LM1117/LM1117I
10091918
FIGURE 2. Typical Application using Fixed Output Regulator
When the adjustable regulator is used (Figure 3), 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 IL. 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.
10091919
FIGURE 3. Best Load Regulation using Adjustable Output Regulator
4.0 PROTECTION DIODES
Under normal operation, the LM1117 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 VIN. In the LM1117 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 4.
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