AN2744 datasheet(36/56 Pages) STMICROELECTRONICS

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Board description

AN2744

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5.3

Thermal design

All heat dissipation is based on the heat exchange between the ST7538Q IC, the PCB and

the environment.

A large PCB copper area under the device is recommended in order to achieve a better heat

transfer from the IC to the environment, see Figure 33.

The metallic slug under the ST7538Q (the exposed pad of the PwTQFP44 package) must

be properly soldered to the ground copper area on the PCB top side, as recommended in

the datasheet. For the ST7538Q dual channel reference design, the dissipating area is

nearly 1.5 cm².

The larger ground layer on the bottom side should be connected to the top side area through

multiple via holes.

Even if the ST7538Q has an integrated thermal shutdown circuitry, turning off the power

stage if the die temperature (TJ) surpasses 170 °C, it is recommended that TJ does not

exceed 125 °C to guarantee a safe condition for IC operation.

The relationship between the junction temperature TJ and the power dissipation during

transmission PD is described by the following formula:

TJ(tTX, d) = TA - PD·θJA(tTX, d)

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

thermal impedance of the ST7538Q IC.

The value of the thermal impedance depends on the length of the transmission (tTX) and on

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

depicted in Figure 34.

Figure 33.

PCB copper dissipating area for the ST7538Q dual channel reference design

Bottom Layer

Top Layer

Copper Area

Soldering Area

Multiple Via

Holes

Large GND

layer

Bottom Layer

Top Layer

Copper Area

Soldering Area

Multiple Via

Holes

Large GND

layer

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