QIn the datasheet the part is specified using the Ambient Temperature. It is
rather difficult to measure the Ambient Temperature precicely, the Case
Temperature is better defined. Coud you please inform me what the Case
Temperature is, when the Ambient Temperature is 85C?
AThe calculation of maximum ambient temperature is a bit complicated and depends
- maximum junction temperature (inside the package on the silicon chip surface)
- power dissipated by the IC =265mW
- thermal impedance between the junction and ambient- theta_ja = 150degC/W
Tj_max = Tamb + theta_ja*Pdiss
Tamb = Tj_max- theta_ja*Pdiss = 125 - 150*0.265 = 85.25.
In this case the absolute maximum power dissipation 265mW has been chosen to
keep the Tj_max below 125degC at 85degC ambient. If the IC dissipates more
power than 265mW then the maximum ambient temperature has to be reduced
proportionally. If you use a power supply of 5.5V then the quiescent power
dissipation can be as high as 286mW, and you also need to account for power
dissipated in the input and output stages, accounting for a few 10s of mW under
normal conditions or >100mW under fault conditions.
The case temperature (I assume you mean the temperature on the top surface to
the IC package) depends on the amount of power dissipated by the IC. I don't
have any data to indicate the temperature of the package surface with the
maximum power dissipation and at +85degC ambient temperature.
Please see the attached documents from package manufacturer Amkor on the use of
the various thermal specs of packages. For the 16 pin TSSOP package
theta_ja = 150degC/W
theta_jc = 28degC/W