LT1206CS8 datasheet(12/16 Pages) LINER

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LT1206

COPPER AREA

power plane layer either inside or on the opposite side of

the board. Although the actual thermal resistance of the

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

resistance between the layer is small. Copper board stiff-

eners and plated through holes can also be used to spread

the heat generated by the device.

Tables 1 and 2 list thermal resistance for each package. For

the TO-220 package, thermal resistance is given for junc-

tion-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 surface mount package. All measurements were

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

data can be used as a rough guideline in estimating

thermal resistance. The thermal resistance for each appli-

cation will be affected by thermal interactions with other

components as well as board size and shape.

Table 1. R Package, 7-Lead DD

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA (JUNCTION-TO-AMBIENT)

2500 sq. mm

°C/W

1000 sq. mm

2500 sq. mm

°C/W

125 sq. mm

2500 sq. mm

°C/W

*Tab of device attached to topside copper

Table 2. S8 Package, 8-Lead Plastic SOIC

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA (JUNCTION-TO-AMBIENT)

2500 sq. mm

°C/W

1000 sq. mm

2500 sq. mm

°C/W

225 sq. mm

2500 sq. mm

°C/W

100 sq. mm

2500 sq. mm

°C/W

100 sq. mm

1000 sq. mm

2500 sq. mm

°C/W

100 sq. mm

225 sq. mm

2500 sq. mm

°C/W

100 sq. mm

2500 sq. mm

°C/W

*Pins 1 and 8 attached to topside copper

Y Package, 7-Lead TO-220

Thermal Resistance (Junction-to-Case) = 5

°C/W

N8 Package, 8-Lead DIP

Thermal Resistance (Junction-to-Ambient) = 100

°C/W

Calculating Junction Temperature

The junction temperature can be calculated from the

equation:

TJ = (PD × θJA) + TA

where:

TJ = Junction Temperature

TA = Ambient Temperature

PD = Device Dissipation

θJA = Thermal Resistance (Junction-to Ambient)

As an example, calculate the junction temperature for the

circuit in Figure 7 for the N8, S8, and R packages assuming

a 70

°C ambient temperature.

APPLICATI

I FOR ATIO

Figure 7. Thermal Calculation Example

The device dissipation can be found by measuring the

supply currents, calculating the total dissipation, and

then subtracting the dissipation in the load and feedback

network.

PD = (39mA × 30V) – (12V)

2/(2k||2k) = 1.03W

Then:

TJ = (1.03W × 100°C/W) + 70°C = 173°C

for the N8 package

TJ = (1.03W × 65°C/W) × + 70°C = 137°C

for the S8 with 225 sq. mm topside heat sinking

TJ = (1.03W × 35°C/W) × + 70°C = 106°C

for the R package with 100 sq. mm topside

heat sinking

Since the Maximum Junction Temperature is 150

°C, the

N8 package is clearly unacceptable. Both the S8 and R

packages are usable.

–

15V

–15V

0.01

µF

330

Ω

300pF

–12V

12V

f = 2MHz

39mA

LT1206 • F07

LT1206

S/D

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