Data Sheet

LT3073

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Rev 0.

30 of 37

Table 7 lists thermal resistance as a function of the copper area on a fixed board size. All measurements were taken

in still air on a 4-layer FR-4 board with 1oz solid internal planes and 2oz top/bottom planes with a total board

thickness of 1.6mm. The four layers were electrically isolated with no thermal vias present. PCB layers, copper

weight, board layout, and thermal vias affect the thermal resistance result. For more information on thermal

resistance and high thermal conductivity test boards, refer to JEDEC standard JESD51, notably JESD51-7 and

JESD51-12. Achieving low thermal resistance necessitates attention to detail and careful PCB layout.

Table 7. Measured Thermal Resistance of LQFN Package

COPPER AREA

BOARD AREA

THERMAL RESISTANCE

(θJA)

TOP SIDE 1

BOTTOM SIDE

2500mm2

2500mm2

2500mm2

33°C/W

1000mm2

2500mm2

2500mm2

34°C/W

225mm2

2500mm2

2500mm2

36°C/W

100mm2

2500mm2

2500mm2

39°C/W

1 Device is mounted on top side

Calculating Junction Temperature

Example: Given an output voltage of 1.2V, input voltage of 1.5V, and BIAS voltage of 5V, output current ranges from

10mA to 3A, and a maximum ambient temperature of 50°C, what is the maximum junction temperature?

The LT3073’s power dissipation is:

������������������������(������������������) × (������������������ − ������������������������) + ������������������������ × ������������������������������

where:

������������������������(������������������) = 3������

������������������������������ = 5������

������������������������(������������ ������������������������ = 3������ ������������������ ������������������������������ = 5������) = 5������������

thus:

������������������������������ = 3������ × (1.5������ − 1.2������) + 5������������ × 5������ = 0.925������

When a 3mm x 4mm 4L LQFN package is used, the thermal resistance is in the range of 33°C/W to 39°C/W. Note that

the θJA numbers will vary beyond the 33°C/W to 39°C/W depending on board composition and layout. Considering a

θJA value of 36°C/W, the junction temperature rise above the ambient approximately equals:

0.925W × 36°C/W = 33.3°C

The maximum junction temperature equals the maximum ambient temperature plus the maximum junction

temperature rise above ambient:

������������������������������ = 50°������ + 33.3°������ = 83.3°������

Paralleling Devices for Higher Output Current

As shown in Figure 75, multiple LT3073s may be paralleled to obtain a higher output current. This paralleling concept

borrows from the scheme employed by the LT3080 product family.

To accomplish this paralleling, connect the IN and OUT pins of the multiple devices together. Also, connect the REF

pins of the multiple devices. This effectively gives an averaged value of multiple reference voltage sources. The OUT