WM8321 datasheet(13/253 Pages) WOLFSON

13 / 253 page

Production Data

WM8321

PD, February 2012, Rev 4.0

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:

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

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.

For WM8321, ӨJA = 24C/W

The quoted ӨJA is based on testing to the EIA/JEDEC-51-2 test environment (ie. 1ft

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.

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.

PD = 500mW (example figure)

ӨJA = 24C/W

TR = PD * ӨJA = 12C

TA = 85°C (example figure)

TJ = TA +TR = 97C

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.

WM8321 데이터시트(PDF) 13 Page - Wolfson Microelectronics plc