AN3213 Datasheet, PDF (43/64 Pages) STMicroelectronics – ST7570 S-FSK power line networking system-on-chip

AN3213
ST7570 demonstration board description
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 (T_J) and power dissipation during
transmission (P_D) is described in the following formula:
Equation 12
where T_A is the ambient temperature (from -40 to +85 °C) and Zth_JA is the junction to
ambient thermal impedance of the ST7570 IC, which is related to the length of the
transmission (t_TX) and to the duty cycle d = t_PKT / (t_PKT + t_IDLE), assuming a packet-
fragmented transmission as illustrated by Figure 38.
Figure 38.
Packet-fragmented transmission
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When soldered to a proper dissipating area on the PCB, as explained above, the ST7570 IC
is characterized by a steady-state thermal resistance Rth_JA 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 Zth_JA 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 t_TX 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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