ST ST7540, Примечание к применению

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Board description AN2451
30/55 Doc ID 12791 Rev 3
Figure 24. Packet-fragmented transmission
When soldered to a proper copper area on the PCB as explained above, the IC is
characterized by a steady-state thermal impedance of about 35 °C/W. The transient of the
thermal impedance
θ
JA
can be estimated by simulating a 6-cell equivalent model, as shown
in Figure 25 . The simulated curve vs. the transmission duration and the duty cycle is also
given. It can be noticed that the transient of
θ
JA
takes several hundreds of seconds, after
which the static value of 35 °C/W is reached.
Figure 25. Equivalent model of the thermal impedance
θ
JA
of the HTSSOP28
package with exposed pad
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 power that is well above the one sustainable at
steady state. For this reason, a complete thermal analysis requires taking into account the
characteristics of the transmission, i.e. duty cycle and duration, determining the value
reached by the thermal impedance and then the allowed power dissipation.
Actual dissipated power P
D
can be calculated as:
Equation 8
where P
IN
=V
CC
*I
CC
and POUT=V
OUT rms *
I
OUT rms
. Note that power consumption by
receiving circuitry and linear regulators is considered negligible for thermal analysis
purposes. The relationship between current absorption from the power supply (I
CC
) and PA
output current to the load (I
OUT
) is shown in Figure 26 . The value of V
in
can be deduced
from the load regulation curve of the SMPS, given in Figure 37 .
t
PKT
t
TX
Transmission
in progress Idle
state
t
IDLE
1E-1 1 1E1 1E2 1E-2 1E3
10
20
30
0
40
(j
)
C1
5e-4
R2
1
C2
6e-3
R3
8
C3
17e-3
R4
11
C4
0.09
R5
9.5
C5
0.8
R6
6
C6
15
T
A
P
D
R1
0.4
R ĺ °C/W
C
ĺ
W*s/°C
t
TX
[s]
ș
JA
(t
TX
, d)
[ºC/
W]
d = 100%
d = 75%
d = 50%
d = 25%
P
D
P
IN
P
OUT
– =