LT1521/LT1521-3
LT1521-3.3/LT1521-5
U
W U U
APPLICATIONS INFORMATION
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, pulledtosomeintermediatevoltageorisleftopen
circuit. Current flow back into the output will vary depend-
ing on the conditions. Many battery-powered circuits
incorporate some form of power management. The fol-
lowing information will help optimize battery life. Table 4
summarizes the following information.
rectified AC source. If the AC source is removed, then the
input of the LT1521 is effectively left floating. The reverse
outputcurrentalsofollowsthecurveinFigure3iftheinput
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.
When the input of the LT1521 is forced to a voltage below
its nominal output voltage and its output is held high, the
outputcurrentwillfollowthecurveshowninFigure3. This
can happen if the input of the LT1521 is connected to a
discharged (low voltage) battery and the output is held up
by either a backup battery or by second regulator circuit.
When the input pin is forced below the output pin or the
output pin is pulled above the input pin, the input current
willtypicallydroptolessthan2µA(seeFigure4). Thestate
of the shutdown pin will have no effect on the reverse
output current when the output is pulled above the input.
The reverse output current will follow the curve in Figure
3 when the input is pulled to ground. This current flows
through the output pin to ground. The state of the shut-
down pin will have no effect on output current when the
input pin is pulled to ground.
Insomeapplicationsitmaybenecessarytoleavetheinput
on the LT1521 unconnected when the output is held high.
This can happen when the LT1521 is powered from a
50
5
T = 25°C
IN
LT1521
J
V
V
OUT
V
OUT
V
OUT
= 3V (LT1521-3)
= 3.3V (LT1521-3.3)
= 5V (LT1521-5)
45
40
35
30
25
20
15
10
5
= 0V
CURRENT FLOWS
INTO OUTPUT PIN
4
3
2
1
0
V
OUT
= V
SENSE
LT1521-3.3
(LT1521-3/LT1521-3.3
LT1521-5)
V
OUT
= V
ADJ
(LT1521)
LT1521-3.3
LT1521-3
LT1521-5
LT1521-3
LT1521-5
0
0
1
2
3
4
5
6
7
8
9
10
0
1
2
3
4
5
OUTPUT VOLTAGE (V)
INPUT VOLTAGE (V)
LT1521 • F03
LT1521 • F04
Figure 4. Input Current
Figure 3. Reverse Output Current
1521335fb
12
Linear [ Linear ]
