LT1963 datasheet(13/28 Pages) LINER

13 / 28 page

LT3089

3089f

For more information www.linear.com/LT3089

APPLICATIONS INFORMATION

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 LT3089

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 applica-

tions,includingseriesresistancebetweenthepowersupply

and the input of the LT3089 also stabilizes the application.

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

ing 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 Frequency Compensation for Linear

Regulator Configurations

The LT3089 does not require an output capacitor for

stability. LTC recommends an output capacitor of 10μF

with an ESR of 0.5Ω or less to provide good transient

performance in linear regulator configurations. Larger

valuesofoutputcapacitancedecreasepeakdeviationsand

provideimprovedtransientresponseforlargerloadcurrent

changes. Bypass capacitors, used to decouple individual

components powered by the LT3089, 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.

Stability and Frequency Compensation for Current

Source Configurations

The LT3089 does not require input or output capacitors

for stability in many current-source applications. Clean,

tight PCB layouts provide a low reactance, well controlled

operating environment for the LT3089 without requiring

capacitors to frequency compensate the circuit. Figure 3

highlights the simplicity of using the LT3089 as a current

source.

Some current source applications use a capacitor con-

nected in parallel with the SET pin resistor to lower the

current source’s noise. This capacitor also provides a

soft-start function for the current source. See Quieting the

Noise section for further details. When operating without

output capacitors, the high impedance nature of the SET

pin as the input of the error amplifier allows signal from

the output to couple in, showing as high frequency ring-

ing during transients. Bypassing the SET resistor with a

capacitorintherangeof20pFto30pFdampenstheringing.

Depending on the pole introduced by a capacitor or other

complex impedances presented to the LT3089, external

compensation may be required for stability. Techniques

are discussed to achieve this in the following paragraphs.

Linear Technology strongly recommends testing stability

insituwithfinalcomponentsbeforebeginningproduction.

Although the LT3089’s design strives to be stable without

capacitors over a wide variety of operating conditions, it is

not possible to test for all possible combinations of input

and output impedances that the LT3089 will encounter.

These impedances may include resistive, capacitive, and

inductive components and may be complex distributed

networks. In addition, the current source’s value will dif-

fer between applications and its connection may be GND

referenced,powersupplyreferenced,orfloatinginasignal

line path. Linear Technology strongly recommends that

stability be tested in situ for any LT3089 application.

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