Data Sheet

LT3077

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Rev. 0

31 of 38

For example, a 1kΩ resistor programs the current limit to 3A, and a 2kΩ resistor programs the current limit to 1.5A.

Kelvin connect this resistor to the LT3077’s GND pin for good accuracy.

As shown in the electrical characteristic table (Table 1), the ILIM pin sources current proportional (1:3000) to output

current; if the external current limit is not used, connect ILIM to GND, which sets the internal current limit to 4.5A.

Thermal Considerations

For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the

LT3077. The IN and OUT pins on the bottom of the package should be soldered to IN and OUT planes accordingly. In

addition, the IN and OUT must be connected to large copper layers below with thermal vias; these layers spread heat

dissipated by the LT3077. Placing additional vias can reduce thermal resistance further. The die temperature is

calculated by multiplying the LT3077 power dissipation by the thermal resistance from the junction to the ambient.

The internal overtemperature protection monitors the junction temperature of the LT3077. If the junction

temperature reaches approximately 168°C, the LT3077 output shuts down until the temperature drops about 7°C.

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 LT3077’s power dissipation is:

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

where:

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

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

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

thus:

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

When a 3mm x 3mm LFCSP-RT package is used, the thermal resistance is in the range of 41°C/W to 47°C/W. Note that

the θJA numbers vary beyond the 41°C/W to 47°C/W depending on board composition and layout. Considering a θJA

value of 44°C/W, the junction temperature rise above the ambient approximately equals:

0.925W × 44°C/W = 40.7°C

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

temperature rise above ambient:

������������������������������ = 50°������ + 40.7°������ = 90.7°������

Paralleling Devices for Higher Output Current

As shown in Figure 70, multiple LT3077s 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

of each LT3077 is connected to the common load using a small piece of PC trace as a ballast resistor (≅2mΩ) or an

actual sense resistor beyond the feedback SENSE tap of each regulator. The ballast resistor ensures output current

sharing. Keep this ballast trace area free of solder to maintain a controlled resistance.

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