LM317

5/14

10V application.

Althou gh t h e LM 3 17 is st able with no ou tput ca pac i tance , l ike any fee d b ack

circuit , certain va lues of e x terna l ca paci ta nce ca n caus e excessive ringing. An output

capac ita nce (Co) in t h e form of a 1.0

µF tantalum or 25µF aluminum electrolytic

capacitor on the output swamps this effect and insures stability.

4. Protection Diodes

When external capacitors are used with an y IC regulator it is some times 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 20A spikes wh en shorted. Although the surge is short,

there is enough energy to dama ge 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 LM 317, this discharge path is through a large junction that

is able to sustain a 2A surge with no probl em. This is not true of other ty pes of

positive regulators. For output capacitors of 25

µF or less, the LM317’s ballast

resistors and output structure limit the peak current to a low enough level so that there

is no need to use a protection diode.

The by pass capacitor on the adjustme nt terminal can discharge through a low

current junction. Discharge occurs when

either the input or output is shorted. Internal

to the LM 317 is a 50

Ω resistor which limits

the peak discharge current. No protection is

needed for output voltages of 25V or less and

10

µF capacitance. Figure in right shows an LM

317 with protection diodes included for

use with outputs greater than 25V and high

values of output capacitance.

Regulator with protection diodes

R1

R2

LM317L

V

ADJ

V

V

OU

T

IN

OU

T

240

V

IN

D1

IN400

D2

IN400

C1

10u

W317L