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AMS1587CD

3A LOW DROPOUT VOLTAGE REGULATOR

Advanced Monolithic Systems Inc 

APPLICATION HINTS

This current is normally the specified minimum load current of

10mA. Because IADJ is very small and constant it represents a

small error and it can usually be ignored.

AMS1587

VIN

IN OUT

VOUT

ADJ

VREF R1

IADJ

50µA

R2

VOUT = VREF (1+ R2/R1)+IADJR2

Figure 1. Basic Adjustable Regulator

Load Regulation

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

AMS1587 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:

RP x ( R2+R1 ) ,

R1

AMS1587

VIN

IN OUT

ADJ

RP = Parasitic Line Resistance

RP

PARASITIC

LINE RESISTANCE

R1*

RL

R2*

AMS1587

Connected as shown, RP is not multiplied by the divider ratio.

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

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 AMS1587 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.

*CONNECT R1 TO CASE

CONNECT R2 TO LOAD

Figure 2. Connections for Best Load Regulation

Advanced Monolithic Systems, Inc. www.advanced-monolithic.com Phone (925) 443-0722 Fax (925) 443-0723

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