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LT1764 Series

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APPLICATIONS INFORMATION

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

components:

1. Output current multiplied by the input/output voltage

differential: (IOUT)(VIN – VOUT), and

2. GND pin current multiplied by the input voltage:

(IGND)(VIN).

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 tables list 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

°C/W

1000mm

2500mm

°C/W

125mm

2500mm

°C/W

Table 2. FE Package, 16-Lead TSSOP

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA

(JUNCTION-TO-AMBIENT)

2500mm

°C/W

1000mm

2500mm

°C/W

225mm

2500mm

°C/W

100mm

2500mm

°C/W

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:

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

where,

IOUT(MAX) = 500mA

VIN(MAX) = 6V

IGND at (IOUT = 500mA, VIN = 6V) = 10mA

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:

TJMAX = 50°C + 39.5°C = 89.5°C

* Device is mounted on topside

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