LT3083MPDF datasheet(12/28 Pages) AD

12 / 28 page

LT3083

Rev. B

For more information www.analog.com

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 out-

put 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, appli-

cations 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

oscillations. The input wire inductance found in many bat-

tery 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 modifi-

cations/solutions 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-

inductance 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 conduc-

tors, reduce the overall self-inductance to approximately

one-fifth that of a single isolated wire.

If wiring modifications are not permissible for the appli-

cations, 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 effec-

tive in damping the LC resonance. If the added imped-

ance between the power supply and the input is unac-

ceptable, 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 imped-

ance 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 pro-

vide improved transient response for larger load current

changes. Bypass capacitors, used to decouple individual

components powered by the LT3083, increase the effec-

tive output capacitor value. For improvement in transient

performance, place a capacitor across the voltage 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

dielectrics, each with different behavior across tempera-

ture 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 Figure 3 and

Figure 4. When used with a 5V regulator, a 16V 10μF Y5V

LT3083MPDF Datasheet(PDF) 12 Page - Analog Devices

LT3083MPDF Datasheet(HTML) 12 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL