10

LT1584/LT1585/LT1587

load current of 10mA. The current out of the adjust pin adds

to the current from R1 and is typically 55

µA. Its output

voltage contribution is small and only needs consideration

when very precise output voltage setting is required.

LT1584

ADJ

IN

R1

R2

OUT

55

µA

LT1585 • F03

C1

10

µF

+

C2

22

µF

+

Figure 3. Basic Adjustable Regulator

Load Regulation

It is not possible to provide true remote load sensing

because the LT1584/LT1585/LT1587 are three-terminal

devices. Load regulation is limited by the resistance of the

wire connecting the regulators to the load. Load regulation

per the data sheet specification is measured at the bottom

of the package.

For fixed voltage devices, negative side sensing is a true

Kelvin connection with the ground pin of the device re-

turned to the negative side of the load. This is illustrated in

Figure 4.

LT1584-3.3

PARASITIC

LINE RESISTANCE

GND

IN

OUT

LT1585 • F04

Figure 4. Connection for Best Load Regulation

For adjustable voltage devices, negative side sensing is a

true Kelvin connection with the bottom of the output divider

returned to the negative side of the load. The best load

regulation is obtained when the top of resistor divider R1

connects directly to the regulator output and not to the

load. Figure 5 illustrates this point. If R1 connects to the

load, the effective resistance between the regulator and the

load is:

per foot with 16-gauge wire. This translates to 4mV per foot

at 1A load current. At higher load currents, this drop

represents a significant percentage of the overall regula-

tion. It is important to keep the positive lead between the

regulator and the load as short as possible and to use large

wire or PC board traces.

Figure 5. Connection for Best Load Regulation

LT1584

OUT

IN

ADJ

PARASITIC

LINE RESISTANCE

R1*

*CONNECT R1 TO CASE

CONNECT R2 TO LOAD

LT1584 • F05

R2*

Thermal Considerations

The LT1584/LT1585/LT1587 family protects the device

under overload conditions with internal power and thermal

limiting circuitry. However, for normal continuous load

conditions, do not exceed maximum junction temperature

ratings. It is important to consider all sources of thermal

resistance from junction-to-ambient. These sources in-

clude the junction-to-case resistance, the case-to-heat

sink interface resistance, and the heat sink resistance.

Thermal resistance specifications have been developed to

more accurately reflect device temperature and ensure safe

operating temperatures. The electrical characteristics sec-

tion provides a separate thermal resistance and maximum

junction temperature for both the control circuitry and the

power transistor. Older regulators, with a single junction-

to-case thermal resistance specification, use an average of

the two values provided here and allow excessive junction

temperatures under certain conditions of ambient tem-

perature and heat sink resistance. Calculate the maximum

junction temperature for both sections to ensure that both

thermal limits are met.

Junction-to-case thermal resistance is specified from the

IC junction to the bottom of the case directly below the die.