AN1139 STMicroelectronics, AN1139 Datasheet - Page 45

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AN1139

Manufacturer Part Number
AN1139
Description
L6254 - L6268 - L6269 12V DISK DRIVE POWER COMBO IC
Manufacturer
STMicroelectronics
Datasheet
7.2
The thermal characteristics are influenced by many factors. These can not be described by a single thermal re-
sistance, in fact a set of experimental curves gives the best presentation. Two thermal resistance are more im-
portant, the Junction to Ambient Thermal Resistance, Rthj-a, and the Junction to case Thermal Resistance,
Rthj-c.
Rthj-a, represents the thermal resistance of the system and comprises the silicon die, the package, and any
thermal mass in contact with the package to dissipate heat to the ambient. At a given dissipation level Pd, the
increase in junction temperature Tj over ambient temperature Ta is given by:
Rthj-a is made up for many elements both within the device and external to it. If the device is considered alone,
Rthj-a is given by the dissipation path from the silicon die to the lead frame, to the molding compound. The Rthj-
a for the TQFP44 package is 40 to 45 C/W. The Figure #29 shows the equivalent thermal circuit simplified.
Figure 29. Equivalent Thermal Circuit Simplified Package
Rthj-c, is the thermal resistance from the junction to a give area of the package’s external surface. The junction
to top case thermal resistance for this package is 11 C/W. However this value is obtained in an extreme envi-
ronment where 100% of the heat is dissipated through package top case. In the application environment, only
less than 10% of the heat is dissipated through this path. Therefore, Tj should be calculated in the following way:
This gives Tj ~ Tc + Pd. Hence the junction temperature is only slightly higher than the case temperature.
Example with:
According to this condition, the Thermal Resistance Junction to Ambient should be around 42-45×°C/W (with-
out any forced air flow). If the board is mounted in some metal enclosure, this can help to remove the heat re-
ducing the Rthj-a to 35-40×°C/W (provided that there is enough space around the package to have an almost
Tj = Rthj-a * Pd.
Tj
THERMAL CHARACTERISTICS
16.
T
j
Multilayer printed circuit Board: 4 Layers.
Board size: 10x10cm ‘L’ shaped.
Die size: 20mm
Dissipating area: 20mm
Spindle Spin-Up: 2W for 3sec.
Average Power Dissipation. Spindle + Vcm: 0.5W
=
T
c
Silicon
Silicon
+
Rth
Cth
10 %
2
P
d
R
t hj c
2
Rth
Frame
Cth
Frame
Junction Temperature
Rth
Mold
Cth
Mold
AN1139 APPLICATION NOTE
Rth
T a m b
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