LM317

4/14

APPLICATION INFORMATION

1. Basic circuit operation

In operation, the LM 317 develops a nominal 1.25V reference voltage, Vref, between

the output and adjustme nt terminal. The reference voltage is impressed across program

resistor R1 and, since the voltage is constant, a constant current I1 then flows through

the output set resistor R2, giving an output voltage of

V o u t = V r e f ( 1 + R 2 / R 1 ) + I a d j * ( R 2 )

Since the 100

µA current from the adjustment

terminal represents an error term, the LM 317

was designed to mi nimi ze Iadj and ma ke it very

constant with line and load changes. To do this,

all quiescent operating current is returned to the

output establishing a mi nimu m load current

requireme nt. If there is insufficient load on the

output, the output will rise.

Figure: Basic circuit configuration

R1

R2

LL

V

V

ADJ

I

V

I

V

AD

J

1

RE

OU

IN

OU

2. Load Regulation

The LM 31 7 is cap ab le of pro v i d in g extre m ely good load re gu latio n, b u t a few

precaut i ons

are

nee d ed

to

obt a in

ma xi m u m

performance.

For

best

performa nce,

the

programmi ng resistor (R1) shoul d be co nne cted

as

clos e

to

t h e

regula t or

as

possi ble

t o

m i ni mi ze line drops which effecti v ely appear in

series with the reference, t h ere b y degradi n g

regulat io n.

The

gro und

end

of

R2

ca n

b e

returned ne ar the loa d groun d to p r ovide rem o te

groun d se n s ing an d i m prove loa d reg u la tio n

Reg u lat o r with li ne resistance

i n

o u t p u t l e a d

R1

R2

LM317L

V

V

ADJ

V

OUT

IN

OUT

Rs

240

W317L

W317L

3.External capacitors

A 0 . 1

µF disc or 1.0µF tantalum input bypass capacitor (Cin) is recommended to

reduce the sensitivity to input line impedance.

T h e ad j u stme nt term inal ma y be by passe d to gro u n d to impro v e ripp le rejecti o n .

This ca paci tor (Cadj) prevents ri pple from bein g amplified as t h e out pu t v o lta ge is

increase d . A 10

µF capacitor should improve ripple rejection about 15dB at 120Hz in a