AN2451 datasheet(29/55 Pages) STMICROELECTRONICS

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AN2451

Board description

Doc ID 12791 Rev 3

29/55

5.3

Thermal design

All heat dissipation is based on the heat exchange between the ST7540 IC, the PCB and the

surrounding environment.

A large PCB copper area under the device is recommended to make an easier heat transfer

from the ST7540 to the environment. The metallic slug under the device (exposed pad of

HTSSOP28 package) must be properly soldered to the copper area on the PCB top side, as

recommended in the datasheet.

The large ground layer on the bottom side of the board must be connected to the top side

layer through multiple via holes.

In the case of ST7540 reference design, an area of about 0.2 cm2 is put on the PCB top side

for exposed pad soldering, while ground layer dissipating area on the bottom side is nearly

1.5 cm2.

Figure 23.

PCB copper dissipating area for ST7540 reference design board

Even if the ST7540 features a built-in thermal shutdown circuitry which turns off the power

amplifier (PA) when the die temperature (TJ) exceeds 170 °C. It is however recommended

not to exceed 125 °C during normal operating conditions to ensure the functionality of the

IC.

The relationship between junction temperature TJ and power dissipation during

transmission PD is described by the following formula:

Equation 7

where TA is the ambient temperature (from -45 to +85 °C) and θJA is the junction to ambient

thermal impedance of the ST7540 IC, which is related to the length of the transmission (tTX)

and to the duty cycle d = tPKT / (tPKT + tIDLE), assuming a packet-fragmented transmission

as illustrated by Figure 24.

Top layer

Bottom layer

Copper area

Soldering area

Multiple Via Holes

Large GND layer

J tTX d

()

•

TX d

()

–

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