ORSPI4 datasheet(258/263 Pages) LATTICE

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Lattice Semiconductor

ORCA ORSPI4 Data Sheet

258

Package Information

Package Thermal Characteristics Summary

There are three thermal parameters that are in common use:

ΘJA, ψJC, and ΘJC. It should be noted that all the

parameters are affected, to varying degrees, by package design (including paddle size) and choice of materials,

the amount of copper in the test board or system board, and system airﬂow.

ΘJA

This is the thermal resistance from junction to ambient (theta-JA, R-theta, etc.):

(1)

where TJ is the junction temperature, TA, is the ambient air temperature, and Q is the chip power.

Experimentally,

ΘJA is determined when a special thermal test die is assembled into the package of interest, and

the part is mounted on the thermal test board. The diodes on the test chip are separately calibrated in an oven. The

package/board is placed either in a JEDEC natural convection box or in the wind tunnel, the latter for forced con-

vection measurements. A controlled amount of power (Q) is dissipated in the test chip’s heater resistor, the chip’s

temperature (TJ) is determined by the forward drop on the diodes, and the ambient temperature (TA) is noted. Note

that

ΘJA is expressed in units of °C/W.

ψJC

This JEDEC designated parameter correlates the junction temperature to the case temperature. It is generally

used to infer the junction temperature while the device is operating in the system. It is not considered a true thermal

resistance and it is deﬁned by:

(2)

where TC is the case temperature at top dead center, TJ is the junction temperature, and Q is the chip power. Dur-

ing the

ΘJA measurements described above, besides the other parameters measured, an additional temperature

reading, TC, is made with a thermocouple attached at top-dead-center of the case.

ψJC is also expressed in units

of °C/W.

ΘJC

This is the thermal resistance from junction to case. It is most often used when attaching a heat sink to the top of

the package. It is deﬁned by:

(3)

The parameters in this equation have been deﬁned above. However, the measurements are performed with the

case of the part pressed against a water-cooled heat sink to draw most of the heat generated by the chip out the

top of the package. It is this difference in the measurement process that differentiates

ΘJC from ψJC. ΘJC is a true

thermal resistance and is expressed in units of °C/W.

ΘJB

This is the thermal resistance from junction to board. It is deﬁned by:

(4)

where TB is the temperature of the board adjacent to a lead measured with a thermocouple. The other parameters

on the right-hand side have been deﬁned above. This is considered a true thermal resistance, and the measure-

ΘJA

–

-------------------

ψJC

–

-------------------

ΘJC

–

-------------------

ΘJB

–

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