Transceiver Output Power
The LPT-11 transceiver power supply circuit performs a number of key functions:
• draws DC power from the twisted pair network without interfering with
communications with other devices;
• regulates the output voltage (Vcc) to +5VDC ±5% with a maximum steady state
current of 100mA;
• limits Vcc output current to prevent a device 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-11 transceiver will be a
function of the network wiring topology and the power loading on the network. The
LPT-11 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-11 transceiver is 100mA
over the full operating temperature range. For applications that come close to this
100mA limit, it is important to measure peak instantaneous 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-11 transceiver’s power
supply circuit will begin to limit current on any peak instantaneous currents that
exceed 100mA, and this will cause a droop in Vcc. Note that the internal circuitry of
the transceiver itself derives a small amount of current from the switching power
supply directly, and this current consumption does not reduce the 100 mA available
current limit.
The power supply is designed to operate without damage in the event of a short
between the +5VDC output and GND. Since the LPT-11 transceiver uses a switching
power supply to regulate Vcc, the filtering and decoupling requirements of the other
powered devices in the device 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 variations in current loading at a
continuous repetition 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-11 transceiver's switching
frequency (about 140kHz), so the IR LED driver's power supply may need to be
isolated from Vcc with an L-C filter. Typical R-C and L-C filters for isolating loads
are shown in figure 4.1
4-2
Power
Output