Cirrus Logic CS5460A - Manual (Page 21)

CS5460A
DS487F5 21
3. FUNCTIONAL DESCRIPTION
3.1 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 ener-
gy can be accumulated. Each EOUT pulse repre-
sents a predetermined quantity of energy. The
quantity of energy represented in one pulse can be
varied by adjusting the value in the Pulse-Rate
Register. Corresponding pulses on the EDIR out-
put pin signify that the sign of the energy is nega-
tive. Note that these pulses are not influenced by
the value of the Cycle-Count Register, and they
have no reliance on the computation cycle, de-
scribed earlier. With MCLK = 4.096 MHz, K = 1,
the pulses will have an average frequency (in Hz)
equal to the frequency setting in the Pulse Rate
Register when the input signals into the voltage
and current channels cause full-scale readings in
the Instantaneous Voltage and Current Registers.
When MCLK/K is not equal to 4.096 MHz, the
pulse-rate should be scaled by a factor of
4.096 MHz / (MCLK/K) to get the actual output
pulse-rate.
EXAMPLE #1: For a power line with maximum rat-
ed levels of 250 V (RMS) and 20 A (RMS), the
pulse-frequency on the EOUT pin needs to be
‘IR’ = 100 pulses-per-second (100 Hz) when the
RMS-voltage and RMS-current levels on the power
line are 220 V and 15 A respectively. To meet this
requirement, the pulse-rate frequency (‘PR’) in the
Pulse-Rate Register must be set accordingly.
After calibration, the first step to finding the value
of ‘PR’ is to set the voltage and current sensor gain
constants, K
V
and K
I
, such that there will be ac-
ceptable voltage levels on the CS5460A inputs
when the power line voltage and current levels are
at the maximum values of 250 V and 20 A. K
V
and
K
I
are needed to determine the appropriate ratios
of the voltage/current transformers and/or shunt
resistor values to use in the front-end voltage/cur-
rent sensor networks.
For a sinewave, the largest RMS value that can be
accurately measured (without over-driving the in-
puts) will register ~0.7071 of the maximum DC in-
put level. Since power signals are often not
perfectly sinusoidal in real-world situations, and to
provide for some over-range capability, the RMS
Voltage Register and RMS Current Register is set
to measure 0.6 when the RMS-values of the
line-voltage and line-current levels are 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. The sensor gain con-
stants, K
V and K I , are determined by demanding
that the voltage and current channel inputs should
be 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

These sensor gain constants are used to calculate
what the input voltage levels will be on the
CS5460A inputs when the line-voltage and
line-current are 220 V and 15 A. These values are
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-
rent inputs are set at V
Vnom and V Inom , the pulse
rate needs to be ‘IR’ = 100 pulses per second. IR
will be some percentage of PR. The percentage is
defined by the ratios of V
Vnom
/250 mV and
V
Inom
/250 mV with the following formula:
From this equation the value of ‘PR’ is shown as:.
Therefore the Pulse-Rate Register is set to
~420.875 Hz, or 0x00349C.
The above equation is valid when current channel
is set to x10 gain. If current channel gain is set to
x50, then the equation becomes:
PulseRate IR PR
V
Vnom
250mV
-------------------
V
Inom
250mV
-------------------
 
= =
PR
IR
V
Vnom
250mV
------------------
V
Inom
250mV
------------------ 
--------------------------------------------
100Hz
132mV
250mV
------------------
112.5mV
250mV
-----------------------

------------------------------------------------ = =
PR
IR
V
Vnom
250mV
------------------
V
Inom
50mV
---------------

----------------------------------------- =