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LT1763 Series

1763fa

APPLICATIONS INFORMATION

The LT1763 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. Copper board stiffeners 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 3/32" FR-4 board with one ounce

copper.

Table 1. Measured Thermal Resistance

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

50mm

2500mm

°C/W

*Device is mounted on topside.

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

250mA 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) = 250mA

VIN(MAX) = 6V

IGND at (IOUT = 250mA, VIN = 6V) = 5mA

So,

P = 250mA(6V – 3.3V) + 5mA(6V) = 0.71W

The thermal resistance will be in the range of 60

°C/W to

°C/W depending on the copper area. So the junction

temperature rise above ambient will be approximately

equal to:

0.71W(75

°C/W) = 53.3°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 + 53.3°C = 103.3°C

Protection Features

The LT1763 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 (typically less than 100

µA) and no negative voltage

will appear at the output. The device will protect both itself

and the load. This provides protection against batteries

which can be plugged in backward.

The output of the LT1763-X can be pulled below ground

without damaging the device. If the input is left open circuit

or grounded, the output can be pulled below ground by

20V. For fixed voltage versions, the output will act like a

large resistor, typically 500k

Ω or higher, limiting current

flow to less than 100

µA. For adjustable versions, the

output will act like an open circuit; no current will flow out

of the pin. If the input is powered by a voltage source, the

output will source the short-circuit current of the device

and will protect itself by thermal limiting. In this case,

grounding the SHDN pin will turn off the device and stop

the output from sourcing the short-circuit current.

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