Decoupling Capacitors - Microchip Technology MEC172 Series Layout Manual

AN3759
1.0 GENERAL LAYOUT CONSIDERATIONS
This section describes layout considerations for the MEC1721/MEC1723 device. This includes the following topics:
• Section 1.1, "Decoupling Capacitors," on page 2
• Section 1.2, "32.768kHz Crystal Oscillator," on page 5
• Section 1.3, "CAP Pins, AVSS/GND Connection," on page 6
• Section 1.4, "BGA Package PCB Layout Considerations," on page 6
1.1

Decoupling Capacitors

This section includes the following topics:
• Section 1.1.1, "MEC1721/MEC1723 WFBGA Capacitors," on page 2
Decoupling capacitors should be placed as close to the chip as possible to keep series inductance low. When the capac-
itors are mounted on the bottom side of the PCB, the capacitors are connected to the ground plane from the bottom
layer directly using the shortest path to the device. Each VCC pin should have a 0.1 F capacitor located as close to
the pin as possible. Bypass capacitors should be placed close to the supply pins of the MEC1721/MEC1723 with short
and wide traces.
The MEC1721/MEC1723 has an integrated voltage regulator to supply the core circuitry. Decoupling this regulator
requires a critical capacitor of 1F on the CAP pin. ESR of this 1F capacitor, including the routing resistance, must be
less than 100 mOhm.
Capacitors may carry large currents that generate magnetic fields, inducing noise on nearby traces. Sensitive traces
such as the 32kHz crystal should be separated by at least five times the trace width from decoupling capacitors when
possible.
Connecting decoupling caps to power and ground planes using two vias per pad will reduce series inductance.
• FIGURE 1-1: on page 3 shows decoupling for the MEC1721/MEC1723 176-pin WFBGA.
The VCC pin decoupling capacitors can use any typical 16V 10% Ceramic. See also the MEC1721/MEC1723 EVB
Schematics and Bill of Materials.
1.1.1 MEC1721/MEC1723 WFBGA CAPACITORS
• Figure 1-1 shows decoupling for the MEC1721/MEC1723 176-pin WFBGA package.
Note: The capacitors can use any typical 16V 10% ceramic.
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