LM317L, NCV317L

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7

CL = 0.3 mF; CAdj = 10 mF

Figure 16. Line Transient Response

Figure 17. Load Transient Response

0.3

0.2

0.1

0

-0.1

-0.2

100

50

0

0

102030

40

t, TIME (

ms)

CL = 1 mF; CAdj = 10 mF

Vin = 15 V

Vout = 10 V

INL = 50 mA

TJ = 25°C

IL

-0.3

CL = 1.0 mF;

CAdj = 10 mF

Vin

1.5

1.0

0.5

0

-0.5

-1.0

-1.5

1.0

0.5

0

010

20

30

40

t, TIME (

ms)

Vout = 10 V

IL = 50 mA

TJ = 25°C

CL = 0;

Without CAdj

APPLICATIONS INFORMATION

Basic Circuit Operation

The LM317L is a 3−terminal floating regulator. In

operation, the LM317L develops and maintains a nominal

1.25 V reference (Vref) between its output and adjustment

terminals. This reference voltage is converted to a

programming current (IPROG) by R1 (see Figure 13), and this

constant current flows through R2 to ground. The regulated

output voltage is given by:

Vout = Vref (1 +

) + IAdj R2

R2

R1

Since the current from the adjustment terminal (IAdj)

represents an error term in the equation, the LM317L was

designed to control IAdj to less than 100 mA and keep it

constant. To do this, all quiescent operating current is

returned to the output terminal. This imposes the

requirement for a minimum load current. If the load current

is less than this minimum, the output voltage will rise.

Since the LM317L is a floating regulator, it is only the

voltage differential across the circuit which is important to

performance, and operation at high voltages with respect to

ground is possible.

+

Vref

Adjust

Vin

Vout

LM317L

R1

IPROG

Vout

R2

IAdj

Vref = 1.25 V Typical

Figure 18. Basic Circuit Configuration

Load Regulation

The LM317L is capable of providing extremely good load

regulation, but a few precautions are needed to obtain

maximum performance. For best performance, the

programming resistor (R1) should be connected as close to

the regulator as possible to minimize line drops which

effectively appear in series with the reference, thereby

degrading regulation. The ground end of R2 can be returned

near the load ground to provide remote ground sensing and

improve load regulation.

External Capacitors

A 0.1

mF disc or 1.0 mF tantalum input bypass capacitor

(Cin) is recommended to reduce the sensitivity to input line

impedance.

The adjustment terminal may be bypassed to ground to

improve ripple rejection. This capacitor (CAdj) prevents

ripple from being amplified as the output voltage is

increased. A 10

mF capacitor should improve ripple

rejection about 15 dB at 120 Hz in a 10 V application.

Although the LM317L is stable with no output

capacitance, like any feedback circuit, certain values of

external capacitance can cause excessive ringing. An output

capacitance (CO) in the form of a 1.0 mF tantalum or 25 mF

aluminum electrolytic capacitor on the output swamps this

effect and insures stability.