Datasheet

LM1117-N, LM1117I
SNOS412M –FEBRUARY 2000–REVISED MARCH 2013 www.ti.com
Figure 20. Typical Application using Fixed Output Regulator
When the adjustable regulator is used (Figure 21), the best performance is obtained with the positive side of the
resistor R1 tied directly to the output terminal of the regulator rather than near the load. This eliminates line drops
from appearing effectively in series with the reference and degrading regulation. For example, a 5V regulator with
0.05Ω resistance between the regulator and load will have a load regulation due to line resistance of 0.05Ω x I
L
.
If R1 (=125Ω) is connected near the load, the effective line resistance will be 0.05Ω (1+R2/R1) or in this case, it
is 4 times worse. In addition, the ground side of the resistor R2 can be returned near the ground of the load to
provide remote ground sensing and improve load regulation.
Figure 21. Best Load Regulation using Adjustable Output Regulator
PROTECTION DIODES
Under normal operation, the LM1117-N regulators do not need any protection diode. With the adjustable device,
the internal resistance between the adjust and output terminals limits the current. No diode is needed to divert
the current around the regulator even with capacitor on the adjust terminal. The adjust pin can take a transient
signal of ±25V with respect to the output voltage without damaging the device.
When a output capacitor is connected to a regulator and the input is shorted to ground, the output capacitor will
discharge into the output of the regulator. The discharge current depends on the value of the capacitor, the
output voltage of the regulator, and rate of decrease of V
IN
. In the LM1117-N regulators, the internal diode
between the output and input pins can withstand microsecond surge currents of 10A to 20A. With an extremely
large output capacitor (≥1000 µF), and with input instantaneously shorted to ground, the regulator could be
damaged.
In this case, an external diode is recommended between the output and input pins to protect the regulator, as
shown in Figure 22.
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