LT1521 datasheet(11/18 Pages) AD

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LT1521/LT1521-3

LT1521-3.3/LT1521-5

Rev. C

For more information www.analog.com

APPLICATIONS INFORMATION

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

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE** BACKSIDE

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

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE** BACKSIDE

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.

Table 3. SOT-223 Package

(Thermal Resistance Junction-to-Tab 20°C/W)

COPPER AREA

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE*

BACKSIDE

2500mm2

50°C/W

1000mm2

2500mm2

50°C/W

225mm2

2500mm2

58°C/W

100mm2

2500mm2

64°C/W

1000mm2

57°C/W

1000mm2

60°C/W

* Tab of device attached to topside copper.

Calculating Junction Temperature

Example: Given an output voltage of 3.3V, an input volt-

age range of 4.5V to 7V, an output current range of 0mA

to 150mA and a maximum ambient temperature of 50°C,

what will the maximum junction temperature be?

The power dissipated by the device will be equal to:

IOUT(MAX)(VIN(MAX) – VOUT) + IGND(VIN(MAX))

Where,

IOUT(MAX) = 150mA

VIN(MAX) = 7V

IGND at (IOUT = 150mA, VIN = 7V) = 2.1mA

So,

P = 150mA(7V – 3.3V) + (2.1mA)(7V) = 0.57W

If we use a SOT-223 package, then the thermal resistance

will be in the range of 50°C/W to 65°C/W depending on

the copper area. So the junction temperature rise above

ambient will be approximately equal to:

0.57W(60°C/W) = 34.2°C

The maximum junction temperature will then be equal to

the maximum junction temperature rise above ambient

plus the maximum ambient temperature or:

TJMAX = 50°C + 34.2°C = 84.2°C

Output Capacitance and Transient Performance

The LT1521 is designed to be stable with a wide range

of output capacitors. A minimum output capacitor of

1.5µF is required to prevent oscillations. The LT1521 is

a micropower device and output transient response will

be a function of output capacitance. See the Transient

Response curves in the Typical Performance Character-

istics. Larger values of output capacitance will decrease

the peak deviations and provide improved output transient

responseforlargerloadcurrentdeltas.Bypasscapacitors,

used to decouple individual components powered by the

LT1521, will increase the effective value of the output

capacitor.

Protection Features

The LT1521 incorporates several protection features

which make it ideal for use in battery-powered circuits.

In addition to the normal protection features associated

with monolithic regulators, such as current limiting and

thermal limiting, the device is protected against reverse

input voltages, reverse output voltages and reverse volt-

ages from output to input.

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