PRODUCT SPECIFICATION

RC1585

7

Thermal Considerations

The RC1585 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 important to consider all

sources of thermal resistance from junction-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 temperature and ensure safe operating

temperatures. The electrical characteristics section

provides a separate thermal resistance and maximum

junction temperature for both the control circuitry and

the power transistor. Calculate the maximum junction

temperature for both sections to ensure that both

thermal limits are met.

For example, look at using an RC1585T to generate

5A @ 1.5V

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

Load Regulation

It is not possible to provide true remote load sensing

because the RC1585 series 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

returned to the negative side of the load. This is

illustrated in Figure 12.

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 the

resistor divider R1 connects directly to the regulator

output and not to the load. Figure 13 illustrates this

point.

If R1 connects to the load, then the effective resistance

between the regulator and the load would be:

∫∫

∫∫

∫∫

Figure 13. Connection for Best Load Regulation

Parasitic

Line Resistance

R2*

R1*

RC1585

IN

OUT

ADJ

*Connect R1 to case

Connect R2 to load

35

µA

C2

22

µF

R2

R1

C1

10

µF

RC1585

IN

OUT

ADJ

+

+

Figure 11. Basic Regulator Circuit

Figure 12. Connection for Best Load Regulation

∫∫

∫∫

∫∫

RC1585 -1.5

IN

OUT

GND

Parasitic

Line Resistance

The connection shown in Figure 13 does not

is about four milliohms 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 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.

The current out of the adjust pin adds to the current

from R1 and is typically 35

µA. Its output voltage

contribution is small and only needs consideration

when a very precise output voltage setting is

required.