NCS5651

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10

Safe Operating Area

The safe operating area (SOA) of an amplifier is the

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

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 14 and 15.

independent parameter. The maximum allowed current is

800 mA RMS. For that reason it is recommended to limit the

W. This current limitation

is plotted as a horizontal line. The maximal output voltage

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 14. Example SOA in VL–IL Space (bottom

left corner is safe) with Rthj−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 15 displays

exactly the same information as Figure 14 but might be

easier to work with. Constant current values are now

represented as canted lines.

Figure 15. 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].

Digital Power Supply GND−Reference and Translators

In many mixed signal applications analog GND and

digital GND are not at the same potential. To minimize GND

loop issues, the NCS5651 has a separate GND pin (pin 20)

which should be used to reference the digital supply and the

warning flags (pins 16, 17, and 18). In most applications this

would be the same GND reference used for the PLC modem.

Please note that at some point in the application digital GND

and analog GND must be tied together.

REFERENCES

In this document references are made to:

1. ON Semiconductor, Design Manual NCN49597/9,

December 2014. The latest version is available

from your sales representative.

2. ON Semiconductor. AND8402/D Thermal

Considerations for the NCS5651 (application

note). 2014−08−01. Online at

http://www.onsemi.com/pub/Collateral/AND8402

−D.PDF

3. ON Semiconductor. AND9165/D. Getting started

with power line communication (application note).

2014−11−01. Online at

http://www.onsemi.com/pub_link/Collateral/AND

9165−D.PDF