LT3085 datasheet(13/28 Pages) LINER

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LT3083

3083f

APPLICATIONS INFORMATION

When longer supply lines, ﬁlters, current sense resistors,

or other impedances exist between the supply and the

input to the LT3083, input bypassing should be reviewed

if stability concerns are seen. Just as output capacitance

supplies the instantaneous changes in load current for

output transients until the regulator is able to respond,

input capacitance supplies local power to the regulator until

the main supply responds. When impedance separates the

LT3083 from its main supply, the local input can droop

so that the output follows. The entire circuit may break

into oscillations, usually characterized by larger amplitude

oscillations on the input and coupling to the output.

Low ESR, ceramic input bypass capacitors are acceptable

for applications without long input leads. However, applica-

tions connecting a power supply to an LT3083 circuit’s IN

and GND pins with long input wires combined with low

ESR, ceramic input capacitors are prone to voltage spikes,

reliability concerns and application-speciﬁc board oscil-

lations. The input wire inductance found in many battery

powered applications, combined with the low ESR ceramic

input capacitor, forms a high-Q LC resonant tank circuit. In

some instances this resonant frequency beats against the

output current dependent LDO bandwidth and interferes

with proper operation. Simple circuit modiﬁcations/solu-

tions are then required. This behavior is not indicative of

LT3083 instability, but is a common ceramic input bypass

capacitor application issue.

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

directly proportional to its length. Wire diameter is not a

major factor on its self-inductance. For example, the self-

inductance of a 2-AWG isolated wire (diameter = 0.26") is

about half the self-inductance of a 30-AWG wire (diameter

= 0.01"). One foot of 30-AWG wire has about 465nH of

self-inductance.

One of two ways reduces a wire’s self-inductance. One

method divides the current ﬂowing towards the LT3083

between two parallel conductors. In this case, the farther

apart the wires are from each other, the more the self-in-

ductance is reduced; up to a 50% reduction when placed

a few inches apart. Splitting the wires basically connects

two equal inductors in parallel, but placing them in close

proximity gives the wires mutual inductance adding to

the self-inductance. The second and most effective way

to reduce overall inductance is to place both forward and

return current conductors (the input and GND wires) in

very close proximity. Two 30-AWG wires separated by only

0.02", used as forward- and return- current conductors,

reduce the overall self-inductance to approximately one-

ﬁfth that of a single isolated wire.

If wiring modiﬁcations are not permissible for the applica-

tions, including series resistance between the power supply

and the input of the LT3083 also stabilizes the application.

As little as 0.1Ω to 0.5Ω, often less, is effective in damping

the LC resonance. If the added impedance between the

power supply and the input is unacceptable, adding ESR to

the input capacitor also provides the necessary damping of

the LC resonance. However, the required ESR is generally

higher than the series impedance required.

Stability and Output Capacitance

The LT3083 requires an output capacitor for stability. It

is designed to be stable with most low ESR capacitors

(typically ceramic, tantalum or low ESR electrolytic). A

minimum output capacitor of 10μF with an ESR of 0.5Ω

or less is recommended to prevent oscillations. Larger

values of output capacitance decrease peak deviations

and provide improved transient response for larger load

current changes. Bypass capacitors, used to decouple

individual components powered by the LT3083, increase

the effective output capacitor value. For improvement in

transient performance, place a capacitor across the volt-

age setting resistor. Capacitors up to 1μF can be used.

This bypass capacitor reduces system noise as well, but

start-up time is proportional to the time constant of the

voltage setting resistor (RSET in Figure 1) and SET pin

bypass capacitor.

Give extra consideration to the use of ceramic capacitors.

Ceramic capacitors are manufactured with a variety of di-

electrics, each with different behavior across temperature

and applied voltage. The most common dielectrics used

are speciﬁed with EIA temperature characteristic codes of

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

good for providing high capacitances in a small package,

but they tend to have strong voltage and temperature

coefﬁcients as shown in Figures 3 and 4. When used with

a 5V regulator, a 16V 10μF Y5V capacitor can exhibit an

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