Microsemi Inc.

Integrated Products Division

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

Page 6

Copyright

© 2000

Rev. 2.0b, 2005-10-25

LX8385x-xx

3A Low Dropout Positive Regulators

TM

®

OVERLOAD RECOVERY (continued)

voltage across the regulator at the time the short circuit is

removed from the output. 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:

28

.

6

1

R

F

C

×

=

where:

C

≡ the value of the capacitor in Farads; select

an equal or larger standard value.

R1 ≡

the value of resistor R1 in ohms

At a Ripple frequency of 120Hz, with R1= 100Ω:

()

μF

Hz

3

.

13

100

120

28

.

6

1

=

Ω

×

×

=

C

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:

REF

OUT

V

V

=

M

where:

M

≡ a multiplier for the ripple seen when the

ADJ pin is optimally bypassed.

For example, if V

2

25

.

1

5

.

2

=

=

V

V

M

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 LX8385/85A/85B 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.

LX8385x

R1

R2

IN

ADJ

OUT

FIGURE 2

- BASIC ADJUSTABLE REGULATOR

V

IN

I

ADJ

50µA

V

REF

V

OUT

2

1

2

1

R

I

R

R

V

V

ADJ

REF

OUT

+

⎜⎜

⎝

⎛

⎟

⎠

⎞

+

=

LOAD REGULATION

Because the LX8385/85A/85B 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:

⎟

⎠

⎞

⎜

⎝

⎛

+

×

=

1

1

2

R

R

R

R

R

P

Peff

where:

≡

Actual parasitic line resistance.

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

parasitic resistance appears as its actual value, rather than the

higher R

LX8385x

R

Resistance

R

L

R2

IN

ADJ

OUT

FIGURE 3

- CONNECTIONS FOR BEST LOAD REGULATION

R1

V

IN

Connect R1 to

Case of Regulator

Connect R2 to

Load