Cirrus Logic AN337 - Manual

Copyright © Cirrus Logic, Inc. 2008
(All Rights Reserved)
http://www.cirrus.com
CS5463: Determining No-load Condition Without
Violating Starting Condition Standards
1. INTRODUCTION
When the CS5463 is used in power meter applications, some customers use the I
RMS
residual value as the
threshold for the external microprocessor to decide if the meter is under the no-load condition, and then find
the meter can not pass the starting test. This applications note gives the reason and the correct solution.
2. NO-LOAD AND STARTING CONDITIONS
According to the standard IEC 62053-22, the definition of a no-load condition is defined as: "When the volt-
age is applied with no current flowing in the current circuit, the test output of the meter shall not produce
more than one pulse.". The test of starting condition is defined as: "The meter shall start and continue to
register at 0.001In and unity power factor.".
To meet these requirements, a power meter has to use the correct no-load threshold in order to restrain the
pulse output and energy accumulation when there is no load current but resume pulse output and energy
accumulation when the load is above the starting current (I
st
).
3. DETERMINING A NO-LOAD CONDITION CORRECTLY
The CS5463 has three energy pulse outputs (E1, E2, and E3). The pulse outputs under no-load or starting
condition has been well manipulated by the CS5463. In addition, the CS5463 has a no-load threshold reg-
ister (LOAD
Min
) to help disable the active energy pulse output under some specified condition. When the
magnitude of the P
Active
register is less than LOAD
Min
, the active energy pulse output is disabled.
Different from the energy pulses, the energy register, either mechanical or electronic, is normally manipu-
lated by a microcontroller. The microcontroller needs to use a variable and a threshold value to decide if the
meter is under a no-load condition, in which case the values in the power registers should be discarded.
There are two options to select the variable and the threshold.
Table 1: Variable & Threshold Values Comparison
Because I
RMS
exhibits much more system noise than P
Active
, I
RMS
should not be used as a variable
to determine the no-load condition.
V aria ble
Use d Thres ho ld V alue
Com pare
Res ult
I
RMS
R esidual value (AC offset) with
inputs of U
n
and no current
BAD
P
Active
Less than the value with inputs of U
n
and starting current
GOOD
AN337
DEC '08
AN337REV1