Cirrus Logic CS5460A, Manual

Page: 26 / 64

CS5460A
26 DS284PP4
4. FUNCTIONAL DESCRIPTION
4.1 Pulse-Rate Output
As an alternative to reading the real energy through
the serial port, the EOUT and EDIR pins provide a
simple interface with which signed energy can be
accumulated. Each EOUT pulse represents a prede-
termined quantity of energy. The quantity of ener-
gy represented in one pulse can be varied by
adjusting the value in the Pulse-Rate Register.
Corresponding pulses on the EDIR output pin sig-
nify that the sign of the energy is negative. Note
that these pulses are not influenced by the value of
the Cycle-Count Register, and they have no reli-
ance on the computation cycle, described earlier.
With MCLK = 4.096 MHz, K = 1, the pulses will
have an average frequency (in Hz) equal to the fre-
quency setting in the Pulse Rate Register when the
input signals into the voltage and current channels
cause full-scale readings in the Instantaneous Volt-
age and Current Registers. When MCLK/K is not
equal to 4.096 MHz, the user should scale the
pulse-rate that one would expect to get with
MCLK/K = 4.096 MHz by a factor of 4.096 MHz /
(MCLK / K) to get the actual output pulse-rate.
EXAMPLE #1: Suppose that we want the
pulse-frequency on the EOUT pin to be ‘IR’ = 100
pulses per second (100 Hz) when the RMS-volt-
age/RMS-current levels on the power line are
220 V and
15 A
respectively, noting that the max-
imum rated levels on the power line are 250 V and
20 A. We also assume that we have calibrated the
CS5460A voltage/current channel inputs such that
a DC voltage level of 250 mV across the volt-
age/current channels will cause full-scale readings
of 1.0 in the CS5460A Instantaneous Voltage and
Current Registers as well as in the RMS-Voltage
and RMS-Current Registers. We want to find out
what frequency value we should put into the
CS5460A’s Pulse-Rate Register (call this value
‘PR’) in order to satisfy this requirement. Our first
step is to set the voltage and current sensor gain
constants, K
V
and K
I
, such that there will be accept-
able input voltage levels on the inputs when the
power line voltage and current levels are at the
maximum values of 250 V and 20 A, respectively.
We need to calculate K
V
and K
I
in order to deter-
mine the appropriate ratios of the voltage/current
transformers and/or shunt resistor values to use in
the front-end voltage/current sensor networks.
We assume here that we are dealing with a sinuso-
idal AC power signal. For a sinewave, the largest
RMS value that can be accurately measured (with-
out over-driving the inputs) will register at ~0.7071
of the maximum DC input level. Since power sig-
nals are often not perfectly sinusoidal in real-world
situations, and to provide for some over-range ca-
pability, we will set the RMS Voltage Register and
RMS Current Register to measure at 0.6 when the
RMS-values of the line-voltage and line-current
levels are at 250 V and 20 A. Therefore, when the
RMS registers measure 0.6, the voltage level at the
inputs will be 0.6 x 250 mV = 150 mV. We now
find our sensor gain constants, K
V
and K
I
, by de-
manding that the voltage and current channel in-
puts should be at 150 mV RMS when the power
line voltage and current are at the maximum values
of 250 V and 20 A.
K
V
= 150 mV / 250 V = 0.0006
K
I
= 150 mV / 20 A = 0.0075
Ω
These sensor gain constants can help determine the
ratios of the transformer or resistor-divider sensor
networks. We now use these sensor gain constants
to calculate what the input voltage levels will be on
the CS5460A inputs when the line-voltage and
line-current are at 220 V and 15 A. We call these
values V
Vnom
and V
Inom.
V
Vnom
= K
V
* 220 V = 132 mV
V
Inom
= K
I
* 15 A = 112.5 mV
The pulse rate on EOUT will be at ‘PR’ pulses per
second (Hz) when the RMS-levels of voltage/cur-
rent inputs are at 250 mV. When the voltage/cur-