NCN5150
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9
of the NCN5150. The R
IDD
resistor used must be at least 1%
accurate. Note that using 5 and 6 Unit Loads is not covered
by the M-BUS standard.
When the voltage on the STC pin reaches V
STC, VDD ON
the LDO is turned on, and will regulate the voltage on the
VDD pin to 3.3 V, drawing current from the storage
capacitor. A decoupling capacitor of minimum 1 mF is
required on the VDD pin for stability of the regulator. On the
STC pin, a minimum capacitance of 10 mF is required.
Furthermore, the ratio C
STC
/C
VDD
must be larger than 9.
The voltage on the STC pin is clamped to V
STC, clamp
by a
shunt regulator, which will dissipate any excess current that
is not used by the NCN5150 or external circuits.
Slave Power Supply (External)
In case the external sensor circuit consumes more than the
allowed bus current or the sensor should be kept operational
when the bus is not present, an external power supply, such
as a battery, is required.
When the external circuitry uses different logical voltage
levels, simply connect the power supply of that voltage level
to V
IO
, so that the RX, RXI, TX, TXI and PFb pins will
respond to the correct voltage levels. The NCN5150 will still
be powered from the bus, but all communication will be
translated to the voltage level of V
IO
.
If the external power supply should be used only as a
backup when the bus power supply fails, a PMOS transistor
can be inserted between the external power supply and VDD
as shown in Figure 5. The gate is connected to VS, and will
be driven high when the voltage on STC goes above the
turn-on threshold of the LDO, nl. V
STC,
VDD
ON
. For more
information see the paragraph on the power on sequence and
corresponding Figure 12 on page 10.
Communication, Master to Slave
M-BUS communication from master to slave is based on
voltage level signaling. To differentiate between master
signaling and voltage drop caused by the signaling of
another slave over cabling resistance, etc., the mark level
V
BUS,
MARK
is stored, and only when the bus voltage drops
to less than V
T
will the NCN5150 detect communication. A
simplified schematic of the receiver is shown in Figure 8.
The received data is transmitted on the pins TX and TXI, as
shown in the waveforms of Figure 7.
An external capacitor must be connected to the SC pin to
store the mark voltage level. This capacitor is charged to V
B
.
Discharging of this capacitor is typically 40x slower, so that
the voltage on SC drops only a little during the time the
master is transmitting a space. The value of C
SC
must be
chosen it the range of 100 nF−330 nF.
Figure 7. Communication, Master to Slave
V
BUS
V
TX
V
TXI
V
IO
V
IO
V
MARK
= [21 V, 42 V]
V
T
= V
MARK
− 6 V
V
SPACE
= V
MARK
− 12 V
t
t
t
Figure 8. Communication, Master to Slave
Encoding
Echo
V
B
TXI
TX
SC
I
CHARGE
I
DISCHARGE
+
−
Communication, Slave to Master
M-BUS communication from slave to master uses bus
current modulation while the voltage remains constant. This
current modulation can be controlled from either the RX or
RXI pin as shown in Figure 10. When transmitting a space
(“0”), the current modulator will draw an additional current
from the bus. This current can be set with a programming
resistor R
RIS
. To achieve the space current required the
M-BUS standard, R
RIS
should be 100 W. A simplified
schematic of the transmitter is shown in Figure 11.
Figure 9. Typical Relationship between RIS and
Current Modulation Level
