AN2744 datasheet(37/56 Pages) STMICROELECTRONICS

37 / 56 page

AN2744

Board description

37/56

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 PD 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 PD can be calculated as:

PD = PIN - POUT

where PIN = VCC x ICC and POUT = VOUTrms x IOUTrms.

The value of VCC can be inferred from the ICC 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 (ICC) results are nearly equal to the

PLI output current to the load (IOUTrms), so PD can be expressed as:

PD = (VCC - VOUTrms) x IOUTrms

Figure 34.

Packet-fragmented transmission

PKT

Transmission

in progress

Idle

state

IDLE

PKT

Transmission

in progress

Idle

state

IDLE

Figure 35.

Thermal impedance typical curve for the ST7538Q mounted on the reference design

board

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

time (s)

d=1

d=0.75

d=0.5

d=0.25

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

time (s)

d=1

d=0.75

d=0.5

d=0.25

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