Datasheet

LT1762 Series
15
Rev B
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APPLICATIONS INFORMATION
The thermal resistance will be in the range of 110°C/W
to 140°C/W depending on the copper area. So the junc-
tion temperature rise above ambient will be approximately
equal to:
0.44W(125°C/W) = 55°C
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 + 55°C = 105°C
Protection Features
The LT1762 regulators incorporate several protection fea-
tures which make them 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 devices are protected
against reverse input voltages, reverse output 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
20V. Current flow into the device will be limited to less
than 1mA (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.
The output of the LT1762-X can be pulled below ground
without damaging the device. If the input is left open cir-
cuit or grounded, the output can be pulled below ground
by 20V. For fixed voltage versions, the output will act like
a large resistor, typically 500k or higher, limiting current
flow to less than 100µA. For adjustable versions, the out-
put 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 100k) 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 1.22V 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 output
and ADJ pin 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7.
When the IN pin of the LT1762-X 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 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.
Figure7. Reverse Output Current
OUTPUT VOLTAGE (V)
100
90
80
70
60
50
40
30
20
10
0
REVERSE OUTPUT CURRENT (μA)
1762 F07
0 1 2 3
4
5
6 7 8 9
10
T
J
= 25°C
V
IN
= 0V
CURRENT FLOWS
INTO OUTPUT PIN
V
OUT
= V
SENSE
(LT1762-2.5/LT1762-3
LT1762-3.3/LT1762-5)
V
OUT
= V
ADJ
(LT1762)
LT1762
LT1762-3
LT1762-5
LT1762-2.5
LT1762-3.3
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