LT1963EST datasheet(12/16 Pages) LINEAR

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LT1963 Series

1963fa

APPLICATIO S I FOR ATIO

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 LT1963 series regulators have internal thermal lim-

iting designed to protect the device during overload

conditions. For continuous normal conditions, the maxi-

mum 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 ambi-

ent. 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. Copper board stiffeners 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

*Device is mounted on topside

Table 2. SO-8 Package, 8-Lead SO

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

*Device is mounted on topside.

Table 3. SOT-223 Package, 3-Lead SOT-223

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

1000mm

°C/W

1000mm

0mm

1000mm

°C/W

*Device is mounted on topside.

T Package, 5-Lead TO-220

Thermal Resistance (Junction-to-Case) = 4

°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

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