D301 datasheet(32/416 Pages) MICROCHIP

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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 inter-

nal voltage regulator of this microcontroller. However,

some care is needed in selecting the capacitor to

ensure that it maintains sufficient capacitance over the

intended operating range of the application.

Typical low-cost, 10 µF ceramic capacitors are available

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

also available, but are less common). The initial tolerance

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 µF nominal rated Y5V type

capacitor may not deliver enough total capacitance to

meet minimum internal voltage regulator stability and

transient response requirements. Therefore, Y5V

capacitors are not recommended for use with the

internal 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 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

internal 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

a minimum of 16V for the 1.8V core voltage. Suggested

capacitors are shown in Table 2-1.

2.5

ICSP Pins

The PGCx and PGDx 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 100Ω.

Pull-up resistors, series diodes and capacitors on the

PGCx and PGDx pins are not recommended as they

will interfere with the programmer/debugger communi-

cations to the device. If such discrete components are

an application requirement, they should be removed

from the circuit during programming and debugging.

Alternatively, 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 Voltage Input High

(VIH) and Voltage Input Low (VIL) requirements.

For device emulation, ensure that the “Communication

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

into the device match the physical connections for the

ICSP to the Microchip debugger/emulator tool.

For

information

available

Microchip

development tools connection requirements, refer to

Section 30.0 “Development Support”.

-80

-70

-60

-50

-40

-30

-20

-10

101112

1415

DC Bias Voltage (VDC)

6.3V Capacitor

10V Capacitor

16V Capacitor

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