Echelon LONWORKS PLT-22, Руководство пользователя

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series with the supply input. Optimal inductor selection will be covered in the
switching supply section of this chapter.
Power Supply Noise
Power supplies have the opportunity to introduce noise both at their inputs and
outputs. Noise conducted out of the input onto the AC line can degrade
communication performance as well as cause the device to violate emissions
regulations. Similarly, output noise can couple into the transceiver and degrade
communication performance.
Due to regulatory constraints, it is important to verify that the total noise conducted
onto the AC mains is adequately contained. Some supplies may require the addition
of an input filter as shown in the switching supply section of this chapter.
For information on conducted emissions test methodology, see chapter 6.
Energy Storage Power Supplies
In cost or size-sensitive devices, it may be desirable to use an energy storage
power supply. These supplies take advantage of the wide supply voltage range
and the disparity between the PLT-22 transceiver’s transmit and receive mode
current requirements, storing energy while the device is in receive mode and
expending it during signal transmission. By using an energy storage system, the
device can use a smaller, less expensive, power supply than an equivalent “full
power” device. In this way, a low-current supply may be used which only has to
supply the required receive mode current, plus an incremental current to
recharge a capacitor between transmissions.
In order for this energy-storage architecture to work, consideration must be
given to the capacitance value required to maintain an acceptably small supply
voltage drop during transmission. The capacitor value must be selected such
that proper node operation can be maintained during the transmission of a
maximum length packet. The definition of proper node operation includes
preserving transmit and receive functionality of the PLT-22 transceiver, as well
as maintaining V
DD5
supply regulation. The size of the capacitor can be
minimized by designing the supply to have the highest possible nominal voltage
while guaranteeing that V
A
will not exceed 16V, the maximum V
A
specification
of the PLT-22 transceiver. See Appendix C for details on how to balance network
messaging requirements with power supply designs.
For nodes set to transmit at 7Vp-p (TXLVL pin grounded), proper node operation is
then maintained when the node power supply meets both of the following conditions:
• V
A
≥ 11.4V after the typical I
A
transmit load of 130mA has been active for
90.7ms
(1)
. This condition ensures adequate transmit amplifier headroom to drive
the full 7Vp-p signal onto the line under lightly loaded conditions. This condition
needs to be met only with nominal line input voltage and at room temperature.
• V
A
≥ 9.0V after the worst case I
A
transmit load of 250mA has been active for
90.7ms
(1)
. This condition ensures adequate headroom when the transmit
L
ON
W
ORKS
PLT-22 Transceiver's User Guide 5-3