LT3085EDCB-PBF datasheet(12/28 Pages) LINER

12 / 28 page

LT3085

3085fa

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,

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

WEIGHT (oz)

10 mil WIDTH

20 mil WIDTH

54.3

27.1

13.6

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.

–

LT3080

VIN

VCONTROL

OUT

SET

10mΩ

–

LT3085

VIN

4.8V TO 28V

VOUT

3.3V

1.5A

VCONTROL

OUT

10μF

1μF

SET

165k

3085 F04

20mΩ

Figure 4. Parallel Devices

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.

LT3085EDCB-PBF Datasheet(PDF) 12 Page - Linear Technology

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