LT1963 Series
W U U
APPLICATIO S I FOR ATIO
U
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
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
fromthe1.21Vreferencewhentheoutputisforcedto20V.
The top resistor of the resistor divider must be chosen to
limitthecurrentintotheADJpintolessthan5mAwhenthe
ADJpinisat7V. The13VdifferencebetweenOUTandADJ
pinsdividedbythe5mAmaximumcurrentintotheADJpin
yields a minimum top resistor value of 2.6k.
T
JMAX = 50°C + 39.5°C = 89.5°C
Protection Features
The LT1963 regulators incorporate several protection
featureswhichmakethemidealforuseinbattery-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.
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.
Current limit protection and thermal overload protection
areintendedtoprotectthedeviceagainstcurrentoverload
conditions at the output of the device. For normal opera-
tion, the junction temperature should not exceed 125°C.
WhentheINpinoftheLT1963isforcedbelowtheOUTpin
ortheOUTpinispulledabovetheINpin, inputcurrentwill
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.
The input of the device will withstand reverse voltages of
20V.Currentflowintothedevicewillbelimitedtolessthan
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
that can be plugged in backward.
The output of the LT1963 can be pulled below ground
withoutdamagingthedevice.Iftheinputisleftopencircuit
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 600µA. 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.
5.0
LT1963
= V
LT1963-1.8
= V
FB
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
V
V
OUT
OUT
ADJ
LT1963-2.5
= V
V
OUT
FB
LT1963-3.3
= V
V
OUT
FB
T
= 25°C
IN
J
V
= 0V
CURRENT FLOWS
INTO OUTPUT PIN
0
1
2
3
4
5
6
7
8
9
10
OUTPUT VOLTAGE (V)
The ADJ pin of the adjustable device can be pulled above
or below ground by as much as 7V without damaging the
device. Iftheinputisleftopencircuitorgrounded, theADJ
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.
1963 F05
Figure 5. Reverse Output Current
1963fa
13
Linear Systems [ Linear Systems ]
