14

LT3028

3028f

The power dissipated by each output will be equal to:

Where for Output 1:

For Output 2:

So for Output 1:

P = 500mA (5V – 3.3V) + 9mA (5V) = 0.90W

For Output 2:

P = 100mA (5V – 2.5V) + 2mA (5V) = 0.26W

The thermal resistance will be in the range of 35°C/W to

55°C/W depending on the copper area. So the junction

temperature rise above ambient will be approximately

equal to:

(0.90W + 0.26W) 50°C/W = 57.8°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 LT3028 regulator incorporates several protection

features which make 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 device is protected

against reverse input voltages and reverse voltages from

output to input. The two regulators have common inputs

and GND pins and are thermally coupled, however, the two

outputs of the LT3028 operate independently. They can be

shut down independently and a fault condition on one

output will not affect the other output electrically.

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

which can be plugged in backward.

The output of the LT3028 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 ADJ 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 ADJ 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 ADJ pins are connected to a

resistor divider that would pull the pins above their 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.

The top resistor of the resistor divider must be chosen to

limit the current into the ADJ pin to less than 5mA when the

ADJ pin is at 7V. The 13V difference between output and

ADJ pin divided by the 5mA maximum current into the ADJ

pin yields a minimum top resistor value of 2.6k.