Datasheet

 2006-2012 Microchip Technology Inc. DS22025C-page 23
MCP3909
EQUATION 4-2: ACTIVE POWER HF
OUT
FREQUENCY OUTPUT
EQUATION
The constant HF
C
depends on the F
OUT0
and F
OUT1
digital settings with the Table 4-3.
The detailed timings of the output pulses are described
in the Timing Characteristics table (see Section 1.0
“Electrical Characteristics” and Figure 1-1).
4.8.1 MINIMAL OUTPUT FREQUENCY
FOR NO-LOAD THRESHOLD
The MCP3909 also includes, on each output fre-
quency, 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. This threshold only applies to the
pulse outputs and does not gate any serial data coming
from either the A/D output or the multiplier output. 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 Ta bl e 4 -2 and Ta bl e 4-3); 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 infor-
mation 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 4-3: 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)
HF
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