LT1963 datasheet(14/26 Pages) LINER

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LT3088

3088fb

For more information www.linear.com/LT3088

APPLICATIONS INFORMATION

capacitor under a fault condition, preventing sparks or

arcs. In many instances, a series RC network is the best

solutionforstabilizingtheapplicationcircuit.Typicalresis-

tor values will range from 100Ω to 5k. Once again, Linear

Technology strongly recommends testing stability in situ

foranyLT3088applicationacrossalloperatingconditions,

especiallyonesthatpresentcompleximpedancenetworks

at the input and output of the current source.

If an application refers the bottom of the LT3088 current

source to GND, it may be necessary to bypass the top

of the current source with a capacitor to GND. In some

cases, this capacitor may already exist and no additional

capacitance is required. For example, if the LT3088 is

used as a variable current source on the output of a power

supply, the output bypass capacitance would suffice to

provide LT3088 stability. Other applications may require

the addition of a bypass capacitor. A series RC network

may also be used as necessary, and depends on the ap-

plication requirements.

In some extreme cases, capacitors or series RC networks

may be required on both the LT3088’s input and output to

stabilize the circuit. Figure 9 depicts a general application

using input and output capacitor networks rather than

an input-to-output capacitor. As the input of the current

source tends to be high impedance, placing a capacitor

on the input does not have the same effect as placing a

capacitoronthelowerimpedanceoutput.Capacitorsinthe

range of 0.1µF to 1µF usually provide sufficient bypassing

on the input, and the value of input capacitance may be

increased without limit. Pay careful attention to using low

ESR input capacitors with long input lines (see the Stabil-

ity and Input Capacitance section for more information).

Using Ceramic Capacitors

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 coef-

ficients as shown in Figures 10 and 11. When used with

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

effective value as low as 1µF to 2µF for the DC bias voltage

appliedandovertheoperatingtemperaturerange.TheX5R

and X7R dielectrics result in more stable characteristics

and are more suitable for use as the output capacitor.

The X7R type has better stability across temperature,

while the X5R is less expensive and is available in higher

values. Care still must be exercised when using X5R and

X7R capacitors. The X5R and X7R codes only specify

operating temperature range and maximum capacitance

change over temperature. Capacitance change due to DC

bias with X5R and X7R capacitors is better than Y5V and

Z5U capacitors, but can still be significant enough to drop

capacitor values below appropriate levels. Capacitor DC

bias characteristics tend to improve as component case

size increases, but expected capacitance at operating

voltage should be verified.

3088 F09

SET

OUT

–

LT3088

IOUT

RSET

ROUT

COUT OR

VIN

COUT

ROUT

CIN

RIN

50µA

Figure 9. Input and/or Output Capacitors May

Be Used for Compensation

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