Recommendations; Printed Circuit Board; Component Position; Ground And Power Supply (Vss , Vdd ) - ST STM32L152-EVAL Application Note

AN3216
6

Recommendations

6.1

Printed circuit board

For technical reasons, it is best to use a multilayer printed circuit board (PCB) with a
separate layer dedicated to ground (V
provides good decoupling and a good shielding effect. For many applications, economical
reasons prohibit the use of this type of board. In this case, the major requirement is to
ensure a good structure for ground and for the power supply.
6.2

Component position

A preliminary layout of the PCB must separate the different circuits according to their EMI
contribution in order to reduce cross-coupling on the PCB, that is noisy, high-current circuits,
low-voltage circuits, and digital components.
6.3 Ground and power supply (V
Every block (noisy, low-level sensitive, digital, etc.) should be grounded individually and all
ground returns should be to a single point. Loops must be avoided or have a minimum area.
The power supply should be implemented close to the ground line to minimize the area of
the supply loop. This is due to the fact that the supply loop acts as an antenna, and is
therefore the main transmitter and receiver of EMI. All component-free PCB areas must be
filled with additional grounding to create a kind of shielding (especially when using single-
layer PCBs).
6.4

Decoupling

All pins need to be properly connected to the power supplies. These connections, including
pads, tracks and vias should have as low an impedance as possible. This is typically
achieved with thick track widths and, preferably, the use of dedicated power supply planes in
multilayer PCBs.
In addition, each power supply pair should be decoupled with filtering ceramic capacitors C
(100 nF) and a chemical capacitor C of about 10 µF connected in parallel on the
STM32L1xxx device. These capacitors need to be placed as close as possible to, or below,
the appropriate pins on the underside of the PCB. Typical values are 10 nF to 100 nF, but
exact values depend on the application needs.
V /V
DD SS
pair.
Doc ID 17496 Rev 5
) and another dedicated to the V
SS
, V )
SS DD
Figure 14 shows the typical layout of such a
Recommendations
supply. This
DD
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