AN2451 datasheet(31/55 Pages) STMICROELECTRONICS

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AN2451

Board description

Doc ID 12791 Rev 3

31/55

Figure 26.

Output current vs. supply current typical curve for ST7540 in Tx mode

The transmission output level VOUT rms of 2 V and the current limit IOUT rms(LIMIT) of 500 mA,

fixed for the ST7540 reference design, correspond to a maximum output power POUT of 1 W

over a 4

Ω load. In these conditions, the required dissipation results equal:

Equation 9

Figure 27 shows the curve of PD vs. the load impedance modulus according to the VOUT rms

and IOUT rms(LIMIT) set for the ST7540 reference design.

Figure 27.

Dissipated power vs. load impedance modulus typical curve for ST7540

reference design board

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

that in a continuous transmission, i.e. with

JA at its steady-state value of 35 °C/W, a 2 W

dissipation can be sustained in safe conditions if the ambient temperature remains below 55

°C. Instead, supposing a transmission time tTX of 1 s and a duty cycle d of 50%, according

to the graph of Figure 25 the

JA would be 15 °C/W only. In this case a power dissipation of

2.7 W (corresponding to a 1

Ω load) is allowed over the entire ambient temperature range of

the ST7540.

D LIMIT

()

IN LIMIT

()

OUT LIMIT

()

11.7V

0.25A

•

()

0.5A

•

() 2W

≅

–

≅

–

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