ST ST7538Q, Примечание к применению

Страница: 37 / 56

AN2744 Board description
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When soldered to a proper copper area on the PCB, according to the suggestions
previously given, the IC is characterized by a steady-state thermal impedance of about 35
°C/W. Nevertheless, as shown in
Figure 35 , the steady-state value is reached after a
transient whose duration depends on the duty cycle (d) of the transmission. In other words,
a higher P
D
can be sustained if the transmission time is less than the transient completion
time and if the duty cycle of the transmission is lower than 100%.
Actual dissipated power P
D
can be calculated as:
P
D
= P
IN
- P
OUT
where P
IN
= V
CC
x I
CC
and P
OUT
= V
OUTrms
x I
OUTrms
.
The value of V
CC
can be inferred from the I
CC
value according to the load regulation curve of
the power supply, shown in Figure 44 on page 45 in Section 5.7 . Considering the power
consumption by receiving circuitry and linear regulators negligible for thermal analysis
purposes, the current absorption from the power supply (I
CC
) results are nearly equal to the
PLI output current to the load (I
OUTrms
), so P
D
can be expressed as:
P
D
= (V
CC
- V
OUTrms
) x I
OUTrms
Figure 34. Packet-fragmented transmission
t
PKT
t
TX
Transmission
in progress
Idle
state
t
IDLE
t
PKT
t
TX
Transmission
in progress
Idle
state
t
IDLE
Figure 35. Thermal impedance typical curve for the ST7538Q mounted on the reference design
board
0
5
10
15
20
25
30
35
40
1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03
time (s)
θ ja
(
º
C/
W
)
d=1
d=0.75
d=0.5
d=0.25
0
5
10
15
20
25
30
35
40
1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03
time (s)
θ ja
(
º
C/
W
)
d=1
d=0.75
d=0.5
d=0.25