PRODUCT SPECIFICATION

FAN1084

REV. 1.0.8 11/10/03

7

The current out of the adjust pin adds to the current from R1.

Its output voltage contribution is small and only needs consid-

eration when a very precise output voltage setting is required.

Figure 3. Connection for Best Load Regulation

Load Regulation

It is not possible to provide true remote load sensing because

the FAN1084 series are three-terminal devices. Load regula-

tion is limited by the resistance of the wire connecting the reg-

ulator 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 4.

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 regula-

tion is obtained when the top of the 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, then the effective resistance

between the regulator and the load would be:

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 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 5. Connection for Best Load Regulation

Thermal Conditions

The FAN1084 series protect themselves under overload con-

ditions 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-heatsink 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. The electrical

characteristics section provides a separate thermal resistance

and maximum junction temperature for both the control cir-

cuitry and the power transistor. Calculate the maximum junc-

tion temperature for both sections to ensure that both thermal

limits are met.

For example, look at using a FAN1084T to generate 4.5A @

1.5V 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:

= 60 + (9.6) * (5 + 3) = 137

°C

The junction temperature is below the maximum thermal limit.

FAN1084

ADJ

C2

22

µF

+

C1

10

µF

35

µA

+

IN

OUT

R1

R2

FAN1084-1.5

GND

Parasitic

Line Resistance

IN

OUT

FAN1084

ADJ

R1*

R2*

*Connect R1 to case

Connect R2 to load

Parasitic

Line Resistance

IN

OUT