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Sheet目录3862

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PIC18F46J11 FAMILY

DS39932D-page 34

2011 Microchip Technology Inc.

2.4.1

CONSIDERATIONS FOR CERAMIC

CAPACITORS

In recent years, large value, low-voltage, surface-mount

ceramic capacitors have become very cost effective in

sizes up to a few tens of microfarad. The low-ESR, small

physical size and other properties make ceramic

capacitors very attractive in many types of applications.

Ceramic capacitors are suitable for use with the

VDDCORE voltage regulator of this microcontroller.

However, some care is needed in selecting the capac-

itor to ensure that it maintains sufficient capacitance

over the intended operating range of the application.

Typical low-cost, 10 F ceramic capacitors are available

in X5R, X7R and Y5V dielectric ratings (other types are

also available, but are less common). The initial toler-

ance specifications for these types of capacitors are

often specified as ±10% to ±20% (X5R and X7R), or -

20%/+80% (Y5V). However, the effective capacitance

that these capacitors provide in an application circuit will

also vary based on additional factors, such as the

applied DC bias voltage and the temperature. The total

in-circuit tolerance is, therefore, much wider than the

initial tolerance specification.

The X5R and X7R capacitors typically exhibit satisfac-

tory temperature stability (ex: ±15% over a wide

temperature range, but consult the manufacturer's data

sheets for exact specifications). However, Y5V capaci-

tors typically have extreme temperature tolerance

specifications of +22%/-82%. Due to the extreme

temperature tolerance, a 10 F nominal rated Y5V type

capacitor may not deliver enough total capacitance to

meet minimum VDDCORE voltage regulator stability and

transient response requirements. Therefore, Y5V

capacitors are not recommended for use with the

VDDCORE regulator if the application must operate over

a wide temperature range.

In addition to temperature tolerance, the effective

capacitance of large value ceramic capacitors can vary

substantially, based on the amount of DC voltage

applied to the capacitor. This effect can be very signifi-

cant, but is often overlooked or is not always

documented.

A typical DC bias voltage vs. capacitance graph for

X7R type and Y5V type capacitors is shown in

Figure 2-4.

FIGURE 2-4:

DC BIAS VOLTAGE vs.

CAPACITANCE

CHARACTERISTICS

When selecting a ceramic capacitor to be used with the

VDDCORE voltage regulator, it is suggested to select a

high-voltage rating, so that the operating voltage is a

small percentage of the maximum rated capacitor volt-

age. For example, choose a ceramic capacitor rated at

16V for the 2.5V VDDCORE voltage. Suggested

capacitors are shown in Table 2-1.

2.5

ICSP Pins

The PGC and PGD pins are used for In-Circuit Serial

Programming (ICSP) and debugging purposes. It

is recommended to keep the trace length between the

ICSP connector and the ICSP pins on the device as

short as possible. If the ICSP connector is expected to

experience an ESD event, a series resistor is recom-

mended, with the value in the range of a few tens of

ohms, not to exceed 100.

Pull-up resistors, series diodes, and capacitors on the

PGC and PGD pins are not recommended as they will

interfere with the programmer/debugger communica-

tions to the device. If such discrete components are an

application requirement, they should be removed from

the circuit during programming and debugging. Alter-

natively, refer to the AC/DC characteristics and timing

requirements information in the respective device

Flash programming specification for information on

capacitive loading limits, and pin input voltage high

(VIH) and input low (VIL) requirements.

For device emulation, ensure that the “Communication

Channel Select” (i.e., PGCx/PGDx pins), programmed

into the device, matches the physical connections for

the ICSP to the Microchip debugger/emulator tool.

For

information

available

Microchip

development tools connection requirements, refer to

Section 28.0 “Development Support”

-80

-70

-60

-50

-40

-30

-20

-10

1011121314

DC Bias Voltage (VDC)

Capacit

nce

Change

(%)

012

16V Capacitor

10V Capacitor

6.3V Capacitor

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