4-4
Power
Output
1. Choose an LC product that sets the filter cutoff frequency at least a
decade below the frequency of the resonant current demand (fd) in the
load. That is, choose
L
o
C
o2
≥
100/(4
π
2
f
d
2
)
;
2. L
o
must be large enough to keep the “effective capacitance” of C
o2
as seen
by the LPT-11 transceiver to less than 5% of the transceiver’s 22µF
output capacitor C2. This can be accomplished by ensuring that
L
o
≥
1mH(C
o2
/1µF)
;
3. The inductor's series resistance must be small enough so that the load
current flowing through it does not generate excessive voltage drop.
Choose an inductor value Lo that has a low enough DC resistance (DCR
in the manufacturer data sheets) for your load current. Generally,
smaller inductors cost less, so you will want to choose the smallest value
of Lo that meets the above criteria;
4. Once
Lo is set, the value of Co
2
can be chosen subject to the conditions in
steps 1 and 2 above.
Finally, it is good design practice to decouple each IC's Vcc pin in the device circuit
using 0.1µF or 0.01µF radial or surface mount capacitors. This decoupling helps to
reduce Vcc noise that could lead to logic noise problems and radiated EMI problems
(see Chapter 6 for more information on design hints to reduce EMI).
WARNING: THE COMBINED CAPACITANCE OF ALL THE DECOUPLING CAPACITORS
(FROM VCC TO GND) SHOULD NOT EXCEED 1.0µF. TOTAL DECOUPLING CAPACITANCE
IN EXCESS OF 1.0µF CAN CAUSE UNSTABLE POWER SUPPLY PERFORMANCE.
Powering Non-Isolated Devices
In order to provide reliable common mode rejection, the link power system operates
with one earth ground connection at the LPI-10 module, and all LPT-11 transceivers
are isolated (floating) relative to ground. The LPT-11 transceiver power supply is
designed to power devices that are ground isolated. If the power supply is connected
to a grounded device, both network communications and power distribution will be
degraded. For devices where the application electronics must be connected to earth
ground, the FT 31xx Smart Transceiver is the best choice, providing both isolation
and communication with LPT-11 devices on the same twisted pair network segment.
Optionally, optical isolation may be used between the application electronics and
Neuron Chip.
Note that only floating measurement instruments should be used when working with
link power devices when power is applied. Only isolated Digital Volt Meters (DVMs),
floating differential probes (e.g., HP 1141A) or battery-powered oscilloscopes (e.g.
THS 720 Series TEKSCOPE) should be used when working with link power devices.
The LPT-11 transceiver may be damaged if its circuitry is inadvertently shorted to
earth while power is applied.