AN3213 datasheet(43/64 Pages) STMICROELECTRONICS

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AN3213

ST7570 demonstration board description

Doc ID 17429 Rev 1

43/64

Connecting the case to ground could be a good practice to reduce the effect of radiated

signals on the oscillator.

7.4

Thermal impedance and power dissipation calculation

The relationship between junction temperature (TJ) and power dissipation during

transmission (PD) is described in the following formula:

Equation 12

where TA is the ambient temperature (from -40 to +85 °C) and ZthJA is the junction to

ambient thermal impedance of the ST7570 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 38.

Figure 38.

Packet-fragmented transmission

When soldered to a proper dissipating area on the PCB, as explained above, the ST7570 IC

is characterized by a steady-state thermal resistance RthJA of about 50 °C/W. The thermal

impedance curve obtained as the power dissipation step response is given in Figure 39.

It can be seen that the transient of ZthJA takes some thousands of seconds, after which the

static value of 50 °C/W is reached. This means that during the transient phase (i.e. if the

transmission time tTX is some seconds or even less) the IC is able to dissipate a power that

is far higher than the one sustainable at steady-state.

For this reason, a complete thermal analysis requires that the characteristics of the

transmission are taken into account, i.e. duty cycle and duration, determining the value

reached by the thermal impedance and then the allowed power dissipation.

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