LT3040 datasheet(18/30 Pages) AD

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LT3040

Rev. 0

For more information www.analog.com

Give extra consideration to the type of ceramic capaci-

tors used. They are manufactured with variety of dielec-

trics, each with different behavior across temperature and

applied voltage. The most common dielectrics used are

specified with EIS temperature characteristic codes of

Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are

good for providing high capacitance in the small pack-

ages, but they tend to have stronger voltage and tempera-

ture coefficients as shown in Figure 5 and Figure 6. When

used with a 5V regulator, a 16V 10µF Y5V capacitor can

exhibit an effective value as low as 1µF to 2µF for the DC

bias voltage applied over the operating temperature range.

Figure 5.

DC BIAS VOLTAGE (V)

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

–100

–80

3040 F05

–60

–40

X5R

Y5V

–20

Ceramic Capacitor DC Bias Characteristics

Figure 6.

TEMPERATURE (°C)

–50

–100

–80

–25

100

3040 F06

–60

–40

Y5V

–20

125

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

X5R

Ceramic Capacitor Temperature Characteristics

APPLICATIONS INFORMATION

X5R and X7R dielectrics result in more stable character-

istics and are thus more suitable for LT3040. The X7R

dielectric has better stability across temperature, while

X5R is less expensive and is available in higher values.

Nonetheless, care must still be exercised when using

X5R and X7R capacitors. The X5R and X7R codes only

specify operating temperature range and the maximum

capacitance change over temperature. While capacitance

change due to DC bias for X5R and X7R is better than

Y5V and Z5U dielectrics, it can still be significant enough

to drop capacitance below sufficient levels. As shown

in Figure 7, capacitor DC bias characteristics tend to

improve as component case size increases, but verifica-

tion of expected capacitance at the operating voltage is

highly recommended.

Figure 7.

DC BIAS (V)

–100

–80

–60

–40

–20

3040 F07

1210, 2.2mm THICK

1206, 1.8mm THICK

0805, 1.4mm THICK

MURATA: X7R, 25V,4.7µF CERAMIC

Capacitor Voltage Coefficient for Different Case Sizes

Voltage and temperature coefficients are not the only

sources of problems. Some ceramic capacitors have a

piezoelectric response. A piezoelectric device generates

voltage across its terminals due to mechanical stress

upon it, similar to how a piezoelectric microphone works.

For a ceramic capacitor, this stress can be induced by

mechanical vibrations within the system or due to

thermal transients.

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