LT3085_10 Datasheet, PDF (12/28 Pages) Linear Technology – Adjustable 500mA Single Resistor Low Dropout Regulator

LT3085
APPLICATIONS INFORMATION
The worst-case offset between the SET pin and the output
of only ±1.5mV allows very small ballast resistors to be
used. As shown in Figure 4, the two devices have a small
10mΩ and 20mΩ ballast resistors, which at full output
current gives better than 80% equalized sharing of the
current. The external resistance of 20mΩ (6.6mΩ for the
two devices in parallel) only adds about 10mV of output
regulation drop at an output of 1.5A. Even with an output
voltage as low as 1V, this only adds 1% to the regulation.
Of course, more than two LT308X’s can be paralleled for
even higher output current. They are spread out on the
PC board, spreading the heat. Input resistors can further
spread the heat if the input-to-output difference is high.
Thermal Performance
In this example, two LT3085 2mm × 3mm DFN devices
are mounted on a 1oz copper 4-layer PC board. They are
placed approximately 1.5 inches apart and the board is
mounted vertically for convection cooling. Two tests were
set up to measure the cooling performance and current
sharing of these devices.
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
V_IN
4.8V TO 28V
1μF
V_IN
V_CONTROL
LT3080
+
–
SET
OUT ^10mΩ
V_IN
V_CONTROL
LT3085
+
–
SET
165k
OUT ^20mΩ
V_OUT
3.3V
1.5A
10μF
3085 F04
Figure 4. Parallel Devices
Figure 5. Temperature Rise at 800mW Dissipation
12
Figure 6. Temperature Rise at 1.7W Dissipation
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