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LT1579

APPLICATIONS INFORMATION

resistance from the junction to a ground lead is 40

°C/Wfor

16-lead SSOP, 32

°C/W for 16-lead SO and 35°C/W for

8-lead S0.

Table 2. 8-Lead SO Package (S8)

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA (JUNCTION-TO-AMBIENT)

2500 sq mm 2500 sq mm

2500 sq mm

°C/W

1000 sq mm 2500 sq mm

2500 sq mm

°C/W

225 sq mm

2500 sq mm

°C/W

100 sq mm

2500 sq mm

°C/W

*Device is mounted on topside.

Table 3. 16-Lead SO Package (S)

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA (JUNCTION-TO-AMBIENT)

2500 sq mm 2500 sq mm

2500 sq mm

°C/W

1000 sq mm 2500 sq mm

2500 sq mm

°C/W

225 sq mm

2500 sq mm

°C/W

100 sq mm

2500 sq mm

°C/W

*Device is mounted on topside.

Table 4. 16-Lead SSOP Package (GN)

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA (JUNCTION-TO-AMBIENT)

2500 sq mm 2500 sq mm

2500 sq mm

°C/W

1000 sq mm 2500 sq mm

2500 sq mm

°C/W

225 sq mm

2500 sq mm

°C/W

100 sq mm

2500 sq mm

°C/W

*Device is mounted on topside.

Calculating Junction Temperature

Example: Given an output voltage of 5V, an input voltage

range of 5V to 7V for VIN1 and 8V to 10V for VIN2, with an

output current range of 10mA to 150mA and a maximum

ambient temperature of 50

°C, what will the maximum

junction temperature be?

When run from the primary input, current drawn from the

secondary input is negligible and worst-case power dissi-

pation will be:

(IOUT(MAX))(VIN1(MAX) – VOUT) + (IGND)(VIN1(MAX))

Where:

IOUT(MAX) = 150mA

VIN1(MAX) = 7V

IGND at (IOUT = 150mA, VIN1 = 7V) = 2mA

Therefore,

P = (150mA)(7V – 5V) + (2mA)(7V) = 0.31W

When switched to the secondary input, current from the

primary input is negligible and worst-case power dissipa-

tion will be:

(IOUT(MAX))(VIN2(MAX) – VOUT) + (IGND)(VIN2(MAX))

Where:

IOUT(MAX) = 150mA

VIN2(MAX) = 10V

IGND at (IOUT = 150mA, VIN2 = 10V) = 2mA

Therefore,

P = (150mA)(10V – 5V) + (2mA)(10V) = 0.77W

Using a 16-lead SO package, the thermal resistance will be

in the range of 55

°C/W to 68°C/W dependent upon the

copper area. So the junction temperature rise above

ambient will be approximately equal to:

(0.77W)(65

°C/W) = 50.1°C

The maximum junction temperature will then be equal to

the maximum temperature rise above ambient plus the

maximum ambient temperature or:

TJMAX = 50.1°C + 50°C = 100.1°C

Protection Features

The LT1579 incorporates several protection features that

make it ideal for use in battery-powered circuits. In addi-

tion to the normal protection features associated with

monolithic regulators, such as current limiting and ther-

mal limiting, the device is protected against reverse input

voltages, reverse output voltages and reverse voltages

from output to input.

Current limit protection and thermal overload protection

are intended to protect the device against current overload

conditions at the output of the device. For normal opera-

tion, the junction temperature should not exceed 125

°C.

Current limit protection is designed to protect the device

if the output is shorted to ground. With the output shorted

to ground, current will be drawn from the primary input

until it is discharged. The current drawn from VIN2 will not

increase until the primary input is discharged. This pre-

vents a short-circuit on the output from discharging both

inputs simultaneously.

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