NCN5110 Datasheet, PDF (14/24 Pages) ON Semiconductor – Transceiver

NCN5110
ANALOG FUNCTIONAL DESCRIPTION
Because NCN5110 follows the KNX standard only a brief
description of the KNX related blocks is given in this
datasheet. Detailed information on the KNX Bus can be
found on the KNX website (www.knx.org) and in the KNX
standards.
KNX Bus Interfacing
Each bit period is 104 ms. Logic 1 is simply the DC level
of the bus voltage which is between 20 V and 33 V. Logic 0
is encoded as a drop in the bus voltage with respect to the DC
level. Logic 0 is known as the active pulse.
The active pulse is produced by the transmitter and is
ideally rectangular. It has a duration of 35 ms and a depth
between 6 and 9 V (V_act). Each active pulse is followed by
an equalization pulse with a duration of 69 ms. The latter is
an abrupt jump of the bus voltage above the DC level
followed by an exponential decay down to the DC level. The
equalization pulse is characterized by its height V_eq and the
voltage V_end reached at the end of the equalization pulse.
See the KNX Twisted Pair Standard (KNX TP1−256) for
more detailed KNX information.
VBUS
DC Level
Active Pulse
35 ms
Equalization Pulse
69m s
104 ms
1
t
104 ms
0
Figure 10. KNX Bus Voltage versus Digital Value
KNX Bus Transmitter
The purpose of the transmitter is to produce an active
pulse (see Figure 10) between 6 V and 10.5 V regardless of
the bus impedance (Note 1). In order to do this the
transmitter will sink as much current as necessary until the
bus voltage drops by the desired amount. The transmitter
will produce an active pulse whenever the TX pin is brought
high. It is up to the microcontroller to provide the bit−level
coding and provide the correct active pulse duration.
KNX Bus Receiver
The receiver detects the beginning and the end of the
active pulse. The detection threshold for the start of the
active pulse is −0.45 V (typ.) below the average bus voltage.
The detection threshold for the end of the active pulse is
−0.2 V (typ.) below the average bus voltage giving a
hysteresis of 0.25 V (typ.). The result of this detection is
available as a pulse on the RXD pin.
Bus Coupler
The role of the bus coupler is to extract the DC voltage
from the bus and provide a stable voltage supply for the
purpose of powering the NCN5110. This stable voltage
supplied by the bus coupler will follow the average bus
voltage. The bus coupler also makes sure that the current
drawn from the bus changes very slowly. For this a large
filter capacitor is used on the VFILT−pin. Abrupt load
current steps are absorbed by the filter capacitor. Long−term
stability requires that the average bus coupler input current
is equal to the average (bus coupler) load current. This is
shown by the parameter DI_coupler/Dt, which indicated the
bus current slope limit. The bus coupler will also limit the
current to a maximum of I_{coupler_lim}. At startup, this current
limit is increased to I_{coupler_lim,startup} to allow for fast
charging of the VFILT bulk capacitance.
There are 4 conditions that determine the dimensioning of
the VFILT capacitor.
First, the capacitor value should be between 12.5 mF and
4000 mF to garantuee proper operation of the part.
The next requirement on the VFILT capacitor is
determined by the startup time of the system. According to
the KNX specification, the total startup time must be below
10s. This time is comprised of the time to charge the VFILT
capacitor to 12 V (where the DCDC convertor becomes
operatonal) and the startup time of the rest of the system
tstartup,system. This gives the following formula:
1. Maximum bus impedance is specified in the KNX Twisted Pair Standard
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