Cirrus Logic CS5460A, Manual

Page: 45 / 64

CS5460A
DS284PP4 45
age/current channel signals due to such
crosstalk/system noise/etc., and then program the
Power Offset Register to nullify the effects of this
unwanted energy.
4.12 Input Protection - Current Limit
In Figures 6, 7, 8, and 9, note the series resistor R
I+
which is connected to the IIN+ input pin. This re-
sistor serves two purposes. First, this resistor func-
tions in coordination with C
Idiff
and/or C
Idiff
to form
a low-pass filter. The filter will a) remove any
broadband noise that is far outside of the frequency
range of interest, and also b) this filter serves as the
anti-aliasing filter, which is necessary to prevent
the A/D converter from receiving input signals
whose frequency is higher than one-half of the
sampling frequency (the Nyquist frequency). The
second purpose of this resistor is to provide cur-
rent-limit protection for the Iin+ input pin, in the
event of a power surge or lightening surge. The
role that R
I+
contributes to input filtering will be
discussed in the Section 4.13. But first the cur-
rent-limit protection requirements for the Iin+/Iin-
and Vin+/Vin- pins are discussed.
The voltage/current-channel inputs have surge-cur-
rent limits of 100 mA. This applies to brief volt-
age/current spikes (<250 ms). The limit is 10 mA
for DC input overload situations. To prevent per-
manent damage to the CS5460A, the designer must
include adequate protection circuitry in the power
meter design, to insure that these pin current limits
are never exceeded, when CS5460A is operating in
the intended power-line metering environment.
Focussing specifically on Figure 7, which shows
how voltage/current transformers can be used to
sense the line-voltage/line-current, suppose for ex-
ample that the requirements for a certain 120 VAC
power system require that the power meter must be
able to withstand up to a 8kV voltage spike on the
power line during normal operating conditions. To
provide a suitable sensor voltage input level to the
voltage channel input pins of the CS5460A, the
turns ratio of the voltage-sense transformer should
be chosen such that the ratio is, for example, on the
order of 1000:1. A voltage-sense transformer with
a 1000:1 turns ratio will provide a 120 mV (rms)
signal to the CS5460A’s differential voltage chan-
nel inputs, when the power line voltage is at the
nominal level of 120 VAC. Therefore, a brief 8kV
surge would be reduced to a 8V surge across R
V+
.
What happens when 8 volts (common-mode) is
present across one of the analog input pins of the
CS5460A? The Vin+/Vin- and Iin+/Iin- pins of the
CS5460A are equipped with internal protection di-
odes. If a voltage is presented to any of these pins
that is larger than approximately ±7V (with respect
to VA- pin) these protection diodes will turn on in-
side the CS5460A. But in order to prevent exces-
sive current levels from flowing through the
device, the value of R
V+
must be large enough that
when a 8V surge is present across the secondaries
of the voltage-sense transformer, the brief surge
current through R
V+
should not be any greater than
100mA. Therefore, a minimum value for R
V+
would be (8V - 7V) / 100mA = 10
Ω
. This value
may be increased as needed, to easily obtain the de-
sired cutoff frequency of the anti-aliasing filter on
the voltage channel (described later), and also to
provide some margin. But the designer should try
to avoid using values for the protection resistors
that are excessively high. A typical value for R
V+
would be 470
Ω
.
The VIN- pin should also have a protection resistor
(called R
V-
in Figure 7). To maintain symmetry,
the value of R
V-
should be made equal to R
V+
.
For the current channel inputs (Iin+ and Iin-), if we
assume that the maximum current rating (I
max
) for
this power line is 30A (RMS), then a suitable turns
ratio for the current-sense transformer might be
200:1. Since the maximum load for a 120 VAC
line rated at 30A would be 4
Ω
(for unity power fac-
tor), a brief 8kV surge across “L” and “N” could
generate as much as 2000A (RMS) of current