PIC18LF4331-ESLASHML datasheet(26/392 Pages) MICROCHIP

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PIC18F2331/2431/4331/4431

DS39616D-page 26

 2010 Microchip Technology Inc.

2.2

Power Supply Pins

2.2.1

DECOUPLING CAPACITORS

The use of decoupling capacitors on every pair of

power supply pins, such as VDD, VSS, AVDD and

AVSS, is required.

Consider the following criteria when using decoupling

capacitors:

• Value and type of capacitor: A 0.1

F (100 nF),

10-20V capacitor is recommended. The capacitor

should be a low-ESR device, with a resonance

frequency in the range of 200 MHz and higher.

Ceramic capacitors are recommended.

• Placement on the printed circuit board: The

decoupling capacitors should be placed as close

to the pins as possible. It is recommended to

place the capacitors on the same side of the

board as the device. If space is constricted, the

capacitor can be placed on another layer on the

PCB using a via; however, ensure that the trace

length from the pin to the capacitor is no greater

than 0.25 inch (6 mm).

• Handling high-frequency noise: If the board is

experiencing high-frequency noise (upward of

tens of MHz), add a second ceramic type capaci-

tor in parallel to the above described decoupling

capacitor. The value of the second capacitor can

be in the range of 0.01

F to 0.001 F. Place this

second capacitor next to each primary decoupling

capacitor. In high-speed circuit designs, consider

implementing a decade pair of capacitances as

close to the power and ground pins as possible

(e.g., 0.1

F in parallel with 0.001 F).

• Maximizing performance: On the board layout

from the power supply circuit, run the power and

return traces to the decoupling capacitors first,

and then to the device pins. This ensures that the

decoupling capacitors are first in the power chain.

Equally important is to keep the trace length

between the capacitor and the power pins to a

minimum, thereby reducing PCB trace

inductance.

2.2.2

TANK CAPACITORS

On boards with power traces running longer than

six inches in length, it is suggested to use a tank capac-

itor for integrated circuits, including microcontrollers, to

supply a local power source. The value of the tank

capacitor should be determined based on the trace

resistance that connects the power supply source to

the device, and the maximum current drawn by the

device in the application. In other words, select the tank

capacitor so that it meets the acceptable voltage sag at

the device. Typical values range from 4.7

F to 47 F.

2.2.3

CONSIDERATIONS WHEN USING

BOR

When the Brown-out Reset (BOR) feature is enabled,

a sudden change in VDD may result in a spontaneous

BOR event. This can happen when the microcontroller

is operating under normal operating conditions, regard-

less of what the BOR set point has been programmed

to, and even if VDD does not approach the set point.

The precipitating factor in these BOR events is a rise or

fall in VDD with a slew rate faster than 0.15V/

s.

An application that incorporates adequate decoupling

between the power supplies will not experience such

rapid voltage changes. Additionally, the use of an

electrolytic tank capacitor across VDD and VSS, as

described above, will be helpful in preventing high slew

rate transitions.

If the application has components that turn on or off,

and share the same VDD circuit as the microcontroller,

the BOR can be disabled in software by using the

SBOREN bit before switching the component. After-

wards, allow a small delay before re-enabling the BOR.

By doing this, it is ensured that the BOR is disabled

during the interval that might cause high slew rate

changes of VDD.

Note:

Not all devices incorporate software BOR

control. See Section 5.0 “Reset” for

device-specific information.

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