Production Data
WM8321
4
THERMAL CHARACTERISTICS
Thermal analysis must be performed in the intended application to prevent the WM8321 from
exceeding maximum junction temperature. Several contributing factors affect thermal performance
most notably the physical properties of the mechanical enclosure, location of the device on the PCB
in relation to surrounding components and the number of PCB layers. Connecting the GND balls
through thermal vias and into a large ground plane will aid heat extraction.
Three main heat transfer paths exist to surrounding air:
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Package top to air (convection and radiation).
Package bottom to PCB (convection and radiation).
Package leads to PCB (conduction).
(Note that radiation is not normally significant at the moderate temperatures experienced in typical
applications.)
The temperature rise TR is given by TR = PD * ӨJA
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PD is the power dissipated by the device.
ӨJA is the thermal resistance from the junction of the die to the ambient temperature
and is therefore a measure of heat transfer from the die to surrounding air.
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For WM8321, ӨJA = 24C/W
The quoted ӨJA is based on testing to the EIA/JEDEC-51-2 test environment (ie. 1ft3
box, still air, with specific PCB stack-up and tracking rules). Note that this is not
guaranteed to reflect all typical end applications.
The junction temperature TJ is given by TJ = TA + TR
TA, is the ambient temperature.
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The worst case conditions are when the WM8321 is operating in a high ambient temperature, and
under conditions which cause high power dissipation, such as the DC-DC converters operating at low
supply voltage, high duty cycle and high output current. Under such conditions, it is possible that the
heat dissipated could cause the maximum junction temperature of the device to be exceeded. Care
must be taken to avoid this situation. An example calculation of the junction temperature is given
below.
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P
= 500mW (example figure)
D
Ө
= 24C/W
JA
TR = PD * ӨJA = 12C
= 85°C (example figure)
T
A
T = TA +TR = 97C
J
The minimum and maximum operating junction temperatures for the WM8321 are quoted in
Section 5. The maximum junction temperature is 125°C. Therefore, the junction temperature in the
above example is within the operating limits of the WM8321.
PD, February 2012, Rev 4.0
13
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
