LT1129CF-3.3 Просмотр технического описания (PDF) - Linear Integrated System

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производитель

LT1129CF-3.3

Micropower Low Dropout Regulators with Shutdown

Linear Integrated System 

LT 1129/LT 1129-3.3/LT 1129-5

APPLICATIO S I FOR ATIO

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.

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 LT1129 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. Experiments have shown that the

heat spreading copper layer does not need to be electri-

cally connected to the tab of the device. The PC material

can be very effective at transmitting heat between the pad

area, attached to the tab of the device, and a ground or

power plane layer either inside or on the opposite side of

the board. Although the actual thermal resistance of the PC

material is high, the length/area ratio of the thermal

resistor between layers is small. Copper board stiffeners

and plated through holes can also be used to spread the

heat generated by power devices.

The following tables list thermal resistances for each

package. For the TO-220 package, thermal resistance is

given for junction-to-case only since this package is

usually mounted to a heat sink. Measured values of

thermal resistance for several different board sizes and

copper areas are listed for each package. All measure-

ments were taken in still air on 3/32" FR-4 board with 1-oz

10

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

112935fc

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