AN1709 datasheet(32/38 Pages) STMICROELECTRONICS

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EMC DESIGN GUIDE FOR ST MICROCONTROLLERS

Figure 19. Reduction of PCB Tracks Loop Surfaces

Note: This test is done with a double sided PCB. Insulator thickness is 1.5mm, copper thick-

ness is 0.13mm. The overall board size is 65 x 200mm.

Power supply filtering

The power supply is used by all parts of the circuit, so it has to be considered with special at-

tention. The supply loops have to be decoupled to make sure that signal levels and power cur-

rents do not interfere. These loops can be separated using star wiring with one node desig-

nated as common for the circuit (Figure 20.).

The decoupling capacitance should be placed very close to the MCU supply pins to minimize

the resultant loop. It should be also large enough to absorb, without significant voltage in-

crease, parasitic currents coming from the MCU via the input protection diodes. The decou-

pling of the board can be done with electrolytic capacitors (typ. 10µF to 100µF) since the die-

lectric used in such capacitors provides a high volumic capacitance. However these capaci-

tors behave like inductances at high frequency (typ. above 10MHz) while ceramic or plastic

capacitors keep a capacitive behaviour at higher frequency. A ceramic capacitance of, for in-

stance, 0.1µF to 1µF should be used as high frequency supply decoupling for critical chips op-

erating at high frequency.

High Inductance

Low Inductance

AN1709 Arkusz danych(PDF) 32 Page - STMicroelectronics