LT3045 datasheet(35/62 Pages) AD

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Data Sheet

LT3074

analog.com

Rev 0.

35 of 62

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.

LT3074 applications in high-vibration environments have three distinct, piezoelectric noise generators; ceramic

output, input, and SETCAP pin capacitors. However, due to the low output impedance over a wide frequency range

for the LT3074, negligible output noise is generated using a ceramic output capacitor. Similarly, due to the high PSRR

of the LT3074, negligible output noise is generated using a ceramic input capacitor.

Nonetheless, given the high SETCAP pin impedance, any piezoelectric response from the SETCAP 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 SETCAP pin capacitor, any non-piezoelectrically responsive (tantalum, electrolytic, or film)

capacitor can be used at the SETCAP pin, although electrolytic capacitors tend to have high 1/f noise. In any case,

use of a surface mount capacitor is highly recommended. Due to its reduced piezoelectric response, Analog Devices

recommends using the Murata GCJ series ceramic capacitors for CSETCAP.

Stability and Input Capacitance

The LT3074 is stable with a minimum 10µF IN pin capacitor. Analog Devices recommends using low ESR ceramic

capacitors to minimize instantaneous voltage drops under large load transient conditions. Large VIN drops during

large load transients may cause the regulator to enter dropout with the corresponding degradation in load transient

response. Therefore, increased input and output capacitance values may be necessary depending on an

application’s requirements. Sufficient input capacitance is critical as the circuit is intentionally operated close to

dropout to minimize power. Ideally, the output impedance of the supply that powers the IN pins should be less than

20mΩ to support a 3A load with large transients.

In cases where long wires connect the power supply to the input and ground terminals of the LT3074, the use of low

value input capacitors combined with 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 LT3074. 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 465nH of self-inductance.

Several methods exist to reduce the self-inductance of a wire. One method divides the current flowing towards the

LT3074 between two parallel conductors. In this case, placing the wires further apart reduces the inductance; up to

50% reduction when placed only a few inches apart. Splitting the wires connects 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 to each

other. Two 30-AWG wires separated by 0.02” reduce the overall inductance to about one-fifth of a single wire.

If the LT3074 is powered by a battery mounted near the ground and power planes on the same circuit board, a 10µF

capacitor suffices. If a distant supply powers the LT3074, use a low ESR, large value input capacitor on the order of

220µF. As power supply output impedance varies, the minimum input capacitance needed for application stability

also varies.

LT3045 Datasheet(PDF) 35 Page - Analog Devices

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