LT3014
W U U
U
APPLICATIO S I FOR ATIO
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
Protection Features
TheLT3014incorporatesseveralprotectionfeatureswhich
make it ideal for use in battery-powered circuits. In addi-
tion to the normal protection features associated with
monolithic regulators, such as current limiting and ther-
mal limiting, the device is protected against reverse-input
voltages, and reverse voltages from output to input.
T
JMAX = 50°C + 26°C = 76°C
Example 2: Given an output voltage of 5V, an input voltage
of 48V that rises to 72V for 5ms(max) out of every 100ms,
and a 5mA load that steps to 20mA for 50ms out of every
250ms, what is the junction temperature rise above ambi-
ent? Using a 500ms period (well under the time constant
of the board), power dissipation is as follows:
Current limit protection and thermal overload protection
areintendedtoprotectthedeviceagainstcurrentoverload
conditions at the output of the device. For normal opera-
tion, the junction temperature should not exceed 125°C.
P1(48V in, 5mA load) = 5mA • (48V – 5V)
+ (100µA • 48V) = 0.22W
The input of the device will withstand reverse voltages of
80V.Currentflowintothedevicewillbelimitedtolessthan
6mA (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.
P2(48V in, 20mA load) = 20mA • (48V – 5V)
+ (0.55mA • 48V) = 0.89W
P3(72V in, 5mA load) = 5mA • (72V – 5V)
+ (100µA • 72V) = 0.34W
P4(72V in, 20mA load) = 20mA • (72V – 5V)
+ (0.55mA • 72V) = 1.38W
The ADJ pin can be pulled above or below ground by as
muchas7Vwithoutdamagingthedevice.Iftheinputisleft
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. Iftheinputispoweredbyavoltagesource, pulling
the ADJ pin below the reference voltage will cause the
device to current limit. This will cause the output to go to
an unregulated high voltage. Pulling the ADJ pin above the
reference voltage will turn off all output current.
Operation at the different power levels is as follows:
76% operation at P1, 19% for P2, 4% for P3, and
1% for P4.
P
EFF = 76%(0.22W) + 19%(0.89W) + 4%(0.34W)
+ 1%(1.38W) = 0.36W
With a thermal resistance in the range of 40°C/W to
62°C/W, this translates to a junction temperature rise
above ambient of 20°C.
3014fb
10
Linear [ Linear ]
