LT3008 データシートの表示（PDF） - Linear Technology
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LT3008 Datasheet PDF : 16 Pages
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LT3008
APPLICATIONS INFORMATION
Protection Features
The LT3008 incorporates several protection features that
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 also protects against reverse-input voltages, reverse-
output voltages and reverse output-to-input voltages.
Current limit protection and thermal overload protection
protect the device against current overload conditions at
the output of the device. For normal operation, do not
exceed a junction temperature of 125°C.
The IN pin withstands reverse voltages of 50V. The device
limits current flow to less than 20μA (typically less than
1μA) and no negative voltage appears at OUT. The device
protects both itself and the load against batteries that are
plugged in backwards.
The SHDN pin cannot be driven below GND unless tied to
the IN pin. If the SHDN pin is driven below GND while IN
is powered, the output will turn on. SHDN pin logic cannot
be referenced to a negative rail.
The LT3008 incurs no damage if OUT is pulled below
ground. If IN is left open circuit or grounded, OUT can be
pulled below ground by 50V. No current flows from the
pass transistor connected to OUT. However, current flows
in (but is limited by) the resistor divider that sets output
voltage. Current flows from the bottom resistor in the
divider and from the ADJ pin’s internal clamp through the
top resistor in the divider to the external circuitry pulling
OUT below ground. If IN is powered by a voltage source,
OUT sources current equal to its current limit capability
and the LT3008 protects itself by thermal limiting if neces-
sary. In this case, grounding the SHDN pin turns off the
LT3008 and stops OUT from sourcing current.
The LT3008 incurs no damage if the ADJ pin is pulled
above or below ground by 50V. If IN is left open circuit or
grounded, ADJ acts like a 100k resistor in series with a
diode when pulled above or below ground.
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 flow back into the output follows the
curve shown in Figure 5.
If the LT3008 IN pin is forced below the OUT pin or the
OUT pin is pulled above the IN pin, input current typically
drops to less than 1μA. This occurs if the LT3008 input is
connected to a discharged (low voltage) battery and either
a backup battery or a second regulator circuit holds up
the output. The state of the SHDN pin has no effect in the
reverse current if OUT is pulled above IN.
100
90
80
70
60
50
40
30
20
10
0
0
ADJ CURRENT
OUT CURRENT
1 2 3 4 5 6 7 8 9 10
OUTPUT AND ADJ VOLTAGE (V)
3008 F05
Figure 5. Reverse Output Current
3008f
12
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