LT1129IST-3.3TRPBF Просмотр технического описания (PDF) - Linear Technology

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LT1129IST-3.3TRPBF

Micropower Low Dropout Regulators with Shutdown

Linear Technology 

LT1129/LT1129-3.3/LT1129-5

OPERATION

copper. This data can be used as a rough guideline in

estimating thermal resistance. The thermal resistance for

each application will be affected by thermal interactions

with other components as well as board size and shape.

Some experimentation will be necessary to determine the

actual value.

Table 1. Q Package, 5-Lead DD

COPPER AREA

THERMAL RESISTANCE

TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)

2500 sq. mm 2500 sq. mm 2500 sq. mm

25°C/ W

1000 sq. mm 2500 sq. mm 2500 sq. mm

27°C/W

125 sq. mm 2500 sq. mm 2500 sq. mm

35°C/W

* Tab of device attached to topside copper

Table 2. ST Package, 3-Lead SOT-223

COPPER AREA

THERMAL RESISTANCE

TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)

2500 sq. mm 2500 sq. mm 2500 sq. mm

45°C/ W

1000 sq. mm 2500 sq. mm 2500 sq. mm

45°C/W

225 sq. mm 2500 sq. mm 2500 sq. mm

53°C/W

100 sq. mm 2500 sq. mm 2500 sq. mm

59°C/W

* Tab of device attached to topside copper

Table 3. S8 Package, 8-Lead Plastic SOIC

COPPER AREA

THERMAL RESISTANCE

TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)

2500 sq. mm 2500 sq. mm 2500 sq. mm

55°C/ W

1000 sq. mm 2500 sq. mm 2500 sq. mm

55°C/W

225 sq. mm 2500 sq. mm 2500 sq. mm

63°C/W

100 sq. mm 2500 sq. mm 2500 sq. mm

69°C/W

* Device attached to topside copper

T Package, 5-Lead TO-220

Thermal Resistance (Junction-to-Case) = 5°C/W

Calculating Junction Temperature

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

range of 4.5V to 5.5V, an output current range of 0mA to

500mA, 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 = 500mA

VIN MAX = 5.5V

IGND at (IOUT = 500mA, VIN = 5.5V) = 25mA

so, P = 500mA • (5.5V – 3.3V) + (25mA • 5.5V)

= 1.24W

If we use a DD package, then the thermal resistance will be

in the range of 25°C/W to 35°C/W depending on copper

area. So the junction temperature rise above ambient will

be approximately equal to:

1.24W • 30°C/W = 37.2°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 + 37.2°C = 87.2°C

Output Capacitance and Transient Performance

The LT1129 is designed to be stable with a wide range

of output capacitors. The minimum recommended value

is 3.3μF with an ESR of 2Ω or less. The LT1129 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

response. Bypass capacitors, used to decouple individual

components powered by the LT1129, will increase the

effective value of the output capacitor.

Protection Features

The LT1129 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, and reverse voltages from output to input.

For fixed voltage devices the output and sense pins are

tied together at the output.

112935ff

11

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