LX8586 / LX8586A

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 5

Copyright

© 1996

Rev. 1.0a, 2005-11-10

6A Low Dropout Positive Regulators

TM

®

APPLICATION NOTE

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 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:

Where:

C

≡ the value of the capacitor in Farads; select an

equal or larger standard value.

≡ 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 * 100

Ω) = 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:

Where:

M

≡ a multiplier for the ripple seen when the ADJ

pin is optimally bypassed.

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.

The LX8586/86A 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

constant when compared with the current through R1, it represents

a small error and can usually be ignored.

LX8586/86A

OUT

IN

ADJ

V

OUT

V

IN

R1

R2

V

REF

I

ADJ

50µA

V

+ I

R2

R1

Figure 2 -

Basic Adjustment Regulator

Because the LX8586/86A 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:

⎟

⎠

⎞

⎜

⎝

⎛

+

=

R1

R1

R2

*

R

R

P

Peff

≡ Actual parasitic line resistance.

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

LX8586/86A

OUT

IN

ADJ

V

IN

R1

R2

R

L

R

P

Parasitic

Line Resistance

Connect

R2

to Load

Connect

R1 to Case

of Regulator

Figure 3 -

Connections for Best Load Regulation