LT1121-3.3 Linear Technology, LT1121-3.3 Datasheet - Page 10

• Manufacturer Part Number: LT1121-3.3
• Description: Micropower Low Dropout Regulators with Shutdown
• Manufacturer: Linear Technology

A
LT1121/LT1121-3.3/LT1121-5
that the ESR of the capacitor is low (ceramic) the sug-
gested series resistor is shown in Table 5. The LT1121 is
a micropower device and output transient response will be
a function of output capacitance. See the Transient Re-
sponse curves in the Typical Performance Characteristics.
Larger values of output capacitance will decrease the peak
deviations and provide improved output transient re-
sponse. Bypass capacitors, used to decouple individual
components powered by the LT1121, will increase the
effective value of the output capacitor.
Table 5.
Protection Features
The LT1121 incorporates several protection features which
make it ideal for use in battery-powered circuits. In addi-
tion to the normal protection features associated with
monolithic regulators, such as current limiting and ther-
mal limiting, the device is 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
30V. Current flow into the device will be limited to less than
1mA (typically less than 100 A) 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.
For fixed voltage versions of the device, the output can be
pulled below ground without damaging the device. If the
input is open circuit or grounded the output can be pulled
below ground by 20V. 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
10
PPLICATI
OUTPUT CAPACITANCE
0.33 F
0.47 F
0.68 F
>1 F
O
U
S
I FOR ATIO
U
SUGGESTED SERIES
W
None Needed
RESISTOR
2
1
1
U
short-circuit current of the device and will protect itself by
thermal limiting. For the adjustable version of the device,
the output pin is internally clamped at one diode drop
below ground. Reverse current for the adjustable device
must be limited to 5mA.
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 cir-
cuits incorporate some form of power management. The
following information will help optimize battery life. Table
6 summarizes the following information.
The reverse output current will follow the curve in Figure
2 when the input pin 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
to the LT1121 unconnected when the output is held high.
This can happen when the LT1121 is powered from a
rectified AC source. If the AC source is removed, then the
input of the LT1121 is effectively left floating. The reverse
output current also follows the curve in Figure 2 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.
100
90
80
70
60
50
40
30
20
10
0
0
Figure 2. Reverse Output Current
T
V
CURRENT FLOWS
INTO OUTPUT PIN
TO GROUND
J
IN
= 25°C
1
< V
OUT
2
OUTPUT VOLTAGE (V)
3
(V
LT1121-3.3
OUT
4
LT1121
= V
5
ADJ
LT1121-5
6
)
7
8
1121• F02
9
10