Cirrus Logic AN366 - Manual (Page 5)

AN366
AN366REV2 5
3.4 Application Processor Scaling Example
The scaling example below demonstrates how to convert from the current register value to the reported current
using the full-scale value. The specified full-load (Current
FULLSCALE
) is 50A. If the AFE current register value
(Current
REGISTER
) is 0.25 (0x40 0000), then the actual current value (ReportedCurrent
ACTUAL
) is calculated by
the application processor using Equation 3.
Use Equation 3 to convert the current register value to the real current:.
Scaling for power requires a change in the denominator to reflect a power scaling ratio of 0.36, which is equal
to the voltage (0.6) multiplied by current (0.6). The input full load (Ich
FULLSCALE
) is 50A and the maximum
voltage (Vch
FULLSCALE
) is 140V. If the present load is applied to the meter results in a power
register (Power
REGISTER
) reading of 0.15 (0x13 3333), then the application processor needs to convert the
power register value to the real current value. Use Equation 4 to convert the power register value to real reported
power.
Cirrus Logic power meters are bidirectional, which allows power to be measured in both directions (consumed
or delivered). This reduces the digital scaling by one bit due to polarity, unlike the unsigned RMS current register.
The 24-bit P1
AVG
and P2
AVG
registers are defined in Figure 3.
ReportedCurrent
ACTUAL
Current
REGISTER
Current
FULLSCALE

0.6
-----------------------------------------------------------------------------------------------------------
0.25 50A 
0.6
----------------------------- 20.8A = = = [Eq: 3]
ReportedPower
ACTUAL
Power
REGISTER
Power
FULLSCALE

0.36
--------------------------------------------------------------------------------------------------- =
Power
REGISTER
Vch
FULLSCALE
Ich
FULLSCALE
   
0.36
--------------------------------------------------------------------------------------------------------------------------------------------- =
0.15 140 50    
0.36
--------------------------------------------- 2916.7W = =
[Eq: 4]
Active Power 1 (P1
AVG
) – Page 16, Address 5
Default = 0x00 0000
Instantaneous power is averaged over each low-rate interval (SampleCount samples) and then added
with power offset (P
OFF
) to compute active power (P
AVG
).
This is a two's complement value in the range of -1.0 value  1.0, with the binary point to the right of the
MSB.
Active Power 2 (P2
AVG
) – Page 16, Address 11
Default = 0x00 0000
Instantaneous power is averaged over each low-rate interval (SampleCount samples) to compute active
power (P2
AVG
).
This is a two's complement value in the range of -1.0  value 1.0, with the binary point to the right of the
MSB.
MSB LSB
-(2 0 ) 2 -1 2 -2 2 -3 2 -4 2 -5 2 -6 2 -7 ..... 2 -17 2 -18 2 -19 2 -20 2 -21 2 -22 2 -23
MSB LSB
-(2 0 ) 2 -1 2 -2 2 -3 2 -4 2 -5 2 -6 2 -7 ..... 2 -17 2 -18 2 -19 2 -20 2 -21 2 -22 2 -23
Figure 3. Example of P1
AVG
and P2
AVG
Registers