MCP3905/06
DS21948E-page 20 © 2009 Microchip Technology Inc.
The high-frequency output HF
OUT
has lower
integration times and, thus, higher frequencies. The
output frequency value can be calculated with the
following equation:
EQUATION 4-2: HF
OUT
FREQUENCY
OUTPUT EQUATION
The constant HF
C
depends on the F
OUT0
and F
OUT1
digital settings with the Table 4-4.
The detailed timings of the output pulses are described
in the Timing Characteristics table (see Section 1.0
“Electrical Characteristics” and Figure 1-1).
MINIMAL OUTPUT FREQUENCY FOR
NO-LOAD THRESHOLD
The MCP3905/06 also includes, on each output
frequency, a no-load threshold circuit that will eliminate
any creep effects in the meter. The outputs will not
show any pulse if the output frequency falls below the
no-load threshold. The minimum output frequency on
F
OUT0/1
and HF
OUT
is equal to 0.0015% of the
maximum output frequency (respectively F
C
and HF
C
)
for each of the F2, F1 and F0 selections (see Table 4-3
and Table 4-4); except when F2, F1, F0 = 011. In this
last configuration, the no-load threshold feature is
disabled. The selection of F
C
will determine the start-up
current load. In order to respect the IEC standards
requirements, the meter will have to be designed to
allow start-up currents compatible with the standards
by choosing the FC value matching these
requirements. For additional applications information
on no-load threshold, startup current and other meter
design points, refer to AN994, "IEC Compliant Active
Energy Meter Design Using The MCP3905/6”,
(DS00994).
TABLE 4-4: OUTPUT FREQUENCY CONSTANT HF
C
FOR HF
OUT
(V
REF
=2.4V)
HF
OUT
Hz()
8.06 V
0
×
V
1
G
××
HF
C
×
V
REF
()
2
--------------------------------------------------------------- -=
Where:
V
0
= the RMS differential voltage on Channel 0
V
1
= the RMS differential voltage on Channel 1
G = the PGA gain on Channel 0
(current channel)
F
C
= the frequency constant selected
V
REF
= the voltage reference
F2 F1 F0 HF
C
HF
C
(Hz)
HF
C
(Hz)
(MCLK = 3.58 MHz)
HF
OUT
Frequency (Hz) with
full-scale AC Inputs
000 64 x F
C
MCLK/2
15
109.25 27.21
001 32 x F
C
MCLK/2
15
109.25 27.21
010 16 x F
C
MCLK/2
15
109.25 27.21
0112048 x F
C
MCLK/2
7
27968.75 6070.12
100128 x F
C
MCLK/2
16
219.51 47.42
101 64 x F
C
MCLK/2
16
219.51 47.42
110 32 x F
C
MCLK/2
16
219.51 47.42
111 16 x F
C
MCLK/2
16
219.51 47.42
