LT3012 LINER [Linear Technology], LT3012 Datasheet - Page 12

• Manufacturer Part Number: LT3012
• Description: 250mA, 4V to 80V Low Dropout Micropower Linear Regulator
• Manufacturer: LINER [Linear Technology]

LT3012
APPLICATIONS INFORMATION
Protection Features
The LT3012 incorporates several protection features which
make it ideal for use in battery-powered circuits. In ad-
dition to the normal protection features associated with
monolithic regulators, such as current limiting and thermal
limiting, the device is protected against reverse-input volt-
ages, and reverse voltages from output to input.
Like many IC power regulators, the LT3012 has safe operat-
ing area protection. The safe area protection decreases the
current limit as input voltage increases and keeps the power
transistor inside a safe operating region for all values of
input voltage. The protection is designed to provide some
output current at all values of input voltage up to the device
breakdown. The SOA protection circuitry for the LT3012
uses a current generated when the input voltage exceeds
25V to decrease current limit. This current shows up as
additional quiescent current for input voltages above 25V.
This increase in quiescent current occurs both in normal
operation and in shutdown (see curve of Quiescent Current
in the Typical Performance Characteristics).
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
(LT3012E) or 140°C (LT3012HFE).
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 refer-
ence 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.
12
In situations where the ADJ pin is connected to a resistor
divider that would pull the ADJ pin above its 7V clamp volt-
age 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
1.24V reference when the output is forced to 60V. 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 53V difference between the OUT and ADJ
pins divided by the 5mA maximum current into the ADJ
pin yields a minimum top resistor value of 10.6k.
In circuits where a backup battery is required, several
different input/output conditions can occur. The output
voltage may be held up while the input is either pulled
to ground, pulled to some intermediate voltage, or is left
open circuit. Current fl ow back into the output will follow
the curve shown in Figure 4. The rise in reverse output
current above 7V occurs from the breakdown of the 7V
clamp on the ADJ pin. With a resistor divider on the
regulator output, this current will be reduced depending
on the size of the resistor divider.
When the IN pin of the LT3012 is forced below the OUT
pin or the OUT pin is pulled above the IN pin, input cur-
rent will typically drop to less than 2μA. This can happen
if the input of the LT3012 is connected to a discharged
(low voltage) battery and the output is held up by either
a backup battery or a second regulator circuit. The state
of the SHDN pin will have no effect on the reverse output
current when the output is pulled above the input.
200
180
160
140
120
100
80
60
40
20
0
0
Figure 4. Reverse Output Current
T
V
V
CURRENT FLOWS
INTO OUTPUT PIN
J
IN
OUT
= 25°C
1
= 0V
= V
2
ADJ
OUTPUT VOLTAGE (V)
3
4
5
6
(SEE ABOVE)
PIN CLAMP
7
ADJ
8
9
3012 F04
10
3012fd