LT1521-3.3_15 datasheet(10/16 Pages) LINER

Part #	LT1521-3.3

Description	300mA Low Dropout Regulators with Micropower Quiescent Current and Shutdown

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Manufacturer	LINER [Linear Technology]

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LT1521/LT1521-3

LT1521-3.3/LT1521-5

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output is turned off and the divider current will be zero.

Curves of Adjust Pin Voltage vs Temperature and Adjust

Pin Bias Current vs Temperature appear in the Typical

Performance Characteristics. The reference voltage at the

adjust pin has a positive temperature coefficient of ap-

proximately 15ppm/

°C. The adjust pin bias current has a

negative temperature coefficient. These effects will tend to

cancel each other.

The adjustable device is specified with the adjust pin tied

to the output pin. This sets the output voltage to 3.75V.

Specifications for output voltages greater than 3.75V will

be proportional to the ratio of the desired output voltage

to 3.75V; (VOUT/3.75V). For example: load regulation for

an output current change of 1mA to 300mA is –20mV

typical at VOUT = 3.75V. At VOUT = 12V, load regulation

would be:

(12V/3.75V)(– 20mV) = – 64mV

Thermal Considerations

The power handling capability of the device will be limited

by the maximum rated junction temperature (125

°C). The

power dissipated by the device will be made up of two

components:

1. Output current multiplied by the input/output voltage

differential: IOUT(VIN – VOUT), and

2. Ground pin current multiplied by the input voltage:

(IGND)(VIN)

APPLICATIONS INFORMATION

Figure 2. Adjustable Operation

+

VIN

VOUT

LT1521

GND

IN

SHDN

OUT

ADJ

LT1521 • F01

VOUT = 3.75V 1 +

+ (IADJ + R2)

VADJ = 3.75V

IADJ = 50nA AT 25°C

OUTPUT RANGE = 3.75V TO 20V

R2

R1

)

R1

R2

The ground pin current can be found by examining the

Ground Pin Current curves in the Typical Performance

Characteristics. Power dissipation will be equal to the sum

of the two components listed above.

The LT1521 series regulators have internal thermal limit-

ing designed to protect the device during overload condi-

tions. For continuous normal load conditions the maxi-

mum junction temperature rating of 125

°C must not be

exceeded. It is important to give careful consideration to

all sources of thermal resistance from junction to ambient.

Additional heat sources mounted nearby must also be

considered.

For surface mount devices, heat sinking is accomplished

by using the heat spreading capabilities of the PC board

and its copper traces. Copper board stiffeners and plated

through-holes can also be used to spread the heat gener-

ated by power devices.

The following tables list thermal resistance for each pack-

age. Measured values of thermal resistance for several

different board sizes and copper areas are listed for each

package. All measurements were taken in still air on

3/32" FR-4 board with one ounce copper. All NC leads were

connected to the ground plane.

Table 1. MS8 Package

COPPER AREA

THERMAL RESISTANCE

TOPSIDE**

BACKSIDE

BOARD AREA

(JUNCTION-TO-AMBIENT)

2500mm2

110

°C/W

1000mm2

2500mm2

115

°C/W

225mm2

2500mm2

120

°C/W

100mm2

2500mm2

130

°C/W

* Pin 4 is ground. ** Device is mounted on topside.

Table 2. S8 Package*

COPPER AREA

THERMAL RESISTANCE

TOPSIDE** BACKSIDE

BOARD AREA (JUNCTION-TO-AMBIENT)

2500mm2

60

°C/W

1000mm2

2500mm2

60

°C/W

225mm2

2500mm2

68

°C/W

100mm2

2500mm2

74

°C/W

* Pins 3, 6, 7 are ground. ** Device is mounted on topside.

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