LM317SSLASHNOPB datasheet(13/48 Pages) TI1

13 / 48 page

LM317-N

LM117, LM317-N

www.ti.com

SNVS774P – MAY 2004 – REVISED OCTOBER 2015

8.3 Feature Description

8.3.1 Load Regulation

The LM317-N is capable of providing extremely good load regulation but a few precautions are needed to obtain

maximum performance. The current set resistor, R1, must be connected near the output terminal of the regulator

rather than near the load. If R1 is placed too far from the output terminal, then the increased trace resistance, RS,

will cause an error voltage drop in the adjustment loop and degrade load regulation performance. Therefore, R1

must be placed as close as possible to the output terminal to minimize RS and maximize load regulation

performance.

Figure 16 shows the effect of the trace resistance, RS, when R1 is placed far from the output terminal of the

regulator. It is clear that RS will cause an error voltage drop especially during higher current loads, so it is

important to minimize the RS trace resistance by keeping R1 close to the regulator output terminal.

Figure 16. Regulator With Line Resistance in Output Lead

With the TO-3 package, it is easy to minimize the resistance from the case to the set resistor, by using two

separate leads to the case. However, with the TO package, care must be taken to minimize the wire length of the

output lead. The ground of R2 can be returned near the ground of the load to provide remote ground sensing

and improve load regulation.

8.4 Device Functional Modes

8.4.1 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 LM317-N 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 capacitors 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 LM317-N 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.

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