PIC24FJ64GA108 datasheet(13/22 Pages) MICROCHIP

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 2008-2013 Microchip Technology Inc.

DS80368N-page 13

PIC24FJ256GA110 FAMILY

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 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 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 inter-

nal 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

16V, 10V and 6.3V rated 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

16V for the 2.5V core voltage. Suggested capacitors

are shown in Table 2-1.

-80

-70

-60

-50

-40

-30

-20

-10

101112

1516

DC Bias Voltage (VDC)

16V Capacitor

10V Capacitor

6.3V Capacitor

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