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LT3089

3089f

For more information www.linear.com/LT3089

The LT3089 uses a unity-gain follower from the SET pin

to the OUT pin. Therefore, multiple possibilities exist

(besides a SET pin resistor) to set output voltage. For

example, using a high accuracy voltage reference from

SET to GND removes the errors in output voltage due to

reference current tolerance and resistor tolerance. Active

driving of the SET pin is acceptable.

The typical noise scenario for a linear regulator is that the

outputvoltagesettingresistordividergainsupthereference

noise, especially if VOUT is much greater than VREF. The

LT3089’s noise advantage is that the unity-gain follower

presents no noise gain whatsoever from the SET pin to the

output. Thus, noise figures do not increase accordingly.

Error amplifier noise is typical 85nV/√Hz(27µVRMS over

a 10Hz to 100kHz bandwidth). The error amplifier’s noise

is RMS summed with the other noise terms to give a final

noise figure for the regulator.

Paralleling of regulators adds the benefit that output noise

is reduced. For n regulators in parallel, the output noise

drops by a factor of √n.

Curves in the Typical Performance Characteristics sec-

tion show noise spectral density and peak-to-peak noise

characteristics for both the reference current and error

amplifier over a 10Hz to 100kHz bandwidth.

Load Voltage Regulation

The LT3089 is a floating device. No ground pin exists on

the packages. Thus, the IC delivers all quiescent current

and drive current to the load. Therefore, it is not possible

to provide true remote load sensing. The connection re-

sistance between the regulator and the load determines

load regulation performance. The data sheet’s load

regulation specification is Kelvin sensed at the package’s

pins. Negative-side sensing is a true Kelvin connection by

returning the bottom of the voltage setting resistor to the

negative side of the load (see Figure 10).

Connected as shown, system load regulation is the sum

of the LT3089’s load regulation and the parasitic line

resistance multiplied by the output current. To minimize

load regulation, keep the positive connection between the

regulator and load as short as possible. If possible, use

large diameter wire or wide PC board traces.

TEMP Pin Operation (Die Temperature Monitor)

TheTEMPpinoftheLT3089outputsacurrentproportional

to average die temperature. At 25°C, the current from the

TEMP pin is 25µA, with a slope of 1µA/°C. The current out

of the TEMP pin is valid for junction temperatures above

0°C (absent initial offset considerations). Below 0°C, the

TEMP pin will not sink current to indicate die temperature.

The TEMP pin output current is valid for voltages up to

40V below and 0.4V above the OUT pin allowing operation

even during short-circuit conditions.

Connecting a resistor from TEMP to ground converts the

TEMP pin current into a voltage to allow for monitoring

by an ADC. With a 1k resistor, 0mV to 150mV indicates

0°C to 150°C.

It should be noted that the TEMP pin current represents an

average temperature and should not be used to guarantee

that maximum junction temperature is not exceeded.

Instantaneous power along with thermal gradients and

time constants may cause portions of the die to exceed

maximum ratings and thermal shutdown thresholds. Be

sure to calculate die temperature rise for steady state

(>1 minute) as well as impulse conditions.

IMON Pin Operation (Current Monitor)

The LT3089’s IMON pin outputs a current proportional to

the load current supplied at a ratio of 1:5000. The IMON

pin current is valid for voltages up to 40V below and

0.4V above the OUT pin, allowing operation even during

short-circuit conditions.

APPLICATIONS INFORMATION

Figure 10. Connections for Best Load Regulation

SET

–

LT3089

50µA

3089 F10

OUT

RSET

PARASITIC

RESISTANCE

LOAD

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