# LT1521CS8 Linear Technology, LT1521CS8 Datasheet - Page 12

#### LT1521CS8

Manufacturer Part Number
LT1521CS8
Description
IC LDO REG W/SHTDWN ADJ 8-SOIC
Manufacturer
Linear Technology
Datasheet

#### Specifications of LT1521CS8

Regulator Topology
Voltage - Output
3.8 ~ 20 V
Voltage - Input
4.3 ~ 20 V
Voltage - Dropout (typical)
0.5V @ 300mA
Number Of Regulators
1
Current - Output
300mA
Current - Limit (min)
320mA
Operating Temperature
0°C ~ 125°C
Mounting Type
Surface Mount
Package / Case
8-SOIC (3.9mm Width)
Lead Free Status / RoHS Status
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 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.
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.
In some applications it may be necessary to leave the input
on the LT1521 unconnected when the output is held high.
This can happen when the LT1521 is powered from a
LT1521/LT1521-3
LT1521-3.3/LT1521-5
APPLICATIONS
12
50
45
40
35
30
25
20
15
10
Figure 3. Reverse Output Current
5
0
0
T
V
CURRENT FLOWS
INTO OUTPUT PIN
V
(LT1521-3/LT1521-3.3
LT1521-5)
V
J
IN
OUT
OUT
1
= 25 C
= 0V
U
= V
= V
2
SENSE
OUTPUT VOLTAGE (V)
3
LT1521-3
INFORMATION
LT1521-3.3
(LT1521)
U
4
5
6
LT1521
LT1521-5
W
7
8
LT1521 • F03
9
10
U
rectified AC source. If the AC source is removed, then the
input of the LT1521 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.
When the input of the LT1521 is forced to a voltage below
its nominal output voltage and its output is held high, the
output current will follow the curve shown in Figure 3. 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
will typically drop to less than 2 A (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
4
3
2
1
0
0
V
V
V
OUT
OUT
OUT
Figure 4. Input Current
= 3V (LT1521-3)
= 3.3V (LT1521-3.3)
= 5V (LT1521-5)
1
INPUT VOLTAGE (V)
LT1521-3
2
3
LT1521-3.3
LT1521-5
4
LT1521 • F04
5
1521335fb