Datasheet

11
LT3012B
3012bf
APPLICATIO S I FOR ATIO
WUUU
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 + 65.5°C = 115.5°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 50mA 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:
P1(48V in, 5mA load) = 5mA • (48V – 5V)
+ (200µA • 48V) = 0.23W
P2(48V in, 50mA load) = 50mA • (48V – 5V)
+ (1mA • 48V) = 2.20W
P3(72V in, 5mA load) = 5mA • (72V – 5V)
+ (200µA • 72V) = 0.35W
P4(72V in, 50mA load) = 50mA • (72V – 5V)
+ (1mA • 72V) = 3.42W
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%(0.23W) + 19%(2.20W) + 4%(0.35W)
+ 1%(3.42W) = 0.64W
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 26°C to 38°C.
Protection Features
The LT3012B 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 ther-
mal limiting, the device is protected against reverse-input
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
80V. 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 ADJ pin of the device 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 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. If the input is powered by a voltage
source, pulling the ADJ pin below the reference voltage
will cause the device to current limit. This will cause the
output to go to a unregulated high voltage. Pulling the ADJ
pin above the reference voltage will turn off all output
current.