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LT1585A

5A Low Dropout Fast Response Positive Adjustable and Fixed Voltage Regulators

ON Semiconductor 

LT1585A

Figure 11. Basic Adjustable Regulator

Vin

C1 +

10 mF

LT1585A

IN

OUT

Vout

ADJ

Vref

+

R1

C2

10 mF

Iadj

55 mA

R2

Vout = Vref (1 + R2/R1) + Iadj (R2)

Load Regulation

It is not possible to provide true remote load sensing

because the LT1585A are 3–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 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 resistor divider R1

connects directly to the regulator output and not to the load.

Figure 4 illustrates this point. If R1 connects to the load, the

effective resistance between the regulator and the load is :

RP (1 + R2/R1), RP = Parasitic Line Resistance

The connection shown in Figure 11 does not multiply RP

by the divider ratio. As an example, RP 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.

Thermal Considerations

The LT1585A protects the device 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. 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

temperature 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. It is strongly recommended to use

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.

For example, using an LT1585ACT (TO–220 package)

with an output adjusted to 3.3 V and assuming:

Vin (Max Continuous) = 5.25V (5V + 5%), Vout = 3.3V,

TJ IOUT = 5A.

TA = 70°C, qHEAT SINK = 3°C/W

qCASE–TO–HEAT SINK = 1°C/W (with Thermal Compound)

Power dissipation under these conditions is equal to:

Figure 12. Connection for

Best Load Regulation

LT1585A

PD

Parasitic

Line Resistance

Vin

IN

OUT

ADJ

R1*

FL

PD = (Vin – Vout) (Iout) = (5.25 – 3.3) (5) = 9.75W

Junction temperature will be equal to:

TJ = TA + PD (qHEAT SINK + qCASE–TO–HEAT SINK + qJC)

For the Control Section:

TJ = 70°C + 9.75W (3°C/W + 1°C/W + 0.7°C/W)

TJ = 115.8°C

115.8°C < 125°C = TJMAX

(Control Section Commercial Range)

For the Power Transistor:

* Connect R1 to Case

R2*

* Connect R2 to Load

TJ = 70°C + 9.75W (3°C/W + 1°C/W + 3°C/W)

TJ = 138.3°C

138.3°C < 150°C = TJMAX

(Control Section Commercial Range)

In both cases the junction temperature is below the

maximum rating for the respective sections, ensuring reliable

operation.

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MOTOROLA ANALOG IC DEVICE DATA

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