7.4.1 External Capacitors

An input-bypass capacitor is recommended. A 0.1-μF disc or 1-μF solid tantalum on the input is suitable input bypassing for almost all applications. The device is more sensitive to the absence of input bypassing when adjustment or output capacitors are used, but the above values will eliminate the possibility of problems.

The adjustment terminal can be bypassed to ground on the LM317A to improve ripple rejection. This bypass capacitor prevents ripple from being amplified as the output voltage is increased. With a 10-μF bypass capacitor, 80-dB ripple rejection is obtainable at any output level. Increases over 10 μF do not appreciably improve the ripple rejection at frequencies above 120 Hz. If the bypass capacitor is used, it is sometimes necessary to include protection diodes to prevent the capacitor from discharging through internal low current paths and damaging the device.

In general, the best type of capacitor to use is solid tantalum. Solid tantalum capacitors have low impedance even at high frequencies. Depending upon capacitor construction, it takes about 25 μF in aluminum electrolytic to equal 1-μF solid tantalum at high frequencies. Ceramic capacitors are also good at high frequencies. However, some types have a large decrease in capacitance at frequencies around 0.5 MHz. For this reason, 0.01-μF disc may seem to work better than a 0.1-μF disc as a bypass.

Although the LM317A is stable with no output capacitors, like any feedback circuit, certain values of external capacitance can cause excessive ringing. This occurs with values between 500 pF and 5000 pF. A 1-μF solid tantalum (or 25-μF aluminum electrolytic) on the output swamps this effect and insures stability. Any increase of the load capacitance larger than 10 μF will merely improve the loop stability and output impedance.