12

LT3023

3023f

Calculating Junction Temperature

Example: Given an output voltage on the first channel of

3.3V, an output voltage of 2.5V on the second channel, an

input voltage range of 4V to 6V, output current ranges of

0mA to 100mA for the first channel and 0mA to 50mA for

the second channel, with a maximum ambient tempera-

ture of 50

°C, what will the maximum junction temperature

be?

The power dissipated by each channel of the device will be

equal to:

where (for the first channel):

so:

P1 = 100mA(6V – 3.3V) + 2mA(6V) = 0.28W

and (for the second channel):

so:

P2 = 50mA(6V – 2.5V) + 1mA(6V) = 0.18W

The thermal resistance will be in the range of 40

°C/W to

60

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

temperature rise above ambient will be approximately

equal to:

(0.28W + 018W)(60

°C/W) = 27.8°C

The maximum junction temperature will then be equal to

the maximum junction temperature rise above ambient

plus the maximum ambient temperature or:

Protection Features

The LT3023 regulator incorporates several protection

features which makes it 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 LT3023 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. 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 SHDN1/SHDN2 pins will turn off the

device and stop the output from sourcing the short-circuit

current.

The ADJ1 and ADJ2 pins 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 ADJ1 and

ADJ2 pins 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 ADJ1 and ADJ2 pins are connected

to a resistor divider that would pull the pins above their 7V

clamp voltage if the output is pulled high, the ADJ1/ADJ2

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. The top resistor of the resistor divider must

be chosen to limit the current into the ADJ pin to less than

5mA when the ADJ1/ADJ2 pin is at 7V. The 13V difference

between output and ADJ1/ADJ2 pin divided by the 5mA

maximum current into the ADJ1/ADJ2 pin yields a mini-

mum top resistor value of 2.6k.