LM1086
SNVS039H –JUNE 2000–REVISED MAY 2013
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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 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.
Figure 21. Best Load Regulation using Adjustable Output Regulator
PROTECTION DIODES
Under normal operation, the LM1086 regulator does not need any protection diode. With the adjustable device,
the internal resistance between the adjustment and output terminals limits the current. No diode is needed to
divert the current around the regulator even with a capacitor on the adjustment terminal. The adjust pin can take
a transient signal of ±25V with respect to the output voltage without damaging the device.
When an output capacitor is connected to a regulator and the input is shorted, 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 LM1086 regulator, 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, shown in Figure 22.
Figure 22. Regulator with Protection Diode
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