User`s guide

4-2
Transceiver Output Power
The LPT-10 transceiver power supply circuit performs a number of key functions:
• draws DC power from the twisted pair network without interfering with
communications with other nodes;
• regulates the output voltage (Vcc) to +5VDC ±10% with a sustained peak current of
100mA;
• limits Vcc output current to prevent a node with a Vcc short circuit from reducing the
network voltage;
• uses an undervoltage shutdown circuit to prevent the transceiver from attempting to
start up when the network voltage is too low.
The upper limit of the twisted pair network voltage is 42.4VDC at the output of the
LPI-10 module. The actual voltage at the input to the LPT-10 transceiver will be a
function of the network wiring topology and the power loading on the network. The
LPT-10 transceiver has a lower input voltage limit of ≈26VDC, and the power supply
includes an undervoltage detection circuit that will prevent the transceiver from
operating at a lower network voltage.
The maximum (sustained peak) output current for the LPT-10 transceiver is 100mA over
the full operating temperature range. For applications that come close to this 100mA
limit, it is important to measure peak application current with a current probe (like the
Tektronix AM503) rather than with a Digital MultiMeter (DMM). DMMs measure the
average current, but they generally cannot follow the rapid current variations
associated with digital circuitry. The LPT-10 transceiver’s power supply circuit will
begin to limit current on any peak application currents that exceed 100mA, and this will
cause a droop in Vcc. Note that the transceiver itself derives power from the switching
power supply directly, and its current consumption does not reduce the available 100mA
limit.
The power supply is designed to operate without damage in the event of a short
between the +5VDC output and GND. However, the power supply output voltage may
overshoot its nominal 5V value when the short condition is cleared. To avoid this
overshoot, a 5.6V zener diode should be installed between the +5VDC output and GND
if there is a possibility that a node's power supply could be put into a short circuit
condition.
Since the LPT-10 transceiver uses a switching power supply to regulate Vcc, the
filtering and decoupling requirements of the other powered devices in the node must be
considered. A power supply output filter may be required to prevent noise generated by
the transceiver's switching power supply from interfering with the operation of these
other devices.
As with all switching power supplies, "resonant" current loads on Vcc should be
avoided. A resonant load is one that presents large changes in current loading at a
continuous repitition rate that is near the switching power supply’s switching frequency
(or its immediate harmonics or sub-harmonics). An example is a circuit that includes an
IR transmitter. The IR LED in a transmitter is typically driven by current pulses ≥
50mA in amplitude, and with a carrier frequency of 39kHz or 42kHz. These frequencies
are close to 1/4 of the LPT-10 transceiver's switching frequency (about 150kHz), so the