NCN5150NGEVB
http://onsemi.com
7
FUNCTIONAL DESCRIPTION
Power Supply
The NCN5150 provides power to be used in the
application. To do this, the NCN5150 draws a fixed current
from the bus. The amount of current drawn is fixed at a
number of unit loads (intervals of 1.5 mA) and can be set by
changing the value of the RIDD resistor. The SOIC version
can support up to 2 UL, while the QFN version can support
up to 6 UL. The required resistors for are shown in table xx.
Also shown in table xx is the maximum amount of
capacitance allowed on the STC pin. This limit will ensure
that the transceiver will start up in less than 3 s as required
by the M-BUS standard. Using a smaller capacitor on STC
is allowed, and will result in faster start-up, but also in a
faster shutdown, reducing the time between the early
warning PFb toggling (when the bus is removed) and the
point where the 3.3V VDD voltage can no longer be
maintained. The minimum allowed capacitance on STC is
10 times the capacitance on the VDD pin. The minimum
required capacitance on the VDD pin is 1
m
F.
Table 4. UL, RIDD VALUES, STC CAPACITANCE
Number of
Unit Loads
RIDD
Value
Maximum STC
Capacitor Value
Min. Avail-
able Current
1
30 k
W
330
m
F
0.88 mA
2
13 k
W
820
m
F
2.10 mA
3
8.45 k
W
1200
m
F
3.10 mA
4
6.19 k
W
1500
m
F
4.20 mA
5
4.87 k
W
2200
m
F
5.30 mA
6
4.02 k
W
2700
m
F
6.50 mA
Shown in Figure 6 is the startup sequence of the device.
Note that the NCN5150 does not require any external
(PMOS) transistor on the STC pin for proper startup. The
yellow waveform is the bus voltage, the purple waveform
the STC voltage, the green waveform the VDD output, and
the blue waveform the TX output.
Figure 6. NCN5150 Startup Sequence
Shown in Figure 7 is the shutdown sequence of the device.
The green waveform is the bus voltage, the blue waveform
is the VDD output, the purple waveform is the PFb output,
and the yellow waveform is the TX output.
