LT3081
17
3081fc
For more information
applicaTions inForMaTion
Voltage and temperature coefficients are not the only
sources of problems. Some ceramic capacitors have a
piezoelectric response. A piezoelectric device generates
voltage across its terminals due to mechanical stress. In a
ceramic capacitor, the stress can be induced by vibrations
in the system or thermal transients.
Paralleling Devices
Higher output current is obtained by paralleling multiple
LT3081s together. Tie the individual SET pins together and
tie the individual IN pins together. Connect the outputs in
common using small pieces of PC trace as ballast resistors
to promote equal current sharing. PC trace resistance in
milliohms/inch is shown in Table 2. Ballasting requires
only a tiny area on the PCB.
Table 2. PC Board Trace Resistance
WEIGHT (oz)
10mil WIDTH
20mil WIDTH
1
54.3
27.1
2
27.1
13.6
Trace resistance is measured in mΩ/in.
The worst-case room temperature offset, only ±1.5mV
between the SET pin and the OUT pin, allows the use of
very small ballast resistors.
As shown in Figure 9, each LT3081 has a small 10mΩ
ballast resistor, which at full output current gives better
than 80% equalized sharing of the current. The external
resistance of 10mΩ (5mΩ for the two devices in parallel)
only adds about 15mV of output regulation drop at an
output of 3A. Even with an output voltage as low as 1V,
this only adds 1.5% to the regulation. Of course, paralleling
more than two LT3081s yields even higher output current.
Spreading the devices on the PC board also spreads the
heat. Series input resistors can further spread the heat if
the input-to-output difference is high.
If the increase in load regulation from the ballast resis-
tors is unacceptable, the I
MON
output can be used to
compensate for these drops (see Using I
MON
Cancels
Ballast Resistor Drop in the Typical Applications section).
Regulator paralleling without the use of ballast resistors is
accomplished by comparing the I
MON
outputs of regula-
tors (see Load Current Sharing Without Ballasting in the
Typical Applications section).
Quieting the Noise
The LT3081 offers numerous noise performance advan-
tages. Every linear regulator has its sources of noise. In
general, a linear regulator’s critical noise source is the
reference. In addition, consider the error amplifier’s noise
contribution along with the resistor divider’s noise gain.
Many traditional low noise regulators bond out the voltage
reference to an external pin (usually through a large value
resistor) to allow for bypassing and noise reduction. The
LT3081 does not use a traditional voltage reference like
other linear regulators. Instead, it uses a 50µA reference
current. The 50µA current source generates noise current
levels of 18pA/√
Hz
(5.7nA
RMS
over a 10Hz to 100kHz
bandwidth). The equivalent voltage noise equals the RMS
noise current multiplied by the resistor value.
The SET pin resistor generates spot noise equal to √
4kTR
(k = Boltzmann’s constant
, 1.38 • 10
–23
J/°K, and T is abso-
lute temperature) which is RMS summed with the voltage
noise. If the application requires lower noise performance,
bypass the voltage setting resistor with a capacitor to GND.
Note that this noise-reduction capacitor increases start-up
time as a factor of the RC time constant.
SET
+
–
LT3081
50µA
10mΩ
10mΩ
IN
V
IN
4.8V TO 40V
V
OUT
3.3V
3A
OUT
10µF
1µF
33k
3081 F09
SET
+
–
LT3081
50µA
IN
OUT
Figure 9. Parallel Devices
