LT1764-1.5 Linear Technology, LT1764-1.5 Datasheet - Page 13

• Manufacturer Part Number: LT1764-1.5
• Description: 3A/ Fast Transient Response/ Low Noise/LDO Regulators
• Manufacturer: Linear Technology

APPLICATIONS
device will protect both itself and the load. This provides
protection against batteries which can be plugged in
backward.
The output of the LT1764-X 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. For fixed voltage versions, the output will act like a
large resistor, typically 5k or higher, limiting current flow
to typically less than 600 A. For adjustable versions, the
output will act like an open circuit; no current will flow 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 SHDN pin will turn off the device and stop
the output from sourcing the short-circuit current.
The ADJ pin of the adjustable 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 5k) in series with a diode
when pulled above ground.
In situations where the ADJ pin is connected to a resistor
divider that would pull the ADJ pin above its 7V clamp
voltage 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 1.21V 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
ADJ pin is at 7V. The 13V difference between OUT and ADJ
U
INFORMATION
U
W
U
pins divided by the 5mA maximum current into the ADJ pin
yields a minimum top resistor value of 2.6k.
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 flow back into the output will follow
the curve shown in Figure 5.
When the IN pin of the LT1764-X is forced below the OUT
pin or the OUT pin is pulled above the IN pin, input current
will typically drop to less than 2 A. This can happen if the
input of the device 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.
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
LT1764
LT1764-1.8
1
2
Figure 5. Reverse Output Current
OUTPUT VOLTAGE (V)
3
4
5
6
7
LT1764-2.5
LT1764-3.3
8
LT1764 Series
9
10
T
V
CURRENT FLOWS INTO
OUTPUT PIN
V
V
LT1764-2.5, LT1764-3.3)
J
IN
OUT
OUT
= 25 C
= OV
= V
= V
ADJ
FB
(LT1764-1.8,
(LT1764)
13
1764 F05
1764fa