LT3081
18
3081fc
The LT3081 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
output voltage setting resistor divider gains up the reference
noise, especially if V
OUT
is much greater than V
REF
. The
LT3081’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
(27µV
RMS
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 LT3081 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 LT3081’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)
The TEMP pin of the LT3081 outputs a current proportional
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.
I
MON
Pin Operation (Current Monitor)
The LT3081’s I
MON
pin outputs a current proportional to
the load current supplied at a ratio of 1:5000. The I
MON
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
IN
SET
+
–
LT3081
50µA
3081 F10
OUT
R
SET
R
P
PARASITIC
RESISTANCE
R
P
R
P
LOAD