AN4068 datasheet(46/63 Pages) STMICROELECTRONICS

46 / 63 page

Design guidelines

AN4068

46/63

Doc ID 022923 Rev 2

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

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

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

The thermal impedance as a response to dissipation at different duty cycle and duration

values can be estimated by simulating a 6-cell equivalent model obtained through the curve

fitting from Figure 37, as shown in Figure 38.

Figure 38.

Simulation model of the thermal impedance ZthJA of the ST7580 mounted

on the reference design board

The actual dissipated power PD can be calculated as:

Equation 11

where

and

. Note that power consumption by the

receiving circuitry and linear regulators is considered negligible for thermal analysis

purposes. The relationship between current absorption from the power supply (ICC) and PA

output current to the load (IOUT) is shown in Figure 2.

A transmission output level VOUT rms of 2.5 V, together with the current limit IOUT rms(LIMIT) of

1 A, corresponds to a maximum output power POUT of 2.5 W over a 1.5 Ω line load

(considering a 1

Ωcoupling series impedance in transmission at 86 kHz frequency). In these

conditions, the required dissipation results as follows:

Equation 12

Referring to the relationship between dissipated power and temperature, it can be proved

that in a continuous transmission, i.e. with ZthJA at its steady-state value of 50 °C/W, with an

ambient temperature of 25 °C, the maximum dissipation can be 2 W. However, by controlling

the transmission duty cycle and total duration it is possible to obtain a higher dissipation.

OUT

−

⋅

OUT

OUTrms

⋅

)

(

)

(

)

LIMIT

(

OU T

)

LIMIT

(

)

LIMIT

(

⋅

−

⋅

≅

−

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