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LT3008 Series

3008fc

APPLICATIONS INFORMATION

So,

P = 20mA(12.6V – 3.3V) + 0.3mA(12.6V) = 189.8mW

The thermal resistance ranges from 65°C/W to 85°C/W

depending on the copper area. So, the junction temperature

rise above ambient approximately equals:

0.1898W(75°C/W) = 14.2°C

The maximum junction temperature equals the maximum

junction temperature rise above ambient plus the maximum

ambient temperature or:

TJ(MAX) = 85°C + 14.2°C = 99.2°C

Protection Features

The LT3008 incorporates several protection features that

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

dition to the normal protection features associated with

monolithic regulators, such as current limiting and thermal

limiting, the device also protects against reverse-input

voltages, reverse-output voltages and reverse output-to-

input voltages.

Current limit protection and thermal overload protec-

tion protect the device against current overload condi-

tions at the output of the device. For normal operation,

do not exceed a junction temperature of 125°C. The

typical thermal shutdown circuitry temperature threshold

is 160°C.

The IN pin withstands reverse voltages of 50V. The device

limits current ﬂow to less than 30μA (typically less than

1μA) and no negative voltage appears at OUT. The device

protects both itself and the load against batteries that are

plugged in backwards.

For example, a 4-layer, 1 ounce copper PCB board with

3 thermal vias from the DFN exposed backside or the

3 fused TSOT-23 GND pins to inner layer GND planes

achieves 45°C/W thermal resistance. Demo circuit DC

1388A’s board layout achieves this 45°C/W performance.

This is approximately a 30% improvement over the lowest

numbers shown in Tables 2 and 3.

Calculating Junction Temperature

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

range of 12V ±5%, an output current range of 0mA to 20mA

and a maximum ambient temperature of 85°C, what will

the maximum junction temperature be for an application

using the DC package?

The power dissipated by the device is equal to:

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

where,

IOUT(MAX) = 20mA

VIN(MAX) = 12.6V

IGND at (IOUT = 20mA, VIN = 12.6V) = 0.3mA

Table 3: Measured Thermal Resistance for TSOT-23 Package

COPPER AREA

BOARD

AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE*

BACKSIDE

2500mm2

65°C/W

1000mm2

2500mm2

67°C/W

225mm2

2500mm2

70°C/W

100mm2

2500mm2

75°C/W

50mm2

2500mm2

85°C/W

*Device is mounted on the topside.

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