LT3085EDCB-PBF datasheet(13/28 Pages) LINER

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LT3085

3085fa

APPLICATIONS INFORMATION

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 ampliﬁer 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 (0.7nARMS over the

10Hz to 100kHz bandwidth). The voltage noise of this is

equal to the noise current multiplied by the resistor value.

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 airﬂow

across the devices will decrease the device temperature

about 20°C providing a margin for higher operating ambi-

ent temperatures.

Figure 6. Temperature Rise at 1.7W Dissipation

Figure 5. Temperature Rise at 800mW Dissipation

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