LT1963 datasheet(13/26 Pages) LINER

13 / 26 page

LT3088

3088fb

For more information www.linear.com/LT3088

APPLICATIONS INFORMATION

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 LT3088’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 LT3088 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 LT3088 application.

In LT3088 applications with long wires or PCB traces, the

inductive reactance may cause instability. In some cases,

adding series resistance to the input and output lines (as

showninFigure 7)maysufficientlydampenthesepossible

high-Q lines and provide stability. The user must evaluate

therequiredresistorvaluesagainstthedesign’sheadroom

constraints. In general, operation at low output current

levels (<20mA) automatically requires higher values of

programming resistors and may provide the necessary

damping without additional series impedance.

If the line impedances in series with the LT3088 are

complex enough such that series damping resistors are

not sufficient, a frequency compensation network may be

necessary. Several options may be considered.

Figure 8 depicts the simplest frequency compensation

networks as a single capacitor across the two terminals

of the current source. Some applications may use the

capacitance to stand off DC voltage but allow the transfer

of data down a signal line.

Forsomeapplications,purecapacitancemaybeunaccept-

able or present a design constraint. One circuit example

typifying this is an “intrinsically-safe” circuit in which an

overload or fault condition potentially allows the capaci-

tor’s stored energy to create a spark or arc. For applica-

tions where a single capacitor is unacceptable, Figure 8

alternately shows a series RC network connected across

the two terminals of the current source. This network has

the added benefit of limiting the discharge current of the

SET

OUT

–

LT3088

50µA

RSET

ROUT

RSERIES

LONG LINE

REACTANCE/INDUCTANCE

3088 F07

LONG LINE

REACTANCE/INDUCTANCE

Figure 7. Adding Series Resistance Decouples

and Dampens Long Line Reactances

Figure 8. Compensation from Input to Output

of Current Source Provides Stability

3088 F08

SET

OUT

–

LT3088

CCOMP OR

RSET

ROUT

RCOMP

CCOMP

50µA

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