WM8351 datasheet(13/330 Pages) WOLFSON

13 / 330 page

Production Data

WM8351

PD, April 2012, Rev 4.5

THERMAL CHARACTERISTICS

Thermal analysis must be performed in the intended application to prevent the WM8351 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 nine 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:

Package top to air (radiation).

Package bottom to PCB (radiation).

Package leads to PCB (conduction).

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 WM8351, ӨJA = 32C/W

The junction temperature TJ is given by TJ = TA + TR

TA, is the ambient temperature.

The worst case conditions are when the WM8351 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.

PD = 1W (example figure)

ӨJA = 32°C/W

TR = PD * ӨJA = 32°C

TA = 85°C (example figure)

TJ = TA +TR = 117°C

The minimum and maximum operating junction temperatures for the WM8351 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 WM8351.

WM8351 Fiches technique(PDF) 13 Page - Wolfson Microelectronics plc