FAN1587A

PRODUCT SPECIFICATION

8

REV. 1.0.5 11/10/03

Figure 12. Basic Regulator Circuit

Load Regulation

It is not possible to provide true remote load sensing because

the FAN1587A series are three-terminal devices. Load regu-

lation 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 returned

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

Figure 13.

Figure 13. 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 regu-

lation is obtained when the top of resistor divider R1 con-

nects directly to the regulator output and not to the load.

Figure 14 illustrates this point.

If R1 connects to the load, then the effective resistance

between the regulator and the load would be:

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

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

sents a significant percentage of the overall regulation. 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 14. Connection for Best Load Regulation

Thermal Considerations

The FAN1587A series protect themselves 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 impor-

tant to consider all sources of thermal resistance from junc-

tion-to-ambient. These sources include 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 tempera-

ture and ensure safe operating temperatures.

For example, look at using an FAN1587AT to generate 3A

@ 1.5V

± 2% from a 3.3V source (3.2V to 3.6V).

Assumptions:

•

a thermally conductive material)

The power dissipation in this application is:

From the specification table:

= 70 + (6.42) * (3 + 3) = 109

°C

The junction temperature is below the maximum rating.

FAN1587A

ADJ

35

µA

R2

R1

C2

22

µF

+

C1

22

µF

+

65-1587-14

IN

OUT

FAN1587A-1.5, -3.3

GND

65-1587-17

PARASITIC

LINE RESISTANCE

IN

OUT

FAN1587A

ADJ

R2*

CONNECT R1 TO CASE

CONNECT R2 TO LOAD

*

R1*

65-1587-15

PARASITIC

LINE RESISTANCE

IN

OUT