LT3023
12
3023fa
APPLICATIONS INFORMATION
Calculating Junction Temperature
Example: Given an output voltage on the fi rst 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 fi rst channel and 0mA to 50mA for the
second channel, with a maximum ambient temperature of
50°C, what will the maximum junction temperature be?
The power dissipated by each channel of the device will
be equal to:
I
OUT(MAX)
(V
IN(MAX)
– V
OUT
) + I
GND
(V
IN(MAX)
)
where (for the fi rst channel):
I
OUT(MAX)
= 100mA
V
IN(MAX)
= 6V
I
GND
at (I
OUT
= 100mA, V
IN
= 6V) = 2mA
so:
P1 = 100mA(6V – 3.3V) + 2mA(6V) = 0.28W
and (for the second channel):
I
OUT(MAX)
= 50mA
V
IN(MAX)
= 6V
I
GND
at (I
OUT
= 50mA, V
IN
= 6V) = 1mA
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:
T
JMAX
= 50°C + 27.8°C = 77.8°C
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 operation,
the junction temperature should not exceed 125°C.
The input of the device will withstand reverse voltages
of 20V. Current fl ow 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 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
fl ow 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 minimum top resistor value of 2.6k.
