensor device in a sensor chamber with two vent apertures" is
shown the integration of the sensor in a sensor chamber isolated from the heating and with
two vent apertures covered with filter membranes. This solution provides at the same time
an efficient response time and a good protection from dust and light. In case of waterproof
device, a sensor chamber with one vent aperture is preferred.
Figure 16: Integration of the digital pressure sensor device in a sensor chamber with two vent
apertures
Based on the above recommendation, Figure 17: "Device integration reference in a
portable device" describes the integration in a portable device of the digital pressure sensor
in the bottom left corner. In this solution, a single vent aperture has been adopted (diameter
in the range of 0.5 mm) placing the sensor in the left corner, to simplify the integration with
the mechanical case and to maintain the right distance from other heating sources. A filter
membrane is also inserted for dust and water protection, depending on the specific
application.

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Reference design: integration and housing on a
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Figure 17: Device integration reference in a portable device

Use case and configuration example for the
LPS22HB
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4 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:
Power Down modality
Continuous modality
In Power Down modality, no measurements are taken; this is the default sensor
configuration:”. ODR bit [6:4] of CTRL_REG1(0x10h) set to 000.
In Continuous mode, the device is able to detect pressure and temperature data,
depending on the defined data output frequency. In particular 2 operation modes are
available as continuous modality:
One-shot mode
Active mode
In continuous modality, measurements data output are available on the output register with
a refresh frequency defined by the selected output data rate (ODR [2, 0] bits of
CTRL_REG1 (0x10)) as described in table:
Table 1: ODR configuration
ODR2
ODR1
ODR0
Pressure (Hz)
Temperature (Hz)
0
0
0
One shot
0
0
1
1 Hz
1 Hz
0
1
0
10 Hz
10 Hz
0
1
1
25 Hz
25 Hz
1
0
0
50 Hz
50 Hz
1
0
1
75 Hz
75 Hz
At device boot, the ODR[2,0] bits default configuration loaded is ‘000’ and the device goes
in Power down mode-To get a single measurement of pressure and temperature the
ONE_SHOT bit in CTRL_REG2 (0x11h) has to be set. Once the measurement is acquired,
the ONE_SHOT bit is self-cleared to the default value “0” (Idle), the new data are available
in the output registers, the STATUS_REG(0x27h) bits are updated and the device goes
back in power-down mode.
In active mode, according to the ODR configuration selected, different pressure and
temperature resolution profiles are available as described in the next section.
4.1 Main device settings
In Active and One shot mode, depending on the specific application, a set of configuration
settings is available, ranging from low power to ultra-high resolution profiles. The right
trade-off among, resolution, output data rate and power consumption has to be identified, in
order to make the sensor suitable for the specific design requirements.
The main parameters that can be configured for the specific usage are as follows:

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Use case and configuration example for the
LPS22HB
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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.
The configuration can be modified for adapting the output resolution and the power
consumption of the sensor to the design specifications.In the first case the resolution is
higher than resolution in Low Current mode. Conversely the low current mode is suggested
for applications in which the power consumption has to be low at disadvantage of the
output resolution.
In addition, to further improving the power saving, the embedded 32-slot of 40-bit data
FIFO, to store the pressure and temperature output values, can be utilized by setting the
FIFO mode. This allows consistent power saving for the system, since the host processor
does not need to continuously poll data from the sensor, but it can wake up only when
needed and burst the significant data out from the FIFO. This buffer can work according to
seven different modes as described in table 1. The FIFO buffer is enabled when the
FIFO_EN bit in CTRL_REG2 (11h) is set to '1' and each mode is selected by the
FIFO_MODE[2:0] bits in FIFO_CTRL (14h). Programmable FIFO threshold status, FIFO
overrun events and the number of unread samples stored are available in the
FIFO_STATUS (26h)register and can be set to generate dedicated interrupts on the
INT_DRDY pad using the CTRL_REG3 (12h) register.
Table 2: FIFO mode selection
F_MODE2
F_MODE1
FMODE0
FIFO mode selection
0
0
0
Bypass mode
0
0
1
FIFO mode
0
1
0
Stream mode
0
1
1
Stream to FIFO mode
1
0
0
Bypass to Stream mode
1
0
1
Reserved
1
1
0
Dynamic stream mode
1
1
1
Bypass to FIFO mode
As far as the ODR setting is concerned, ranging in active mode among 1,10,25,50, 75 Hz,
it has to be defined looking at the target power consumption, and the appropriate data
output refresh time. Finally, a low pass filter on pressure data can be enabled by setting the
EN_LPFP bit in CTRL_REG1 (10h) with two possible selectable cutoff ODR/9 and ODR/20
(bit LPF_CFG of CTRL_REG1). Keep in mind that in order to modify CTRL_REG1 register
the procedure described in figure 15 has to be used.

Use case and configuration example for the
LPS25HB
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5 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.
The LPS25HB can operate in two different modalities:
Power Down modality
Continuous modality
In Power Down modality, no measurements are taken; this is the default sensor
configuration PD [7] bit of CTRL_REG1 (0x20) set to “0”.
In Continuous mode, the device can detect pressure and temperature data, depending on
the defined data output frequency.
In particular, two operation modes are available as continuous modality:
One-shot mode
Active mode
In continuous modality, measurements data output are available on the output register with
a refresh frequency defined by the selected output data rate (ODR [2, 0] bits of
CTRL_REG1 (0x20)) as described in table
Table 3: ODR configuration
ODR2
ODR1
ODR0
Pressure (Hz)
Temperature (Hz)
0
0
0
One shot
0
0
1
1 Hz
1 Hz
0
1
0
7 Hz
7 Hz
0
1
1
12.5 Hz
12.5 Hz
1
0
0
25 Hz
25 Hz
At device boot, the ODR[2,0] bits default configuration loaded is ‘000’ and the device goes
in one-shot mode, that allows to get a single measurement of pressure and temperature, by
toggling the ONE_SHOT bit in CTRL_REG2 (0x21). Once the measurement is acquired,
the ONE_SHOT bit is self-cleared to the default value “0” (Idle), the new data are available
in the output registers, the STATUS_REG(0x27) bits are updated and the device goes back
in power-down mode.
In active mode, power-down bit set to “1”, according to the ODR configuration selected,
different pressure and temperature resolution profiles are available as described in the next
section.
5.1 Main device settings
In Active and One shot mode, depending on the specific application, a set of configuration
settings is available, ranging from low power to ultra-high resolution profiles. The right
trade-off among, resolution, output data rate and power consumption has to be identified,
to make the sensor suitable for the specific design requirements.