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LT1585-1.5

Fixed 1.5V, 4.6A and 5A Low Dropout, Fast Response GTL+ Regulators

Linear Technology 

LT1585-1.5/LT1585A-1.5

APPLICATIONS INFORMATION

Load Regulation

It is not possible to provide true remote load sensing

because the LT1585-1.5/LT1585A-1.5 are 3-terminal de-

vices. 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 GND pin of the device returned

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

Figure 3.

LT1585-1.5

RP

PARASITIC

LINE RESISTANCE

VIN IN

OUT

GND

RL

1585-1.5 F03

Figure 3. Connection for Best Load Regulation

Thermal Considerations

The LT1585-1.5/LT1585A-1.5 protect the device under

overload conditions with internal power and thermal limit-

ing circuitry. However, for normal continuous load condi-

tions, do not exceed maximum junction temperature rat-

ings. It is important to consider all sources of thermal

resistance from junction-to-ambient. These sources in-

clude 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 sec-

tion provides a separate thermal resistance and maximum

junction temperature for both the control circuitry and the

power transistor. Older regulators with a single junction-

to-case thermal resistance specification, use an average of

the two values provided here and allow excessive junction

temperatures under certain conditions of ambient tem-

perature and heat sink resistance. Calculate the maximum

junction temperature for both sections 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 heat flow. Proper

mounting ensures the best thermal flow from this area of

the package to the heat sink. Linear Technology strongly

recommends thermal compound at the case-to-heat sink

interface. Use a thermally conductive spacer if the case of

the device must be electrically isolated and include its

contribution to the total thermal resistance. Please consult

“Mounting Considerations for Power Semiconductors”

1990 Linear Applications Handbook, Volume I, Pages

RR3-1 to RR3-20. The output connects to the case of the

device in the LT1585-1.5/LT1585A-1.5.

For example, using an LT1585ACT-1.5 (TO-220, commer-

cial) and assuming:

VIN (Max Continuous) = 3.465V (3.3V + 5%), VOUT = 1.5V

IOUT = 5A

TA = 70°C, θHEAT SINK = 3°C/W

θCASE-TO-HEAT SINK = 1°C/W (with Thermal Compound)

Power dissipation under these conditions is equal to:

PD = (VIN – VOUT)(IOUT) = (3.465 – 1.5)(5A) = 9.825W

Junction temperature will be equal to:

TJ = TA + PD(θHEAT SINK + θCASE-TO-HEAT SINK + θJC)

For the Control Section:

TJ = 70°C + 9.825W (3°C/W + 1°C/W + 0.7°C/W) = 116.2°C

116.2°C < 125°C = TJMAX (Control Section Commercial

Range)

For the Power Transistor:

TJ = 70°C + 9.825W (3°C/W + 1°C/W + 3°C/W) = 138.8°C

138.8°C < 150°C = TJMAX (Power Transistor Commercial

Range)

In both cases the junction temperature is below the maxi-

mum rating for the respective sections, ensuring reliable

operation.

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