User`s guide

LONWORKS LPT-10 Transceiver User’s Guide 2-7
(generally the solder side of the board) should contain as much ground plane as
possible.
The switching power supply circuit in the LPT-10 transceiver uses the external
components L1, C1 and C2 as part of its switching regulator. Because moderate currents
are switched at approximately 150kHz, it is very important that L1, C1 and C2 are
placed close to the LPT-10 transceiver and oriented as shown in the figure. The inductor
L1 and the capacitors C1 and C2 should be placed with minimum gaps to the body of the
transceiver.
The ground connections between the LPT-10 transceiver and L1, C1 and C2 should be as
similar as possible to those shown in figure 2.3. The wide ground traces and the ground
plane on the other layer of the board serve two functions. First, the wide ground traces
reduce inductance to provide a low-impedance path for the power supply switching
currents. Second, the wide ground areas minimize electric and magnetic field noise
generated by the power supply circuit. The "INDUCTOR" trace from pin 4 of the LPT-
10 transceiver to the input of L1 carries a 35V, 150kHz switching waveform, and this
can generate moderate levels of electric field noise that can capacitively couple into
any nearby high-impedance circuitry. The ground plane is shown close to the
"INDUCTOR" trace in order to absorb some of the electric field noise generated by the
trace.
Note that L1 is shown in figure 2.3 with a dot marking that is oriented toward the
transceiver. In the Taiyo-Yuden LHL08 series of inductors, the dot identifies which pin
is connected to the inner portion (beginning) of the cylindrical wire winding on the
ferrite slug. Since the input to L1 is a 35V switching waveform and the output is a
smooth +5VDC, it is best to orient the inductor so that the windings with the noisy 35V
switching waveform are in the inner part of the inductor coil. This uses the inductor
coils themselves as part of the electric field shielding. Consult the manufacturer’s
data sheet for the inductor you are using to determine if polarity marking is available,
and whether the marked pin is connected to the inner or outer portion of the coil
winding.
If inductor L1 is an "open slug" type without shielding, it will generate moderate levels
of magnetic field noise during normal power supply operation. Ground guarding and a
ground plane on the other PC board layer will help to contain the magnetic field noise
in a smaller volume near L1. Since the switching frequency of the power supply is near
150kHz, the copper ground plane serves as a fairly effective magnetic field shield.
The electric and magnetic field noise generated by any switching power supply circuit
may interfere with the operation of sensitive circuitry nearby. The magnetic field
noise can be minimized by using a toroidal inductor for L1, or by using a slug inductor
with an integral magnetic shield. Sensitive circuits on a link power node should be laid
out to minimize the loop area of any amplifier inputs or high-impedance lines.
Minimizing these loop areas reduces the amount of voltage that can be induced in the
circuits from the magnetic switching noise that is present. Note that the traces from
the network connector to the LPT-10 transceiver as shown in figure 2.3 are spaced as
closely together as possible in order to minimize their loop area. Circuits that are
sensitive to electric field noise should be kept away from L1 and pin 4 of the
transceiver, and ground guarding should be employed to shield them from the electric
field noise.