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Data Sheet

LT3074

analog.com

Rev 0.

42 of 62

Calculating Junction Temperature

Example: Given an output voltage of 1.2V, input voltage of 1.5V, and BIAS voltage of 5V, output ranges from 10mA to

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

The LT3074’s power dissipation is:

IOUT(MAX) × (VIN – VOUT) + IGND × VBIAS

where: IOUT(MAX) = 3A; VBIAS = 5V; IGND (at VBIAS = 5V and IOUT = 3A) = 5mA

thus, PDISS = 3A × (1.5V – 1.2V) + 5mA × 5V = 0.925W

When a 3mm × 4mm 4-layer LQFN package is used, the thermal resistance is in the range of 30°C/W to 36°C/W. Note

that the θJA numbers vary beyond the 30°C/W to 36°C/W depending on board composition and layout. Considering a

θJA value of 33°C/W, the junction temperature rise above the ambient approximately equals:

0.925W × 33°C/W = 30.525°C

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

temperature rise above ambient:

TJMAX = 50°C + 30.525°C = 80.525°C

Protection Features

The LT3074 incorporates internal current limit and thermal shutdown protection features. The internal current of

the LT3074 typically clamps the output current to 4.5A. The die junction temperature can exceed the 125°C maximum

operating temperature if the ambient temperature is high. If this occurs, the LT3074 relies on an internal thermal

safety feature (thermal shutdown). Typically, at 168°C, the LT3074 thermal shutdown engages, and the output is shut

down until the temperature falls below its thermal hysteresis limit.

Serial Bus Addressing

The LT3074 responds to PMBus global addresses of 7’h00 (general call address) and 7’h0C (SMBus alert response

address). In addition, the ASEL pin allows the user to program 16 unique addresses for the LT3074. The three most

significant bits (MSBs) of the 7-bit address are hardwired to 3’b110. A ±1% resistor divider from BIASAF, ASEL, and

AGND sets the lower 4 bits of the 7-bit address. Table 10 gives the recommended values of the resistors to be used

for programming the corresponding address.

The LT3074 also supports the MFR_RAIL_ADDRESS register that allows the user to program a common address for

all devices connected on a rail. The lower seven bits of the MFR_RAIL_ADDRESS register set the address to be used

for communication and the MSB acts as the enable bit for this address. The rail address is enabled when the MSB of

the MFR_RAIL_ADDRESS is set to 0 and disabled when the bit is set to 1. By default, the MSB of the

MFR_RAIL_ADDRESS register is set to 1 and the lower 7 bits of this register are set to 7’h00. The MFR_RAIL_ADDRESS

should only be used to write data to device registers connected on a shared rail. If the MFR_RAIL_ADDRESS is used

to read data from the registers, data corruption is possible if all devices do not respond with the same data.

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