AM2908

Motor Driver ICs

AMtek semiconductors

July. 2020 V0.2

- 9 -

Θja

junction-to-ambient thermal resistance

??℃/W

Ψjt

junction-to-top characterization parameter

??℃/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:

Tj

Tt

P

T

T

d

t

j

jt

/

)

(

−

=

ψ

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.