Cirrus Logic CS5460A, Руководство пользователя

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CS5460A
50 DS284PP4
cause the voltage signal to be shifted by as much as
~0.100 degrees ahead of the current signal.
We
can see that the component value tolerance may
cause even more discrepancy between the two sig-
nal delays than if we had decided to leave the
PC[6:0] bits in their “0000000” setting. Thus, ad-
justment of the PC[6:0] bits to more closely match
the two time-constants/delays may only be useful if
a precise calibration operation can be performed on
each individual power meter, during final calibra-
tion/test of the meter. Indeed, such variation in the
R and C component values demonstrates an even
more important reason for the designer to take ad-
vantage of the CS5460A’s phase compensation
feature, as can be seen when one compares the ef-
fect that component value tolerance has on channel
phase-matching to the relatively slight difference in
phase-match that is calculated if one assumes ze-
ro-value tolerance for the R and C values.
4.14 Protection Against High-Voltage
and/or High-Current Surges
In many power distribution systems, it is very like-
ly that the power lines will occasionally carry brief
but large transient spikes of voltage/current. Two
common sources of such high-energy disturbances
are 1) a surge in the line during a lightning storm,
or 2) a surge that is caused when a very inductive
or capacitive load on the power line is suddenly
turned on (“inductive kick”). In these situations,
the input protection resistors and corresponding in-
put filter capacitors (discussed in the previous sec-
tions) may not be sufficient to protect the CS5460A
from such high-frequency voltage/current surges.
The surges may still be strong enough to cause per-
manent damage to the CS5460A. Because of this,
the designer should consider adding certain addi-
tional components within the voltage/current chan-
nel input circuitry, which can help to protect the
CS5460A from being permanently damaged by the
surges.
Referring to Figure 24, the addition of capacitors
C1 and C2 can help to further attenuate these
high-frequency power surges, which can greatly
decrease the chances that the CS5460A will be
damaged. Typical values for C1 and C2 may be on
the order of 10pF, although the exact value is relat-
ed to the reactive and resistive properties of the us-
er’s voltage and current sensor devices.
In
addition, diodes D1 - D4 can help to quickly clamp
a high voltage surge voltage presented across the
voltage/current inputs, before such a surge can
damage the CS5460A. An example of a suitable
diode part number for this application is BAV199,
which has the ability to turn on very quickly (very
small turn-on time). A fuse could potentially serve
this purpose as well (not shown). R3 and R4 can
provide protection on the “-” sides of the two input
pairs. Set R3 = R1 and R4 = R5. Finally, placing
50-
Ω
resistors in series with the VA+ and VD+ pins
is another technique that has sometimes proven to
be effective in protecting the CS5460A from such
high-level, high-frequency voltage/current surges.
However, these 50-
Ω
resistors may not be neces-
sary if the protection on the analog input channels
is sufficient, and this is not the most attractive solu-
tion, because these resistors will dissipate what can
be a significant amount of power, and they will
cause an undesirable voltage drop which decreases
the voltage level presented to the VA+ and VD+
supply pins.
4.15 Improving RFI Immunity
During EMC acceptance testing of the user’s pow-
er metering assembly, the performance of the
CS5460A’s A/D converters can be adversely af-
fected by external radio frequency interference
(RFI). Such external RFI can be coupled into the
copper traces and/or wires on the designer’s PCB.
If RFI is coupled into any of the traces which tie
into the CS5460A’s Vin+/Vin- or Iin+/Iin- input
pins, then errors may be present in the CS5460A’s
power/energy registration results.