LX8382-xx

PRODUCT DA T ABOOK 1996/1997

5

Copyright © 1997

Rev. 1.0

8/97

APPLICA TION NOTES

FIGURE 2 — BASIC ADJUSTABLE REGULATOR

R2+R1

R1





LX8382/82A-xx

OUT

IN

ADJ

V

OUT

V

IN

R1

R2

V

REF

I

ADJ

50µA

V

1 +

+ I

R2

R1

OVERLOAD RECOVERY (continued)

If this limited current is not sufficient to develop the designed

voltage across the output resistor, the voltage will stabilize at some

lower value, and willnever reach the designed value. Under these

circumstances, it may be necessary to cycle the input voltage

down to zero in order to make the regulator output voltage return

to regulation.

RIPPLE REJECTION

Ripple rejection can be improved by connecting a capacitor

between the ADJ pin and ground. The value of the capacitor should

be chosen so that the impedance of the capacitor is equal in

magnitude to the resistance of R1 at the ripple frequency. The

capacitor value can be determined by using this equation:

C = 1 / (6.28 * F

where: C

≡ the value of the capacitor in Farads;

select an equal or larger standard value.

F

R

≡ the ripple frequency in Hz

R1

≡ the value of resistor R1 in ohms

At a ripple frequency of 120Hz, with R1 = 100

Ω:

C = 1 / (6.28 * 120Hz * 100Ω) = 13.3µF

The closest equal or larger standard value should be used, in this

case, 15µF.

When an ADJ pin bypass capacitor is used, output ripple

amplitude will be essentially independent of the output voltage. If

an ADJ pin bypass capacitor is not used, output ripple will be

proportional to the ratio of the output voltage to the reference

voltage:

M = V

where: M

≡ a multiplier for the ripple seen when the

ADJ pin is optimally bypassed.

V

For example, if V

M = 2.5V/1.25V= 2

Output ripple will be twice as bad as it would be if the ADJ pin

were to be bypassed to ground with a properly selected capacitor.

OUTPUT VOLTAGE

The LX8382/82A ICs develop a 1.25V reference voltage between the

output and the adjust terminal (See Figure 2). By placing a resistor, R1,

between these two terminals, a constant current is caused to flow

through R1 and down through R2 to set the overall output voltage.

Normally this current is the specified minimum load current of 10mA.

Because I

through R1, it represents a small error and can usually be ignored.

LOAD REGULATION

Because the LX8382/82A regulators are three-terminal devices, it

is not possible to provide true remote load sensing.

Load

regulation will be limited by the resistance of the wire connecting

the regulator to the load. The data sheet specification for load

regulation is measured at the bottom of the package. Negative

side sensing is a true Kelvin connection, with the bottom of the

output divider returned to the negative side of the load. Although

it may not be immediately obvious, best load regulation is

obtained when the top of the resistor divider, (R1), is connected

directly to the case of the regulator, not to the load.

This is

illustrated in Figure 3. If R1 were connected to the load, the

effective resistance between the regulator and the load would be:

R

where: R

When the circuit is connected as shown in Figure 3, the parasitic

resistance appears as its actual value, rather than the higher R

LX8382/82A-xx

OUT

IN

ADJ

V

IN

R1

R2

R

L

R

P

Parasitic

Line Resistance

Connect

R1 to Case

of Regulator

Connect

R2

to Load

FIGURE 3 — CONNECTIONS FOR BEST LOAD REGULATION