LT1129/LT1129-3.3/LT1129-5
APPLICATIONS INFORMATION
Current limit protection and thermal overload protection
areintendedtoprotectthedeviceagainstcurrentoverload
conditionsattheoutputofthedevice.Fornormaloperation,
the junction temperature should not exceed 125°C.
ThereverseoutputcurrentwillfollowthecurveinFigure 3
when the input pin is pulled to ground. This current flows
throughtheoutputpintoground.Thestateoftheshutdown
pin will have no effect on output current when the input
pin is pulled to ground.
The input of the device will withstand reverse voltages
of 30V. 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 that can be plugged in backwards.
In some applications it may be necessary to leave the
input to the LT1129 unconnected when the output is held
high. This can happen when the LT1129 is powered from
a rectified AC source. If the AC source is removed, then
the input of the LT1129 is effectively left floating. The
reverse output current also follows the curve in Figure 3
if the input pin is left open. The state of the shutdown pin
will have no effect on the reverse output current when the
input pin is floating.
For fixed voltage versions of the device, the sense pin
is internally clamped to one diode drop below ground.
For the adjustable version of the device, the output pin
is internally clamped at one diode drop below ground. If
the output pin of an adjustable device, or the sense pin
of a fixed voltage device, is pulled below ground, with the
input open or grounded, current must be limited to less
than 5mA.
When the input of the LT1129 is forced to a voltage below
its nominal output voltage and its output is held high, the
reverse output current will still follow the curve shown
in Figure 3. This can happen if the input of the LT1129 is
connected to a discharged (low voltage) battery and the
output is held up by either a backup battery or by a second
regulator circuit.
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 vary
depending on the conditions. Many battery-powered
circuits incorporate some form of power management.
The following information will help optimize battery life.
Table 4 summarizes the following information.
When the input pin is forced below the output pin or the
outputpinispulledabovetheinputpin,theinputcurrentwill
typically drop to less than 2μA (see Figure 4). The state of
the shutdown pin will have no effect on the reverse output
current when the output is pulled above the input.
5
100
T
= 25°C
J
V
V
= 3.3V (LT1129-3.3)
= 5V (LT1129-5)
OUT
OUT
90
80
V
= 0V
IN
SENSE
V
= V
OUT
4
3
2
1
0
CURRENT FLOWS
70 INTO DEVICE
60
50
40
30
20
10
0
LT1129
LT1129-3.3
LT1129-5
0
1
2
3
4
5
0
2
4
6
8
10
1
3
5
7
9
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
112935 F04
112935 F03
Figure 3. Reverse Output Current
Figure 4. Input Current
112935ff
12
Linear [ Linear ]
