PCIF0 datasheet(269/294 Pages) ATMEL

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269

9219A–RFID–01/11

Atmel ATA5505 [Preliminary]

24. Decoupling Capacitors

The operating frequency (i.e. system clock) of the processor determines in 95% of cases the

value needed for microcontroller decoupling capacitors.

The hypotheses used as first evaluation for decoupling capacitors are:

• The operating frequency (

fop) supplies itself the maximum peak levels of noise. The main

peaks are located at

fop and 2

• fop.

• An SMC capacitor connected to 2 micro-vias on a PCB has the following characteristics:

– 1.5 nH from the connection of the capacitor to the PCB,

– 1.5 nH from the capacitor intrinsic inductance.

Figure 24-1. Capacitor description

According to the operating frequency of the product, the decoupling capacitances are chosen

considering the frequencies to filter,

fop and 2

• fop.

The relation between frequencies to cut and decoupling characteristics are defined by:

and

where:

– L: the inductance equivalent to the global inductance on the V

CC/Gnd lines.

–C

1 & C2: decoupling capacitors (C1 = 4 • C2).

Then, in normalized value range, the decoupling capacitors become:

These decoupling capacitors must to be implemented as close as possible to each pair of

power supply pins:

– 16-17 for logic sub-system,

– 5-6 for analogical sub-system.

Nevertheless, a bulk capacitor of 10-47µF is also needed on the power distribution network of

the PCB, near the power source.

For further information, please refer to Application Notes Atmel

® AVR040 “EMC Design Con-

siderations“ and Atmel AVR042 “Hardware Design Considerations“ on the Atmel web site.

Table 24-1.

Decoupling Capacitors vs. Frequency

fop , operating frequency

16MHz

33nF

10nF

12MHz

56nF

15nF

10MHz

82nF

22nF

8MHz

120nF

33nF

6MHz

220nF

56nF

4MHz

560nF

120nF

PCB

Capacitor

1.5 nH

0.75 nH

fop

Π LC

-----------------------

fop

•

Π LC

-----------------------

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