lt3085ems8e Linear Technology Corporation, lt3085ems8e Datasheet - Page 13

• Manufacturer Part Number: lt3085ems8e
• Description: Adjustable 500ma Single Resistor Low Dropout Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The first test was done with approximately 1.6V
input- to-output and 0.5A per device. This gave a 800mW
dissipation in each device and a 1A output current. The
temperature rise above ambient is approximately 28°C
and both devices were within plus or minus 1°C. Both the
thermal and electrical sharing of these devices is excel-
lent. The thermograph in Figure 5 shows the temperature
distribution between these devices and the PC board
reaches ambient temperature within about a half an inch
from the devices.
The power is then increased with 3.4V across each device.
This gives 1.7 watts dissipation in each device and a device
temperature of about 90°C, about 65°C above ambient
as shown in Figure 6. Again, the temperature matching
between the devices is within 2°C, showing excellent
tracking between the devices. The board temperature has
reached approximately 40°C within about 0.75 inches of
each device.
While 90°C is an acceptable operating temperature for these
devices, this is in 25°C ambient. For higher ambients, the
temperature must be controlled to prevent device tempera-
ture from exceeding 125°C. A 3-meter-per-second airfl ow
across the devices will decrease the device temperature
about 20°C providing a margin for higher operating ambi-
ent temperatures.
Figure 5. Temperature Rise at 800mW Dissipation
Both at low power and relatively high power levels de-
vices can be paralleled for higher output current. Current
sharing and thermal sharing is excellent, showing that
acceptable operation can be had while keeping the peak
temperatures below excessive operating temperatures on
a board. This technique allows higher operating current
linear regulation to be used in systems where it could
never be used before.
Quieting the Noise
The LT3085 offers numerous advantages when it comes
to dealing with noise. There are several sources of noise
in a linear regulator. The most critical noise source for any
LDO is the reference; from there, the noise contribution
from the error amplifi er must be considered, and the gain
created by using a resistor divider cannot be forgotten.
Traditional low noise regulators bring the voltage refer-
ence out to an external pin (usually through a large value
resistor) to allow for bypassing and noise reduction of
reference noise. The LT3085 does not use a traditional
voltage reference like other linear regulators, but instead
uses a reference current. That current operates with typi-
cal noise current levels of 2.3pA/√Hz (0.7nA
10Hz to 100kHz bandwidth). The voltage noise of this is
equal to the noise current multiplied by the resistor value.
Figure 6. Temperature Rise at 1.7W Dissipation
LT3085
RMS
over the
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3085f