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 V
OUT rms
of 2 V and the current limit I
OUT rms(LIMIT)
of 500 mA,
fixed for the ST7540 reference design, correspond to a maximum output power P
OUT
of 1 W
over a 4
Ω
load. In these conditions, the required dissipation results equal:
Equation 9
shows the curve of P
D
vs. the load impedance modulus according to the V
OUT rms
and I
OUT 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
θ
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 t
TX
of 1 s and a duty cycle d of 50%, according
to the graph of
θ
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.
P
D LIMIT
(
)
P
IN LIMIT
(
)
P
OUT LIMIT
(
)
11.7V
0.25A
•
(
)
2V
0.5A
•
(
)
2W
≅
–
≅
–
=
