12

LT1764 Series

1764fa

Thermal Considerations

The power handling capability of the device is limited by

the maximum rated junction temperature (125

°C). The

power dissipated by the device is made up of two compo-

nents:

1. Output current multiplied by the input/output voltage

2. GND pin current multiplied by the input voltage:

The GND pin current can be found using the GND Pin

Current curves in the Typical Performance Characteris-

tics. Power dissipation will be equal to the sum of the two

components listed above.

The LT1764 series regulators have internal thermal limit-

ing designed to protect the device during overload condi-

tions. For continuous normal conditions, the maximum

junction temperature rating of 125

°C must not be

exceeded. It is important to give careful consideration to

all sources of thermal resistance from junction to ambient.

Additional heat sources mounted nearby must also be

considered.

For surface mount devices, heat sinking is accomplished

by using the heat spreading capabilities of the PC board

and its copper traces. Surface mount heatsinks and plated

through-holes can also be used to spread the heat gener-

ated by power devices.

The following table lists thermal resistance for several

different board sizes and copper areas. All measurements

were taken in still air on 1/16" FR-4 board with one ounce

copper.

Table 1. Q Package, 5-Lead DD

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA

(JUNCTION-TO-AMBIENT)

2500mm

2

2500mm

2

2500mm

2

23

°C/W

1000mm

2

2500mm

2

2500mm

2

25

°C/W

125mm

2

2500mm

2

2500mm

2

33

°C/W

*Device is mounted on topside.

T Package, 5-Lead TO-220

Thermal Resistance (Junction-to-Case) = 2.5

°C/W

Calculating Junction Temperature

Example: Given an output voltage of 3.3V, an input voltage

range of 4V to 6V, an output current range of 0mA to

500mA and a maximum ambient temperature of 50

°C,

what will the maximum junction temperature be?

The power dissipated by the device will be equal to:

where,

So,

P = 500mA(6V – 3.3V) + 10mA(6V) = 1.41W

Using a DD package, the thermal resistance will be in the

range of 23

°C/W to 33°C/W depending on the copper

area. So the junction temperature rise above ambient will

be approximately equal to:

1.41W(28

°C/W) = 39.5°C

The maximum junction temperature will then be equal to

the maximum junction temperature rise above ambient

plus the maximum ambient temperature or:

Protection Features

The LT1764 regulators incorporate several protection

features which make them ideal for use in battery-powered

circuits. In addition to the normal protection features

associated with monolithic regulators, such as current

limiting and thermal limiting, the devices are 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.

The input of the device will withstand reverse voltages of

20V. Current flow into the device will be limited to less than

1mA and no negative voltage will appear at the output. The