ADM7151 datasheet(36/45 Pages) AD

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Data Sheet

LT3097

APPLICATIONS INFORMATION

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Rev. 0 | 36 of 45

put capacitance do decrease peak output deviations during a load

transient. Note that bypass capacitors used to decouple individual

components powered by the LT3097 increase the effective output

capacitance.

Give extra consideration to the type of ceramic capacitors used.

The capacitors are manufactured with a variety of dielectrics, each

with different behaviors across temperature and applied voltage.

The most common dielectrics used are specified with Electronic

Industries Alliance (EIA) temperature characteristic codes of Z5U,

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

providing high capacitance in the small packages, but these dielec-

trics tend to have stronger voltage and temperature coefficients,

as shown in Figure 127 and Figure 128. When used with a 5 V

regulator, a 16 V, 10 μF Y5V capacitor can exhibit an effective value

as low as 1 μF to 3 μF for the DC bias voltage applied over the

operating temperature range.

Figure 127. Ceramic Capacitor DC Bias Characteristics

Figure 128. Ceramic Capacitor Temperature Characteristics

X5R and X7R dielectrics result in more stable characteristics and

are thus more suitable for the LT3097. The X7R dielectric has better

stability across temperature, while the 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 the operating temperature range and the maximum

capacitance change over temperature. While capacitance changes

due to DC bias for X5R and X7R are better than Y5V and Z5U

dielectrics, it can still be significant enough to drop capacitance

below sufficient levels. As shown in Figure 129, capacitor DC bias

characteristics tend to improve as component case size increases.

However, verification of expected capacitance at the operating

voltage is highly recommended. Due to its good voltage coefficient

in small case sizes, Analog Devices, Inc., recommends using the

Murata GCM series ceramic capacitors.

Figure 129. Capacitor Voltage Coefficient for Different Case Sizes

HIGH VIBRATION ENVIRONMENTS

Voltage and temperature coefficients are not the only sources of

problems. Some ceramic capacitors have a piezoelectric response.

A piezoelectric device generates a voltage across its terminals due

to mechanical stress, similar to how a piezoelectric microphone

works. For a ceramic capacitor, this stress can be induced by me-

chanical vibrations within the system or due to thermal transients.

LT3097 applications in high-vibration environments have three dis-

tinct, piezoelectric noise generators: ceramic output, input, and

SETP/SETN pin capacitors. However, due to the low output impe-

dance over a wide frequency range of the LT3097, negligible output

noise is generated using a ceramic-output capacitor. Similarly, due

to the ultrahigh PSRR of the LT3097, negligible output noise is

generated using a ceramic-input capacitor. Nonetheless, given the

high SETP/SETN pin impedance, any piezoelectric response from

a ceramic SETP/SETN pin capacitor generates significant output

noise, and peak-to-peak excursions of hundreds of mV. However,

due to the high ESR and ESL tolerance of the SETP/SETN pin

capacitor, any non-piezoelectrically responsive (tantalum, electrolyt-

ic, or film) capacitor can be used at the SETP/SETN pin, although

electrolytic capacitors tend to have high 1/f noise. In any case, the

use of a surface-mount capacitor is highly recommended.

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