LX8117-xx/8117A-xx/8117B-xx

PRODUCT DA T ABOOK 1996/1997

7

Copyright © 1999

Rev. 1.4

3/99

APPLICA TION NOTES

FIGURE 2 — BASIC ADJUSTABLE REGULATOR

R2+R1

R1





LX8117-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 will never 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 LX8117 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 LX8117 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

LX8117-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