DocID025978 Rev 1

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AN4450

Hardware (designing PCB schematics and layout)

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In SPI the CS pin is the chip select and it is controlled by SPI master. It goes low at the start

of the communication and returns high at the end.

SPI communication can work in 2 different modes: 3-wires and 4-wires. In the first case the

pins used are: CS, SPC and SDIO; in the latter: CS, SPC, SDI and SDO. SPI mode

selection is done through bit 0 of register CTRL_REG1: 0=4-wires; 1=3-wires.

Both SPI and I²C interfaces are active by default. In case of SPI use only, I²C interface can

optionally be disabled by programming the corresponding bit in a control register

(CTRL_REG2[3]=1).

2.2

Pressure sensor PCB layout and soldering

recommendations

The LPS25H has an aperture on top of the package, so special care is required since

sensor performance could be compromised by:



Mechanical stress coming from the PCB board

–

The whole package surface + air should have minimum temperature gradient

–

Avoid placement in long and narrow PCB area, warp-free area



Temperature gradients (non-uniform/rapidly changing temperature around sensor)



Strong electrical field / light source



Localized air pressure stability (unwanted fast air pressure variation, fans)



soldering heat resistance according to JEDEC J-STD-020.

2.2.1

PCB design rules

The pressure sensor is affected by mechanical stress coming from the PCB board, hence it

should be minimized. A typical suggestion is to place the pressure sensor at the edge of the

PCB where warping is minimal.

PCB land and solder masking general recommendations are shown below. Refer to the

LPS25H datasheet for pad count, size and pitch.



It is recommended to open a solder mask external to PCB land.



The area below the sensor (on the same side of the board) must be defined as a keep-

out area. It is strongly recommended not to place any structure on the top metal layer

underneath the sensor.



This means that it is possible to place the ground plane in the PCB middle layer under

the LPS25H, but not in the plane immediately under the LPS25H.



Traces connected to pads should be as symmetrical as possible. Symmetry and

balance for pad connection will help component self-alignment and will lead to better

control of solder paste reduction after reflow.



For better performance over temperature, it is strongly recommended not to place large

insertion components like buttons or shielding boxes at distances less than 2 mm from

the sensor.



The pin 1 indicator must be left unconnected to ensure proper device operation.