AM8919

Motor Driver ICs

http://www.amtek-semi.com

1

Specifications subject to change without notice

- 5 -

Θja

junction-to-ambient thermal resistance

32.98℃/W

Ψjt

junction-to-top characterization parameter

0.82℃/W



Θja is obtained in a simulation on a JEDEC-standard 2s2p board as specified in JESD-51.



The Θja number listed above gives an estimate of how much temperature rise is expected if the

device was mounted on a standard JEDEC board.



When mounted on the actual PCB, the Θja value of JEDEC board is totally different than the

Θja value of actual PCB.



Ψjt is extracted from the simulation data to obtain Θja using a procedure described in JESD-51,

which estimates the junction temperature of a device in an actual PCB.



The thermal characterization parameter, Ψjt, is proportional to the temperature difference

between the top of the package and the junction temperature. Hence, it is useful value for an

engineer verifying device temperature in an actual PCB environment as described in JEDEC

JESD-51-12.



When Greek letters are not available, Ψjt is written Psi-jt.



Definition:

Where :

Ψjt (Psi-jt) = Junction-to-Top(of the package)

C/W

Tj= Die Junction Temp.

C

Tt= Top of package Temp at center.

C

Pd= Power dissipation. Watts



Practically, most of the device heat goes into the PCB, there is a very low heat flow through top

of the package, So the temperature difference between Tj and Tt shall be small, that is any error

caused by PCB variation is small.



This constant represents that Ψjt is completely PCB independent and could be used to predict

the Tj in the environment of the actual PCB if Tt is measured properly.