9

LT1083/LT1084/LT1085

108345fe

The problem occurs with a heavy output load when the

input voltage is high and the output voltage is low, such as

immediately after removal of a short. The load line for such

a load may intersect the output current curve at two points.

If this happens, there are two stable output operating

points for the regulator. With this double intersection, the

power supply may need to be cycled down to zero and

brought up again to make the output recover.

Ripple Rejection

The typical curves for ripple rejection reflect values for a

bypassed adjustment pin. This curve will be true for all

values of output voltage. For proper bypassing and ripple

rejection approaching the values shown, the impedance of

the adjust pin capacitor at the ripple frequency should be

less than the value of R1, (normally 100

Ω to 120Ω). The

size of the required adjust pin capacitor is a function of the

input ripple frequency. At 120Hz the adjust pin capacitor

should be 25

µF if R1 = 100Ω. At 10kHz only 0.22µF is

needed.

For circuits without an adjust pin bypass capacitor, the

ripple rejection will be a function of output voltage. The

output ripple will increase directly as a ratio of the output

with the output voltage equal to 5V and no adjust pin

capacitor, the output ripple will be higher by the ratio of 5V/

1.25V or four times larger. Ripple rejection will be de-

graded by 12dB from the value shown on the typical curve.

Output Voltage

The LT1083 develops a 1.25V reference voltage between

the output and the adjust terminal (see Figure 1). 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.

with the current through R1, it represents a small error and

can usually be ignored.

Figure 1. Basic Adjustable Regulator

R1

R2

IN

OUT

50

µA

ADJ

LT1083

1083/4/5 ADJ F01

R2

R1

( )

Load Regulation

Because the LT1083 is a three-terminal device, it is not

possible to provide true remote load sensing. Load regu-

lation will be limited by the resistance of the wire connect-

ing 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 connec-

tion, with the bottom of the output divider returned to the

negative side of the load. Although it may not be immedi-

ately obvious, best load regulation is obtained when the

top of the resistor divider R1 is connected

directly to the

case

not to the load. This is illustrated in Figure 2. If R1

were connected to the load, the effective resistance be-

tween the regulator and the load would be:

R

RR

R

R

Parasitic Line

PP

×

+

⎛

⎝⎜

⎞

⎠⎟

=

21

1

,

Resistance

Figure 2. Connections for Best Load Regulation

LT1083

OUT

IN

ADJ

PARASITIC

LINE RESISTANCE

R1*

*CONNECT R1 TO CASE

CONNECT R2 TO LOAD

1083/4/5 ADJ F02

R2*