LT3041 datasheet(23/36 Pages) AD

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Data Sheet

LT3041

APPLICATIONS INFORMATION

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Rev. 0 | 23 of 36

Figure 75. Ceramic Capacitor Temperature Characteristics

X5R and X7R dielectrics result in more stable characteristics and

are thus more suitable for the LT3041. 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 operating temperature range and the maximum

capacitance change over temperature. While capacitance changes

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

dielectrics, it can still be significant enough to drop capacitance to

below sufficient levels. As shown in Figure 76, 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 recommends using the Murata

GCM series ceramic capacitors.

Figure 76. 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 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 mechanical

vibrations within the system or due to thermal transients.

LT3041 applications in high-vibration environments have three dis-

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

pin capacitors. However, due to the low output impedance over

a wide frequency range of the LT3041, negligible output noise is

generated using a ceramic-output capacitor. Similarly, due to the

ultrahigh PSRR of the LT3041, negligible output noise is generat-

ed using a ceramic-input capacitor. Nonetheless, given the high

SET pin impedance, any piezoelectric response from a ceramic

SET pin capacitor generates significant output noise, peak-to-peak

excursions of hundreds of mV. However, due to the high ESR and

ESL tolerance of the SET pin capacitor, any nonpiezoelectrically

responsive (tantalum, electrolytic, or film) capacitor can be used

at the SET pin, although electrolytic capacitors tend to have high

1/f noise. In any case, use of a surface-mount capacitor is highly

recommended.

STABILITY AND INPUT CAPACITANCE

The LT3041 is stable with a minimum 10 μF IN pin capacitor.

Analog Devices recommends using low ESR ceramic capacitors.

In cases where long wires connect the power supply to the input

and ground terminals of the LT3041, the use of low value input ca-

pacitors combined with a large load current can result in instability.

The resonant LC tank circuit formed by the wire inductance and the

input capacitor is the cause of this instability and not the LT3041.

The self-inductance, or isolated inductance, of a wire is directly

proportional to its length. The wire diameter, however, has less

influence on its self-inductance. For example, the self-inductance of

a 2-AWG isolated wire with a diameter of 0.26" is about half the

inductance of a 30-AWG wire with a diameter of 0.01". One foot of

30-AWG wire has 465 nH of self-inductance.

Several methods exist to reduce the self-inductance of a wire. One

method divides the current flowing toward the LT3041 between

the two parallel conductors. In this case, placing the wires further

apart reduces the inductance; up to a 50% reduction when placed

only a few inches apart. Splitting the wires connect two equal

inductors in parallel. However, when placed close to each other,

their mutual inductance adds to the overall self inductance of the

wires; therefore, a 50% reduction is not possible in such cases.

The second and more effective technique to reduce the overall

inductance is to place the forward and return current conductors

(the input and ground wires) close. Two 30-AWG wires separated

by 0.02" reduce the overall inductance to about one-fifth of a single

wire.

If a battery mounted close powers the LT3041, a 10 μF input

capacitor suffices for stability. However, if a distantly located supply

powers the LT3041, use a larger value input capacitor. Use a rough

guideline of 1 μF (in addition to the 10 μF minimum) per 6" of

wire length. The minimum input capacitance required to stabilize

the application also varies with the output capacitance as well as

the load current. Place additional capacitance on the output of the

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