MCP3909 datasheet(23/44 Pages) MICROCHIP

23 / 44 page

DS22025B-page 23

MCP3909

The

high-frequency

output

HFOUT has lower

integration times and, thus, higher frequencies. The

output frequency value can be calculated with the

following equation:

EQUATION 4-2:

ACTIVE POWER HFOUT

FREQUENCY OUTPUT

EQUATION

The constant HFC depends on the FOUT0 and FOUT1

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

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. 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 FOUT0/1 and HFOUT is

equal to 0.0015% of the maximum output frequency

(respectively FC and HFC) for each of the F2, F1 and F0

selections (see Table 4-2 and Table 4-3); except when

F2, F1, F0 = 011. In this last configuration, the no-load

threshold feature is disabled. The selection of FC 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

AN994,

"IEC Compliant Active Energy Meter Design Using The

MCP3905/6”, (DS00994).

TABLE 4-3:

OUTPUT FREQUENCY CONSTANT HFC FOR HFOUT (VREF =2.4V)

TABLE 4-2:

ACTIVE POWER OUTPUT FREQUENCY CONSTANT FC FOR FOUT0/1 (VREF =2.4V)

FC (Hz)

(MCLK = 3.58 MHz)

FOUT Frequency (Hz)

with Full-Scale

DC Inputs

FOUT Frequency (Hz)

with Full-Scale

AC Inputs

MCLK/221

1.71

0.74

0.37

MCLK/220

3.41

1.48

0.74

MCLK/219

6.83

2.96

1.48

MCLK/218

13.66

5.93

2.96

HFOUT Hz

()

8.06 V0

V1 G

HFC

VREF

()

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

Where:

V0 = the RMS differential voltage on

Channel 0

V1 = the RMS differential voltage on

Channel 1

G = the PGA gain on Channel 0

(current channel)

HFC = the frequency constant selected

VREF = the voltage reference

HFC

HFC (Hz)

(MCLK = 3.58 MHz)

HFOUT Frequency (Hz) with

full scale AC Inputs

64 x FC

MCLK/215

109.25

27.21

32 x FC

MCLK/215

109.25

27.21

16 x FC

MCLK/215

109.25

27.21

2048 x FC

MCLK/27

27968.75

6070.12

128 x FC

MCLK/216

219.51

47.42

64 x FC

MCLK/216

219.51

47.42

32 x FC

MCLK/216

219.51

47.42

16 x FC

MCLK/216

219.51

47.42

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