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LT1206

1206fa

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 stiffen-

ers 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

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 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 application will

be affected by thermal interactions with other components

as well as board size and shape.

Table 1. R Package, 7-Lead DD

COPPER AREA

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE*

BACKSIDE

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. S8 Package, 8-Lead Plastic SO

COPPER AREA

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE*

BACKSIDE

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

60°C/W

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

62°C/W

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

65°C/W

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

69°C/W

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

73°C/W

100 sq. mm

225 sq. mm 2500 sq. mm

80°C/W

100 sq. mm

100 sq. mm 2500 sq. mm

83°C/W

*Pins 1 and 2 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.

APPLICATIONS INFORMATION

–

15V

–15V

0.01

µF

330

Ω

300pF

–12V

12V

f = 2MHz

39mA

LT1206 • F07

LT1206

S/D

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.

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