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LT1761 Series

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

125

°C/W

1000mm

2500mm

125

°C/W

225mm

2500mm

130

°C/W

100mm

2500mm

135

°C/W

50mm

2500mm

150

°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 50mA

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) = 50mA

VIN(MAX) = 6V

IGND at (IOUT = 50mA, VIN = 6V) = 1mA

So,

P = 50mA(6V – 3.3V) + 1mA(6V) = 0.14W

The thermal resistance will be in the range of 125

°C/W to

150

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

temperature rise above ambient will be approximately

equal to:

0.14W(150

°C/W) = 21.2°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 + 21.2°C = 71.2°C

APPLICATIONS INFORMATION

Protection Features

The LT1761 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 LT1761-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 typically 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.

The ADJ pin of the adjustable device can be pulled above

or below ground by as much as 7V without damaging the

device. If the input is left open circuit or grounded, the ADJ

pin will act like an open circuit when pulled below ground

and like a large resistor (typically 100k) in series with a

diode when pulled above ground.

In situations where the ADJ pin is connected to a resistor

divider that would pull the ADJ pin above its 7V clamp

voltage if the output is pulled high, the ADJ pin input

current must be limited to less than 5mA. For example, a

resistor divider is used to provide a regulated 1.5V output

from the 1.22V reference when the output is forced to 20V.

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