CS5460A datasheet(21/54 Pages) CIRRUS

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CS5460A

DS487F4

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

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

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, KV and KI, 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. KV and

KI 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, KV and KI, 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.

KV = 150 mV / 250 V = 0.0006

KI = 150 mV / 20 A = 0.0075

Ω

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

VVnom and VInom.

VVnom =KV * 220 V = 132 mV

VInom =KI * 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 VVnom and VInom, 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 VVnom/250 mV and

VInom/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

Vnom

250mV

-------------------

Inom

250mV

-------------------

⋅⋅

Vnom

250mV

------------------

Inom

250mV

------------------

--------------------------------------------

100Hz

132mV

250mV

------------------

112.5mV

250mV

-----------------------

------------------------------------------------

Vnom

250mV

------------------

Inom

50mV

---------------

-----------------------------------------

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