lt3085ems8e Linear Technology Corporation, lt3085ems8e Datasheet - Page 12

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

LT3085
APPLICATIONS INFORMATION
Voltage and temperature coeffi cients 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,
ceramic capacitor the stress can be induced by vibrations
in the system or thermal transients.
Paralleling Devices
LT3085’s may be paralleled with other LT308X devices to
obtain higher output current. The SET pins are tied together
and the IN pins are tied together. This is the same whether
it’s in three terminal mode or has separate input supplies.
The outputs are connected in common using a small piece
of PC trace as a ballast resistor to equalize the currents.
PC trace resistance in milliohms/inch is shown in Table
1. Only a tiny area is needed for ballasting.
Table 1. PC Board Trace Resistance
Trace resistance is measured in mΩ/in
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.
12
WEIGHT (oz)
1
2
10 mil WIDTH
54.3
27.1
20 mil WIDTH
27.1
13.6
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.
4.8V TO 28V
V
IN
1μF
V
V
CONTROL
CONTROL
Figure 4. Parallel Devices
V
V
IN
IN
SET
SET
165k
LT3080
LT3085
+
–
+
–
OUT
OUT
10mΩ
20mΩ
3085 F04
10μF
V
3.3V
1.5A
OUT
3085f