IDT82V3255TFG【PDF 15/132 页】IC PLL WAN SMC STRATUM 3 64-TQFP

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IDT82V3255

WAN PLL

Thermal Management

111

December 3, 2008

THERMAL MANAGEMENT

The device operates over the industry temperature range -40°C ~

+85°C. To ensure the functionality and reliability of the device, the maxi-

mum junction temperature Tjmax should not exceed 125°C. In some

applications, the device will consume more power and a thermal solution

should be provided to ensure the junction temperature Tj does not

exceed the Tjmax.

7.1

JUNCTION TEMPERATURE

Junction temperature Tj is the temperature of package typically at the

geographical center of the chip where the device's electrical circuits are.

It can be calculated as follows:

Equation 1: Tj = TA + P X θJA

Where:

θJA = Junction-to-Ambient Thermal Resistance of the Package

Tj = Junction Temperature

TA = Ambient Temperature

P = Device Power Consumption

In order to calculate junction temperature, an appropriate

θJA must

be used. The

θJA is shown in Table 36:

Power consumption is the core power excluding the power dissipated

in the loads. Table 35 provides power consumption in special environ-

ments.

7.2

EXAMPLE OF JUNCTION TEMPERATURE

CALCULATION

Assume:

TA = 85°C

θJA = 21.7°C/W (TQFP/EDG64 Soldered & when airfow rate is 0 m/

P = 1.57W

The junction temperature Tj can be calculated as follows:

Tj = TA + P X θJA = 85°C + 1.57W X 21.7°C/W = 119.1°C

The junction temperature of 119.1°C is below the maximum junction

temperature of 125°C so no extra heat enhancement is required.

In some operation environments, the calculated junction temperature

might exceed the maximum junction temperature of 125°C and an exter-

nal thermal solution such as a heatsink is required.

7.3

HEATSINK EVALUATION

A heatsink is expanding the surface area of the device to which it is

attached.

θJA is now a combination of device case and heat-sink thermal

resistance, as the heat flowing from the die junction to ambient goes

through the package and the heatsink.

θJA can be calculated as follows:

Equation 2:

θJA = θJC + θCH+ θHA

Where:

θJC = Junction-to-Case Thermal Resistance

θCH = Case-to-Heatsink Thermal Resistance

θHA = Heatsink-to-Ambient Thermal Resistance

θCH+ θHA determines which heatsink and heatsink attachment can

be selected to ensure the junction temperature does not exceed the

maximum junction temperature. According to Equation 1 and 2,

θCH+ θHA can be calculated as follows:

Equation 3:

θCH+ θHA = (Tj - TA) / P - θJC

Assume:

Tj = 125°C (Tjmax)

TA = 85°C

P = 1.57W

θJC = 12.6°C/W (TQFP/EDG64)

θCH+ θHA can be calculated as follows:

θCH+ θHA = (125°C - 85°C ) / 1.57W - 12.6°C/W = 12.9°C/W

That is, if a heatsink and heatsink attachment whose θCH+ θHA is

below or equal to 12.9°C/W is used in such operation environment, the

junction temperature will not exceed the maximum junction temperature.

Table 35: Power Consumption and Maximum Junction Temperature

Package

Power

Consumption (W)

Operating

Voltage

(V)

TA (°C)

Maximum

Junction

Temperature (°C)

LQFP/PP64

1.57

3.6

125

TQFP/EDG64

1.57

3.6

125

Table 36: Thermal Data

Package

Pin Count Thermal Pad

θJC (°C/W)

θJB (°C/W)

θJA (°C/W) Air Flow in m/s

012345

LQFP/PP64

12.3

35.1

43.1

38.1

37.3

36.5

36.1

TQFP/EDG64

Yes/Exposed

12.6

35.3

37.0

32.1

30.4

29.4

28.7

28.1

TQFP/EDG64

Yes/Soldered

12.6

1.3

21.7

17.3

16.2

15.6

15.2

14.9

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