LT1963 datasheet(12/28 Pages) LINER

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LT3089

3089f

For more information www.linear.com/LT3089

APPLICATIONS INFORMATION

Again, the lower current levels used in the LT3089 neces-

sitate attention to board leakages as error sources (see the

Programming Linear Regulator Output Voltage section).

In a current source configuration, programmable cur-

rent limit and current monitoring functions are often

unused. When not used, tie IMON to OUT and leave ILIM

open. The TEMP pin is still available for use, if unused tie

TEMP to OUT.

Selecting RSET and ROUT in Current Source Applications

In Figure 3, both resistors RSET and ROUT program the

value of the output current. The question now arises: the

ratio of these resistors is known, but what value should

each resistor be?

ThefirstresistortoselectisRSET.Thevalueselectedshould

generate enough voltage to minimize the error caused by

the offset between the SET and OUT pins. A reasonable

startinglevelis~200mVofvoltageacrossRSET(RSETequal

to 4.02k). Resultant errors due to offset voltage are a few

percent. The lower the voltage across RSET becomes, the

higher the error term due to the offset.

From this point, selecting ROUT is easy, as it is a straight-

forwardcalculationfromRSET.Takenote,however,resistor

errors must be accounted for as well. While larger voltage

drops across RSET minimize the error due to offset, they

also increase the required operating headroom.

Obtainingthebesttemperaturecoefficientdoesnotrequire

the use of expensive resistors with low ppm temperature

coefficients.Instead,sincetheoutputcurrentoftheLT3089

is determined by the ratio of RSET to ROUT, those resis-

tors should have matching temperature characteristics.

Less expensive resistors made from the same material

provide matching temperature coefficients. See resistor

manufacturers’ data sheets for more details.

Higher output currents necessitate the use of higher watt-

age resistors for ROUT. There may be a difference between

the resistors used for ROUT and RSET. A better method to

maintain consistency in resistors is to use multiple resis-

tors in parallel to create ROUT, allowing the same wattage

and type of resistor as RSET.

Programming Current Limit Externally

A resistor placed between ILIM and OUT on the LT3089

externallysetscurrentlimittoalevellowerthantheinternal

current limit. Connect this resistor directly at the OUT pins

for best accuracy. The value of this resistor calculates as:

RILIM = ILIMIT/175mA/kΩ + 300Ω

Theresistorfora0.5Acurrentlimitis:RILIM=0.5A/175mA/

kΩ + 300Ω = 3.16k. Tolerance over temperature is ±15%,

so current limit is normally set 20% above maximum load

current. The 300Ω offset resistance built in to the pro-

grammable current limit allows for lowering the maximum

outputcurrenttoonlybiascurrents(seecurveofMinimum

Load Current in Typical Performance Characteristics) us-

ing external switches.

The LT3089’s internal current limit overrides the pro-

grammed current limit if the input-to-output voltage dif-

ferential in the power transistor is excessive. The internal

currentlimitis≈1Awithafoldbackcharacteristicdependent

on input-to-output differential voltage, not output voltage

per se (see Typical Performance Characteristics).

Stability and Input Capacitance

The LT3089 does not require an input capacitor to main-

tain stability. Input capacitors are recommended in linear

regulator configurations to provide a low impedance input

source to the LT3089. If using an input capacitor, low

ESR, ceramic input bypass capacitors are acceptable for

applications without long input leads. However, applica-

tions connecting a power supply to an LT3089 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

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 modifica-

tions/solutions are then required. This behavior is not

indicative of LT3089 instability, but is a common ceramic

input bypass capacitor application issue.

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