Figure 2.2 illustrates the connections between the LPT-11 transceiver and its four
power supply-related components on one layer of a two-layer PC board. The other
layer (generally the solder side of the board) should contain as much ground plane as
possible.
The switching power supply circuit in the LPT-11 transceiver uses the external
components L1, C1, and C2 as part of its switching regulator. Because moderate
currents are switched at approximately 140kHz, it is very important that L1, C1, and
C2 are placed close to the LPT-11 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. If L1 has exposed ferrite, care should be taken to
avoid contact between L1 and the LPT-11 SIP.
The ground connections between the LPT-11 transceiver and L1, C1, and C2 should
be as similar as possible to those shown in figure 2.2. 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-11 transceiver to the input of inductor L1 can have voltage
signals as high as 35Vp-p at 140kHz. This DC-DC switching waveform may 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 2.2 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 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 often can 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 140kHz, 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 device
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-11 transceiver as shown
2-6
Electrical Interface