( )

OUT

ADJ

R

V

= 1.25 V

I

R

R

2

2

1

1

æ

ö

+

+

ç

÷

è

ø

LM117HV, LM317HV

www.ti.com

SNVS773D – APRIL 2000 – REVISED SEPTEMBER 2015

Device Functional Modes (continued)

In general, the best type of capacitors to use are 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 of solid tantalum at high frequencies. Ceramic capacitors are also good at high frequencies, but some

types have a large decrease in capacitance at frequencies around 0.5 MHz. For this reason, a 0.01-

μF disc may

seem to work better than a 0.1-

μF disc as a bypass.

Although the LM317HV 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 ensures stability. Any increase of

load capacitance larger than 10

μF will merely improve the loop stability and output impedance.

8.4.2 Protection Diodes

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. Most 10-

μF capacitors have

low enough internal series resistance to deliver 20-A spikes when shorted. Although the surge is short, there is

enough energy to damage parts of the IC.

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 the rate of decrease of VIN. In the LM317HV, this discharge path is through a large junction

that is able to sustain 15-A surge with no problem. This is not true of other types of positive regulators. For

output capacitors of 25

μF or less, there is no need to use diodes.

The bypass capacitor on the adjustment terminal can discharge through a low current junction. Discharge occurs

when either the input or output is shorted. Internal to the LM317HV is a 50-

Ω resistor which limits the peak

discharge current. No protection is needed for output voltages of 25 V or less and 10-

μF capacitance. Figure 15

shows an LM317HV with protection diodes included for use with outputs greater than 25 V and high values of

output capacitance.

D1 protects against C1

D2 protects against C2

Figure 15. Regulator With Protection Diodes

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