The LM317 is capable of providing extremely good load
regulation, but a few precautions are needed to obtain
maximum performance. For best performance, the
programming resistor (R1) should be connected as close to
the regulator as possible to minimize line drops which
effectively appear in series with the reference, thereby
degrading regulation. The ground end of R2 can be returned
near the load ground to provide remote ground sensing and
improve load regulation.
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A 0.1 µF disc or 1.0 µF tantalum input bypass capacitor
(Cin) is recommended to reduce the sensitivity to input line
The adjustment terminal may be bypassed to ground to
improve ripple rejection. This capacitor (CAdj
ripple from being amplified as the output voltage is
increased. A 10 µF capacitor should improve ripple
rejection about 15 dB at 120 Hz in a 10 V application.
Although the LM317 is stable with no output capacitance,
like any feedback circuit, certain values of external
capacitance can cause excessive ringing. An output
capacitance (CO) in the form of a 1.0 µF tantalum or 25 µF
aluminum electrolytic capacitor on the output swamps this
effect and insures stability.
When external capacitors are used with any IC regulator
it is sometimes necessary to add protection diodes to prevent
the capacitors from discharging through low current points
into the regulator.
Figure 18 shows the LM317 with the recommended
protection diodes for output voltages in excess of 25 V or
high capacitance values (CO > 25 µF, CAdj
> 10 µF). Diode
D1 prevents CO from discharging thru the IC during an input
short circuit. Diode D2 protects against capacitor CAdj
discharging through the IC during an output short circuit.
The combination of diodes D1 and D2 prevents CAdj
discharging through the IC during an input short circuit.
The LM317 is an adjustable 3terminal...
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