wm8400 Wolfson Microelectronics plc, wm8400 Datasheet - Page 9

• Manufacturer Part Number: wm8400
• Description: Wolfson Audioplustm Hi-fi Audio Codec And Power Management Unit For Mobile Multimedia
• Manufacturer: Wolfson Microelectronics plc

w
Pre-Production
THERMAL CHARACTERISTICS
Thermal analysis must be performed in the intended application to prevent the WM8400 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 ten central
GND balls through thermal vias and into a large ground plane will aid heat extraction.
Three main heat transfer paths exist to surrounding air as illustrated below:
The temperature rise T
The junction temperature T
The worst case conditions are when the WM8400 is operating in a high ambient temperature, with
low supply voltage, high duty cycle and high output current. Under such conditions, it is possible that
the heat dissipated could exceed the maximum junction temperature of the device. Care must be
taken to avoid this situation. An example calculation of the junction temperature is given below.
The minimum and maximum operating junction temperatures for the WM8400 are quoted in the
“Absolute Maximum Ratings” section. The maximum recommended operating junction temperature is
125°C. Therefore, the junction temperature in the above example exceeds the operating limits of the
WM8400, and is not recommended for sustained operation.
-
-
-
-
-
-
-
-
-
-
-
-
Package top to air (radiation).
Package bottom to PCB (radiation).
Package leads to PCB (conduction).
P
Ө
and is therefore a measure of heat transfer from the die to surrounding air.
For WM8400, Ө
T
P
Ө
T
T
T
A
R
A
J
D
D
JA
JA
, is the ambient temperature.
= T
= P
is the power dissipated by the device.
= 85°C (example figure)
= 0.98W (example figure)
is the thermal resistance from the junction of the die to the ambient temperature
= 46°C/W
A
D
R
+T
* Ө
is given by T
R
JA
J
= 130.08°C
is given by T
= 45.08°C
JA
= 46°C/W
R
= P
J
D
= T
* Ө
A
JA
+T
R
PP, April 2009, Rev 3.0
WM8400
9