9

LT1529

LT1529-3.3/LT1529-5

S

APPLICATI

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

2. Ground pin current multiplied by the input voltage:

The GND pin current can be found by examining the GND

Pin Current curves in the Typical Performance Character-

istics. Power dissipation will be equal to the sum of the two

components listed above.

The LT1529 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 copper areas are

listed for the DD package. All measurements were taken in

still air on 3/32" FR-4 board with 1-oz copper. This data can

be used as a rough guideline in estimating thermal resis-

tance. 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

TOPSIDE*

BACKSIDE

BOARD AREA

2500 sq. mm 2500 sq. mm

2500 sq. mm

23

°C/W

1000 sq. mm 2500 sq. mm

2500 sq. mm

25

°C/W

125 sq. mm

2500 sq. mm

2500 sq. mm

33

°C/W

* Device is mounted on topside.

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

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:

so,

P = 500mA • (5.5V – 3.3V) + (3.6mA • 5.5V)

= 1.12W

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

in the range of 23

°C/W to 33°C/W depending on copper

area. So the junction temperature rise above ambient will

be approximately equal to:

1.12W • 28

°C/W = 31.4°C

The maximum junction temperature will then be equal to

the maximum junction temperature rise above ambient

plus the maximum ambient temperature or:

Output Capacitance and Transient Performance

The LT1529 is designed to be stable with a wide range of

output capacitors. The minimum recommended value is

3.3

µF with an ESR of 2Ω or less. The LT1529 is a

T Package, 5-Lead TO-220

Thermal Resistance (Junction-to-Case) = 2.5

°C/W