14
LT3028
3028f
The power dissipated by each output will be equal to:
I
OUT(MAX)
(V
IN(MAX)
– V
OUT
) + I
GND
(V
IN(MAX)
)
Where for Output 1:
I
OUT(MAX)
= 500mA
V
IN(MAX)
= 5V
I
GND
at (I
OUT
= 500mA, V
IN
= 5V) = 9mA
For Output 2:
I
OUT(MAX)
= 100mA
V
IN(MAX)
= 5V
I
GND
at (I
OUT
= 100mA, V
IN
= 5V) = 2mA
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:
T
JMAX
= 50°C + 57.8°C = 107.8°C
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.
APPLICATIONS INFORMATION
WUU
U
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.
