NCN49599
www.onsemi.com
25
Figure 25. R
LIM
in Function of the I
LIM
Thermal Protection
Excessive dissipation inside the amplifier, for instance
during overload conditions, can result in damaging junction
temperatures. A thermal shutdown protection monitors the
junction temperature to protect against this.
When the internal junction temperature reaches
approximately 160_C, the amplifier is disabled and placed in
a high−impedance state. The amplifier will be re−enabled −
assuming the Enable input is still active − when the junction
temperature cools back down to approximately 135_C.
Safe Operating Area
The safe operating area (SOA) of an amplifier is the
collection of output currents I
L
and the output voltages V
L
that will result in normal operation with risk of destruction
due to overcurrent or overheating.
In a normal application only the output amplifier of the
line driver must be considered; the load on the small−signal
amplifier is usually negligible.
The output amplifier SOA depends on the thermal
resistance from junction to ambient Rth
j−a
, which in turn
strongly depends on board design. Rth
j−a
= 50 K/W in free
air is a typical value, which may be used even if the host
printed circuit board (PCB) is mounted in a small closed
box, provided the transmission of frames are infrequent and
widely spread in time.
This typical value is also used in the generation of the
curves plotted in Figures 26 and 27.
Figure 27 shows the SOA in function of output current I
L
and output voltage V
L
with the ambient temperature as
independent parameter. The maximum allowed current is
800 mA RMS. For that reason it is recommended to limit the
output current by using R
LIM
= 5 kW. This current limitation
is plotted as a horizontal line. The maximal output voltage is
limited by V
CC,max
, V
OH
and V
OL
. This results in the straight
line on the right hand side of the V
L
–I
L
plot. The area below
and left from these limitations is considered as safe. The
relation between output voltage and current is the impedance
as seen at the output of the power operational amplifier.
Constant impedance lines are represented by canted lines.
Figure 26. Example SOA in VL–IL space (bottom
left corner is safe) with Rth
j−a
= 50 K/W
Although voltage−versus−current is the normal
representation of safe operating area, a PLC line driver can
only control one of these variables: voltage and current are
linked through the mains impedance. Figure 28 displays
exactly the same information as Figure 27 but might be
easier to work with. Constant current values are now
represented as canted lines.
Figure 27. Example SOA in ZL–VL space (bottom
right corner is safe)
Again, the safe operating area depends on PCB layout.
Thus, the designer must verify the performance of her
particular design [1].
Receiver Path Description
The receiver demodulates the signal on the
communication channel. Typically, an external line
coupling circuit is required to filter out the frequencies of
interest on the communication channel.
The receiver block (Figure 29 and Figure 32) filters,
digitalizes and partially demodulates the output signal of the
coupling circuit. Subsequently, the embedded
microcontroller core will demodulate the resulting digital
stream. The demodulation is described in the fact sheets of
the various firmware solutions.
