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LT1963A Series

1963aff

For more information www.linear.com/LT1963A

applicaTions inForMaTion

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 3. Q Package, 5-Lead DD

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE BOARD AREA

(JUNCTION-TO-AMBIENT)

2500mm2

23°C/W

1000mm2

2500mm2

25°C/W

125mm2

2500mm2

33°C/W

*Device is mounted on topside

Table 4. S0-8 Package, 8-Lead SO

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE BOARD AREA

(JUNCTION-TO-AMBIENT)

2500mm2

55°C/W

1000mm2

2500mm2

55°C/W

225mm2

2500mm2

63°C/W

125mm2

2500mm2

69°C/W

*Device is mounted on topside

Table 5. SOT-223 Package, 3-Lead SOT-223

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE BOARD AREA

(JUNCTION-TO-AMBIENT)

2500mm2

42°C/W

1000mm2

2500mm2

42°C/W

225mm2

2500mm2

50°C/W

100mm2

2500mm2

56°C/W

1000mm2

49°C/W

1000mm2

0mm2

1000mm2

52°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 volt-

age 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

Protection Features

The LT1963A regulators incorporate several protection

featureswhichmakethemidealforuseinbattery-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 (typically less than 100µA) and no negative

voltage will appear at the output. The device will protect

both itself and the load. This provides protection against

batteries that can be plugged in backward.

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