Advanced Monolithic Systems, Inc.

6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

AMS1086

APPLICATION HINTS

This current is normally the specified minimum load current of

small error and it can usually be ignored.

AMS1086

IN

OUT

ADJ

R1

R2

V

OUT

V

REF

V

IN

I

ADJ

50

µA

(1+ R2/R1)+I

Figure 1. Basic Adjustable Regulator

Load Regulation

True remote load sensing it is not possible to provide, because the

AMS1086 is a three terminal device. The resistance of the wire

connecting the regulator to the load will limit the load regulation.

The data sheet specification for load regulation is measured at the

bottom of the package. 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 is connected directly to the case not to the load. If R1

were connected to the load, the effective resistance between the

regulator and the load would be:

R1

AMS1086

IN

OUT

ADJ

R

L

R1*

R2*

V

IN

R

P

PARASITIC

LINE RESISTANCE

CONNECT R2 TO LOAD

Figure 2. Connections for Best Load Regulation

Using 16-gauge wire the parasitic line resistance is about 0.004

Ω

per foot, translating to 4mV/ft at 1A load current. It is important

to keep the positive lead between regulator and load as short as

possible and use large wire or PC board traces.

Thermal Considerations

The AMS1086 series have internal power and thermal limiting

circuitry

designed

to

protect

the

device

under

overload

conditions. However maximum junction temperature ratings

should not be exceeded under continuous normal load conditions.

Careful consideration must be given to all sources of thermal

resistance from junction to ambient, including junction-to-case,

case-to-heat sink interface and heat sink resistance itself. To

ensure safe operating temperatures and reflect more accurately

the device temperature, new thermal resistance specifications

have been developed. Unlike older regulators with a single

junction-to-case thermal resistance specification, the data section

for these new regulators provides a separate thermal resistance

and maximum junction temperature for both the Control Section

and the Power Transistor. Calculations for both temperatures

under certain conditions of ambient temperature and heat sink

resistance and 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. This is

the lowest resistance path for the heat flow. In order to ensure the

best possible thermal flow from this area of the package to the

heat sink proper mounting is required. Thermal compound at the

case-to-heat

sink

interface

is

recommended.

A

thermally

conductive spacer can be used, if the case of the device must be

electrically isolated, but its added contribution to thermal

resistance has to be considered.